Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: pompeiisneaks on December 25, 2015, 04:28:16 pm
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Hello all,
I had another thread going in the schematics area about what I could do to reproduce a vox AC100/2 '65 circuit. That thread is here:
http://el34world.com/Forum/index.php?topic=19426.0 (http://el34world.com/Forum/index.php?topic=19426.0)
I've actually completed the build, and am making a video of it, for all to see, but before I do, I have something going on, and I need to have some input on what may be up.
I've got the tubes out right now, and am slowly bringing the power up on a variac to make sure nothing goes wrong. As I've been doing this, I noted one of my 2W resistors is starting to smoke, so I shut it down. This is a resistor that goes to ground off of the B+ rail, (the thread above has the circuit diagram, but if you look at my schematic I'm attaching to this post, it shows the current build very close to how I made it.
Is this resistor getting a lot of current because the tubes aren't in it, so all the current is going down that resistor? The resistor on the circuit I'm attaching is R23, on the B+ rail between B and C.
Any thoughts about why there would be such current going through that?
Here's the measurements I made:
225V and an 18k resistor gives 0.0125A or 12.5mA and watts is about 2.8, that's more than the 2watter wants obviously so I see why its getting hot. (This is still way below the max wattage of the amp too, but again, I'm thinking it is just because the tubes arent' there to conduct the current right?)
Is there any point at which I should be stopping voltage tests as things are 'ok' and then connect in the tubes? Or do most people just do one quick test with a little current to make sure its not grounding out and then put in the tubes?
~Phil
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You need to replace that 18K/2W with a 18K/25W resistor. You're already smoking it with the B+ only up to 225V. Guess what's gonna happen when you take it off the variac and the B+ comes up to 450V!
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Ok, is there any reason that the schematic would put it as a 2 watter (the original Vox one that I copied for my schematic?) why was that schematic so far off? You'd think they tested the design out? Or is this original '65 schematic also not that great? I thought this was the one the Beattles used no?
BTW link to original schematic I used here: http://el34world.com/charts/Schematics/files/Vox/Vox_ac100.pdf (http://el34world.com/charts/Schematics/files/Vox/Vox_ac100.pdf)
~Phil
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I don't see any mention of that resistor being only 2 watts. Where do you see that? Do the math. 4502/18000 = 11.25 watts.
I looked at several pics of '60s AC100s. That resistor is about the size of a 10W sand block resistor (maybe a bit larger), only it's round.
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D'oh! You're right, okay, I'll get that replaced :)
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Okay so finding 25watt 18k resistors isn't proving easy. I guess I could just do 4 18k 10W in series and then parallel to get 18k at 20 watt right?
(i.e. two parallel sets, then put into series with one another)
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The resistance value is not critical. I would just use this...
http://www.mouser.com/ProductDetail/Vishay-Dale/RH02520K00FE02/?qs=sGAEpiMZZMtlubZbdhIBIMow1WQ9W2ngj8pozixWgko%3d (http://www.mouser.com/ProductDetail/Vishay-Dale/RH02520K00FE02/?qs=sGAEpiMZZMtlubZbdhIBIMow1WQ9W2ngj8pozixWgko%3d)
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Ok cool, 'not critical' as in 'at least 18k' or how much variance can you get by with on something like that? is say 10k or 15k too low etc? More for my knowledge, not for this specific case.
~Phil
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I would not go lower because the bleed current will increase and you would need a higher wattage resistor. 20K is only 11% more than 18K. Slightly less bleed current. 20K is an easier value to find.
The reason I initially suggested 18K @ 25W was because of an old rule of thumb that I was taught a long time ago. Calculate power and double that for the resistor wattage to provide a safety margin. Then choose the next higher wattage that is commonly available. Specifically, that 18K will dissipate 11.25 watts when 450V is applied. So, double 11.25W is 22.5W. Next commonly available size is 25W.
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Excellent, makes perfect sense, thanks! I'll get one and proceed after I've replaced it.
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I had one of those AHA! moments after this reply. I got all the science and math of it just fine, but for whatever reason this simple fact was eluding me... there was nothing wrong with the circuit, the simple fact that an 18k resistor in a 450V circuit will always generate 25 mA and 11.25 Watts is unchangeable no matter how you modify the circuit. I don't know why I was trying to over complicate it. I had done all the math already but somehow was thinking that the other components of the circuit would have some play in this. They don't.
Thanks Sluckey!
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I tend to over think stuff all the time too!
That amp will work without that resistor if you want to continue testing while waiting for a big resistor.
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If I can say, the AC100 circuit is known as to have problems about the bias section of the circuit
if you can, give particoular attention to this section, also, sometime you can find mods on the web
Franco
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Franco,
Thanks, I actually modified that circuit, if you look at my schematics I have a different bias in there. Please let me know if that looks okay.
So next steps (I was able to buy a 22.5k 25w resistor locally), I'm seeing something quite bad/odd in the power. I've got my variac up to about 60v AC and the power on the B+ is already up to 500 VDC. That's not good. Does this mean somethings wrong with the PT? Why would it be putting out such high voltages at half power? Even the Vox AC100CPH schematic that has the voltages for this tranny show it at 497V right before the caps/choke at A, and then at B its down to 488.
When PT's go bad do they sometimes increase their power output? on the other side?
The heaters even are running at like 70V instead of 6! All this at 60ish VAC.
~Phil
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Which PT are you using ?
Are you sure about primary connections ?
Franco
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I'm using the PT from the Vox AC100CPH, linked in the other thread at the top, This is obviously the right rails, or it would be one of the lower windings right? (I've removed the boards from the 100CPH and am making my own circuit in there, just reusing the PT/OT/Chokes from the existing one)
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I'm using the PT from the Vox AC100CPH
WHOA!!!
That PT is meant to be used with a full wave conventional rectifier that uses a center tap to ground circuit. But you have connected a full wave bridge rectifier to that PT, so it will produce exactly twice the B+ you desire.
If you want to use that PT then copy the rectifier circuit from the AC100CPH. The filters can be the same as the orig '60s AC100.
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Ditto
Franco
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Oh my, I'm glad
a. I used a variac to bring it up
b. I learned this before I did something really bad :) Okay, I'll sort that out now.
So why does a full wave only output half the power vs bridge rectifier?
~Phil
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Okay so this is what you mean, I'm attaching a shot of the circuit with the rectifier. The problem I have now is that I don't have a board to attach the diodes to. Do I also need the 1n 1kv caps on this? I have the 22R 7W ones still from the old one, I just don't know if I'll have those caps. (I can buy some of course). What is the purpose of the caps in a situation like this? Does it protect the diodes?
Edit: Helps if I actually attach it... side note I DO have all the parts from this schematic so I'm golden :)
~Phil
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What is the purpose of the caps in a situation like this? Does it protect the diodes?
diode rectifiers tend to generate hi-freq spikes, the caps tend to suppress those spikes
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So why does a full wave only output half the power vs bridge rectifier?
Both rectifier circuits output the same power. But the bridge will output twice the voltage (at half the current, for same-power).
The non-bridge rectifier has a grounded center-tap, and only half the secondary winding is passing current at any one time. The bridge passes current from the entire winding, all the time. So if you take a center-tapped secondary and use a bridge rectifier, you will wind up with twice the normal output voltage.
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Cool thanks to both of you, makes sense.
~Phil
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Oh I also figured out why one of my heaters was at 70ish VAC. There is a power rail running 150V that I'm not going to need, but I confused it with the heaters and connected that, I'll be able to desolder those and hook it up correctly now as well. what fun :)
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it's very good that you go easy on the first start-up and find these things before anything puffs out it's last smoke :icon_biggrin:
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You will also need to use a different bias circuit with that PT. I'd just copy the AC100CPH circuit.
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it's very good that you go easy on the first start-up and find these things before anything puffs out it's last smoke :icon_biggrin:
Also use your light bulb limiter with the variac.
Variac > light bulb limiter > amp.
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You will also need to use a different bias circuit with that PT. I'd just copy the AC100CPH circuit.
Why is that? So the circuit I have made for the other doesn't work with the PT? I thought it was just about having a variable resistor to adjust the bias resistance, and the other parts provide circuit protection. Is that current setup not going to provide the protection I need? Even with the class X cap there? I guess I'm needing to learn a lot more about how PT interacts with the circuit. I was thinking that it just provides the input voltage and the other parts of the circuit were what modified how it worked in each stage? (Not sure if that makes sense?)
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it's very good that you go easy on the first start-up and find these things before anything puffs out it's last smoke :icon_biggrin:
Also use your light bulb limiter with the variac.
Variac > light bulb limiter > amp.
I still haven't built one heh, maybe I need to do that first thing tomorrow before I go to next steps.
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Your bias circuit with the class x cap works well with a Bridge rectifier. But since you will not be using a bridge with that PT you need to use a more common bias supply. It's easy to modify your existing circuit. Just remove R35 and replace C16 with a 220K 1W resistor. You may need to fiddle with the value of this resistor to get a bias range of about -25 to -50v. Everything else in your bias circuit remains the same. This is a very standard circuit used by many Marshall amps.
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Oh ok cool, thanks. I'll do that.
~Phil
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Ok got that sorted out today, all seems fine so far, with one new twist. I'm attaching the heater schematic information, maybe i'm missing something, but I don't think so. In bringing it up again, I ran into a smoking resistor (This time just a 330Ohm on my power LED line, that part was pretty non important), but then I measured heater voltage, and the ac voltage at about 60VAC is about 11VAC when its supposed to be 6.3VAC at full 120VAC. I reviewed the circuit and I don't see anything 'wrong' here, I've used the right wires that were connected only to the preamp before, but now have them to the power amp heaters as well. In the section of the original schematic I'm attaching, it shows that both preamp and power amp heaters c ome from the same line, but that it splits off the preamp to a bridged rectifier so its dc power. There are a few other caps etc. I don't see that I'd really need that, as all the other amps I've seen on this forum so far just use AC for heaters.
Is there something else I'm missing? Why are my heaters running so high voltage? (I added 2 100 Ohm 1W resistors to ground on the circuit like I see many current ones do, but that doesn't increase voltage right?).
Ideas?
~Phil
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D'oh! Nevermind, I figured it out, I disconnected the 'wrong' one, and then disconnected the 'right' one to mix the two together, but accidentally closed off the 'right' one and reconnected them all to the 'wrong' one :BangHead: Oh well, I'll rewire those now and then see where I'm at ;)
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Ok sanity restored, but now I think I may need the full bridge rectifier that I was using before. They sent it as DC because they wanted to convert it to 6VDC because it seems to run about 3VAC. Is there any other reason I'd only have 3VAC on the heaters? If I look at the schematic I posted back two comments ago, it doesn't have a rectifier for the power amp heaters, only for the Preamp. Anyone have ideas? Or is it that I'm using the resistors early in the circuit and that is causing the voltage to drop? Are the resistors to ground supposed to only be at the end of the ac rails or is putting them in the start okay so long as they are there?
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Is there any other reason I'd only have 3VAC on the heaters?
How are you measuring them? 1htr wire to ground? That = about 3vac. If you measure one htr wire to the other htr wire, THAT should = about 6vac
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If you are measuring filament voltage with one probe on chassis ground, then 3VAC is what you should measure. You should measure filament voltage with one probe on a filament pin of a socket and the other probe on the other filament pin of a socket. IOW measure filament voltage BETWEEN pins 2 and 7 of one of the EL34s, or BETWEEN pins 4/5 and 9 of a 12AX7.
I would not bother with the DC heater supply in that amp.
Those two resistors can be connected anywhere along the filament string. Makes no difference whether at the start or finish.
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Oh excellent, then I'm in great shape. I'm going to give the tubes a shot now, and hook er up to the oscilloscope and give it a test
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Yup that was the case, when I tested against the two it came up 6.3, we're golden. I've run it with the oscilloscope at full power, and it seems to have some really odd distortion going on, I'm even seeing it in what appears to be in the volume stage where I'm getting a lot of really weird distortion that seems to come and go. If I have the volume at a very low setting it seems mostly gone, but as soon as I get it up even a little, maybe 3/10 it starts really doing odd stuff.
Also, the input voltage to the grid is at -50ish and when I biased it down a bit I got it to about -35 I think was the lowest I could go, it seems like from the EL34 datasheet it really only wants about -36 max, is that right? Does that mean that maybe its just biased too hot and getting really odd distortion? I think maybe you mentioned I may have to modify the main resistor to get it to vary between --25 and -50, right now its about -55 or -60 down to about -35.
I've yet to hook it up to speakers and a guitar, just using dummy load, and a phone app sending in a sine wave. It def shows a lot of distrortion on the scope as well.
Any ideas on where to poke first?
~Phil
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Plug in a speaker and guitar.
If you want the bias voltage to be smaller, increase the value of that resistor I mentioned.
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Sounds good, I'll give that all a go tomorrow.
~Phil
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Another easy way to lower the bias voltage is to LOWER the value of R33 from 47K. You have it labeled as 'bias range' on your schematic.
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really odd distortion going on
Make sure you *verify* your *source* signal after plugging it in, I got burnt because I started at the spkr, chased the problem back to a flakey cable bringing in the sine wave :think1:
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Shooter, that's exactly what I started thinking too, but I tested on the input jack with the 'scope and it was perfect there and only there. I always do taht to eliminate input as the source of the issue. I do think it could be possible though, that my dummy load was being silly, (not sure if having poor connection there can cause bad behavior) so I think as sluckey said, I'll plug in the speakers and a guitar and see what it 'sounds' like for real :)
Edit: In fact that's part of the reason I bought a 4 input 'scope so I can keep one on the input to ensure its not 'off' :)
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What is the voltage level of your input signal?
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I'll have to take a look when I get home, I'm trying to remember, I think it was 200mV /div and only covered maybe 4 divisions, so 800mV to maybe 1V? Is that too hot?
Edit: Googled it and it seems most guitars that are really hot will not exceed 1V, but I'll have to double check, it may be that I was a bit over it, and this input wasn't happy with that volume level.
I'll let you know what I see on the 'scope at home.
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I usually use 200mV peak to peak applied to the input jack.
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Ok I was definitely pretty hot on my phone., (forget that headphone output is pretty hot). I lowered it down to 200mV peak to peak and it was going up to almost 5 or more before the weirdness started happening. Also this sine wave or input cable may be adding some noise that I'm not able to hear without speakers so after work I'm going to take the speakers to the garage, and a guitar, and give er a whirl.
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First test drive, and it works,
BUUUTTTT... at low volumes its sounding fine except the massive ground hum. I'll monkey with that a bit.
The other issue is still that as soon as I get it up to maybe 5 or so there is a horrible pop type noise that seems cyclical, is that a kind of parasitic oscillation? Or what else would make a cycling pop/chirp sound? I also tapped around the circuitry with a chopstick and near one area where the exit of the phase inverter and it makes a nice crackle noise. Is that just that the solder joints in that area may need touch ups? It is odd that i can't get it to happen when I tap one of the specific areas, but if I hit two things at once it seems to make the pop noise.
Any pointers on what would cause that? It does sound pretty soft still up til I hit that point and then it just becomes overwhelming with the repeated clicky/pop type (or maybe even chirp like I said).
~Phil
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Recheck your drawings. Make sure your schematic agrees with the original (except for the mods you've had to make to use that PT). Make sure your layout drawing agrees with your schematic. Make sure your amp is wired IAW your schematic and layout.
Recheck all solder joints.
Swap tubes.
Measure (and post) voltage readings for all filter cap nodes and every tube pin (even the pins that should be zero volts). What voltage is on pin 5 of each EL34? Pull the B+ fuse and measure bias voltage at pin 5 with the bias pot cranked to each end. What voltages do you get?
Divide and conquer. If you have added reverb, disconnect it. Any better? Pull V1 and V2. Temporarily connect an input jack to the treble control wiper and plug a guitar in. This connects the guitar directly to the input of the power amp. It won't be nearly as loud but should be hum free and very clean sounding. Is it? If so, the problem is in the preamp. If not, the problem is in the power amp.
Post some hi rez pics of your amp. Post your layout drawing.
PS... Please put all pertinent layout and schematic drawing in this thread so we don't have to go chasing them down in other threads.
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Oh sorry I'll get the schematic, (i'll update it first to the latest changes though) and post here. I'll give all of that a go as well, and post here, I do recall the power at A,B,C,D,E all being very close to what I recall it should have been from memory, but do need to go down that logical path of writing it all down etc.
I'll get the pin readings as well, and such.
Thanks, I should have thought of a lot of this stuff from all the reading I've done but didn't recall seeing a specific sticky thread that talks about some of this kind of tribal knowledge, or did I just miss it?
If there isn't one, maybe I'll start one up to start distilling you guys wisdom into something more central for quick review by newbs like me :)
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Here's the latest schematic, the diode and cap area is really ugly, but to the best I could do this late :P
I've also done some testing and measurement.
1. removed the reverb pan completely, no end of the hum
2. removed the second reverb tube, no end of hum
I've not yet gotten to more, as I ran out of time, I did get measurements though.
Power Tubes
pins 1/8, 4, 5
tubes
5 184mv, 480V,-45V
6 0mv, 480V, -46V
7 0mv, 480V, -45V
8 0mV, 481V, -45V
Preamp Tubes
pins 1,3,6,8
1 143V,2.0V,477V,4.0V
2 199V,1.6V,318V,198V
3 200V,1.7V,Not,Used
4 411V,3.9V,409V,3.95V
Now I didn't get consistent readings on Power tube pins 1/8 because its really weird, when I try to test it, it seems to have some level of mV for just a second, like say 184 mV and then drops to 0 really quickly, same with any of them, not sure why? 1/8 go to a 1 ohm to ground, so I should be able to measure voltage and get plate dissipation there, but something seems odd
Similarly for the preamp tubes Pins 2/7 I can Measure like .36V or so for a short time and then it dissipates to 0. Not sure if that is expected or a sign of something wrong.
For the attached schematic you can see that the second half of V1 goes to the reverb and first half of V3 returns from it.
As for the power rail, here's the measurements:
A: 504V, B: 484V, C: 481V C': 476V, D: 320V, E: 293V
The C' is a spot in the schematic where I'm tying in the cap as required but there's another resistor after it going to C, so I labelled it 'C Prime' or C', not sure if that's normal/copacetic, but I just wanted to remind myself the filter cap went in there, not at C directly.
I haven't had a chance to take photo's either, I'll try to find time to get that tomorrow, but with this debugging, I've still not found a way to get the hum to stop, I even blew a fuse today as well while I was doing some testing, not sure what I may have touched, but Its been fine again with a new fuse.
~Phil
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Oh one other thing I've noticed that was odd that happened again last night. The half of the power tubes end up not getting their heaters lit up sometimes. If I turn it back of, jiggle them about a bit, they often come back on, but sometimes that doesn't even do it. It makes me wonder if it is one of two issues:
1. the pins are a bit too loose on those two, so they're not making good contact?
2. the heater power lines can't quite generate enough amps for the entire circuit? This one is pretty hot, its taking:
4x 1.5A for the EL34's
4x 300mA for the 12AX7's
= 7.2 Amps. Is it possible this isn't handling that well? I would think it should because the same setup was used in the other amp, the only difference is that it used a full wave bridged rectifier to provide DC Voltage to the preamp's only. That shouldn't increase the amps the heater rail can produce right? Amp handling is based upon the PT itself and that's pretty much it no?
Also, as noted, instead of using 2 ECC82/12AU7's I'm using all ECC83/12AX7's in the amp to get a feel for what it will sound like. Per discussions I've heard on a lot of other threads this shouldn't be a major issue, it will modify the tone, and increase the potential preamp gain to get more crunch.
~Phil
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The heater issue is likely a loose or bad connection. Could be sockets but that would mean two sockets have the same issue. Could also be a bad wire or solder connection.
Still waiting for your layout drawing. I did find an old layout scattered across several drawings but it is confusing how they actually connect together. And there were enough errors on the drawings to make me think they are preliminary. It took about 30 minutes of sifting through all the various topics dealing with this amp. It would be nice if you would pull all that stuff together and post it in this thread. Hi rez pics would be very helpful at this time too.
Take a look at the link in my signature line at the bottom of any of my replies...
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Oh, sorry, I'll get that in here too, it was late last night when I got it together.
I've gone over it like that link shows, but it was after I finished before power on, and it doesn't mean I didn't make a mistake. I'm sorry that the layout is ugly, it was my first try, and its def changed a bit. I'll also get some pictures. Its an ugly mess, because I'm still learning this, and my lead dress leaves a lot to be desired, and that may be part of my issues, not sure. One thing you'll see in the pics is that my heater line coming from the power tube side to the preamp side has to cross the lines to/from the preamp tubes to the board, so I'm thinking I may want to try a different path for that as well, still trying to figure that out.
~Phil
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Okay here are some images, https://goo.gl/photos/tkdWfVc9nQ43hn339. I'm also attaching screenshots of the latest visio, also not perfect, but the best I can do with visio and limited time. I've also been meaning to ask, as I've heard mention of it, how do you clean the flux remaining on the board? As you can see on mine, its still filthy with it.
I plan on also filling the top areas of the filter caps with GE Silicone II (The non acid type curing type). to ensure nothing can touch the connectors, but only after I complete the build. I've not figured out yet the best way to handle the rca jacks for the reverb either, they're just hanging up there, I may get a terminal strip for those, but am open to suggestions.
Really please be kind, as I'm sure a lot of this looks horrible, but I'm open to suggestions on what I need to improve to make this better in any way.
Edit: this is only my second build as well, thus the really baaaad stuff :P
~Phil
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Lots of questionable looking solder joints in those photos. What kind of iron, tip, and solder are you using, and at what temp if it's adjustable
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Please repost your layout pics, but turn off grids and guides in Visio.
I use denatured alcohol in a spray bottle and acid brushes to clean flux from a board or chassis. I use one acid brush with full length bristles (approx. 1" long) and another acid brush with bristles trimmed to about 3/8" for a more vigorous scrub.
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Lots of questionable looking solder joints in those photos. What kind of iron, tip, and solder are you using, and at what temp if it's adjustable
I've got a hakko 888 http://smile.amazon.com/Hakko-FX888D-23BY-Digital-Soldering-FX-888D/dp/B00ANZRT4M/ref=sr_1_1?s=industrial&ie=UTF8&qid=1451541702&sr=1-1&keywords=hakko+soldering+iron (http://smile.amazon.com/Hakko-FX888D-23BY-Digital-Soldering-FX-888D/dp/B00ANZRT4M/ref=sr_1_1?s=industrial&ie=UTF8&qid=1451541702&sr=1-1&keywords=hakko+soldering+iron) at the default of 750degF. I have a blade tip i think size 2? and I've got 5 core really thin solder. I'd have to double check the exact size though. I can adjust the Hakko to whatever is usually best, I've just left it at hte default.
~Phil
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Please repost your layout pics, but turn off grids and guides in Visio.
I use denatured alcohol in a spray bottle and acid brushes to clean flux from a board or chassis. I use one acid brush with full length bristles (approx. 1" long) and another acid brush with bristles trimmed to about 3/8" for a more vigorous scrub.
Here are the layouts with grids and guides off.
~Phil
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Hello, I usually hang out on TAG (rp) but I often check the posts here. I finally registered just to ask, why is that 18K resistor even there? Is it just a typical bleeder for safety after the amp is shut off? If so why use something that draws 11W? If not just getting rid of it, wouldn’t a more common 220K 2W be better?
Suggestion to the OP: if I got it right looks like the original AC100 has an unused triode. Most guitar amps using 12AU7s didn’t really leave a lasting impression on history especially in V1, lower gain is a part of the chimey sound but you can also turn your guitar volume down and the amp up for pretty much the same effect. A common mod to the 2 channel AC50 was to swap in a 12AX7 for one channel. If it’s not too late, and an extra pot doesn’t ruin your front panel aesthetic, and you have some room for a little terminal strip, maybe add a second channel and use a 12DW7 which is a 12AU7/12AX7 in one tube, and you can tweak-up the two sides to taste. Also, thinking you could use a stereo pot, though the unused side would go up in tandem and might add some hiss.
Happy 2016
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Thanks for the input Ugly Distortion, I only have half of a tube left over (and I may end up using that for a second half of a tube in the reverb stack, not yet sure), but I've replaced the 12AU7's with 12AX7's already, and as far as that resistor, I'm not sure if its for more than just a bleeder resistor, but I've got bleeders on the A and B power filter caps already anyway. If that's the case and others can confirm, I may very well just remove that resistor.
~Phil
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I'd leave that resistor in the circuit just because the original has one. Once all the bugs are worked out I would measure B+ node voltages and then remove that resistor and recheck B+ node voltages. They will increase, but if they don't increase too much I'd be tempted to leave the resistor out.
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Thanks,
So does a resistor like that, that goes to ground instead of being inline, still lower voltages? I thought voltage dropping resistors were inline?
Is there anything else I should get? specific pictures you'd like?
I hope to have some more time tonight to monkey around with it. (Its been chaos with the holidays and inlaws over etc)
~Phil
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Regarding the solder: if you were using the really thin stuff intended for working on printed circuit boards, you may not havve been have been feeding enough of it in, fast enough, to get the nice shiny domes on the tops of the turrets. Try using .032 .
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So does a resistor like that, that goes to ground instead of being inline, still lower voltages?
Yes, unless you have a perfect zero loss PT.
Is there anything else I should get?
Yes. Get some responses to all the stuff I've asked for already. Your responses will likely determine which direction to go. Heck, they might even solve the issue.
I do suspect that your soldering will be a factor in the problems you're having.
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Oh yeah sorry that's right I did have a few more things you asked to do that I've yet to do. I'll get on that.
I also found one other issue, it seems like my input jack is causing the lion's share of the noise, so I'll work on that first. I may have to remove the old circuit board from it and just connect it directly to the pins.
As for the solder size, I think I may order some of the .032 you mentioned drew, I bought this size on a recommendation I saw online on a youtube video but I do think it was for small soldering not things like this, and that may be why its such a chore to use it.
Is there an optimal temperature for soldering? Is my 750 too high/low?
Thanks,
~Phil
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I don't have a soldering station, but your tip will tell you what you need to know. It should be nice and shiny; to get it that way I have to wipe it on a damp (not wet) sponge just before hitting the joint. If it gets too hot, it will turn a funky brown color and will not transfer the heat well. I've also taken to using a tiny bit of flux on all the joints, even though I'm using rosin core solder just like you.
For me, to get those nice domes on the turrets I usually have to use .050 gauge solder; the thinner .032 doesn't "fill in" as nice. So, since I like using .032 better, I don't worry about the domes. :icon_biggrin: But the solder on the joint should not look all frosty, it should still be nice and shiny, looking not-quite wet.
I will say, a 100 watt amp is pretty ambitious for only your second build. Cheers to ya!
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Nice to see someone else doing an AC100 build. They are a really nice sounding circuit, though on mine I stuck with the 12AU7 setup for the stock channel and just added a higher gain channel to it with switching and LED indicators. I built mine out of a gutted Sovtek MIG 100 U, keeping the power transformer but using a choke and OT sourced from Heyboer. I got it all functioning and had an oscillation issue at the phase inverter due to a layout problem and need to gut it and build a new and revised layout but haven't gotten around to it yet. I used DC heaters on the first tube in each channel and the amp is dead quiet at full blast. It sounded great overall except for the oscillation on my higher gain channel.
Good luck on completing your amp!
Greg
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I don't have a soldering station, but your tip will tell you what you need to know. It should be nice and shiny; to get it that way I have to wipe it on a damp (not wet) sponge just before hitting the joint. If it gets too hot, it will turn a funky brown color and will not transfer the heat well. I've also taken to using a tiny bit of flux on all the joints, even though I'm using rosin core solder just like you.
For me, to get those nice domes on the turrets I usually have to use .050 gauge solder; the thinner .032 doesn't "fill in" as nice. So, since I like using .032 better, I don't worry about the domes. :icon_biggrin: But the solder on the joint should not look all frosty, it should still be nice and shiny, looking not-quite wet.
I will say, a 100 watt amp is pretty ambitious for only your second build. Cheers to ya!
Thanks,
I've already ordered the .032, mines a lot smaller, 0.017, so already that's almost double the size :).
Thanks for that, I'm sure I'll get it :)
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Nice to see someone else doing an AC100 build. They are a really nice sounding circuit, though on mine I stuck with the 12AU7 setup for the stock channel and just added a higher gain channel to it with switching and LED indicators. I built mine out of a gutted Sovtek MIG 100 U, keeping the power transformer but using a choke and OT sourced from Heyboer. I got it all functioning and had an oscillation issue at the phase inverter due to a layout problem and need to gut it and build a new and revised layout but haven't gotten around to it yet. I used DC heaters on the first tube in each channel and the amp is dead quiet at full blast. It sounded great overall except for the oscillation on my higher gain channel.
Good luck on completing your amp!
Greg
I'll keep the thread updated, let y'all know how its coming, I rewired the input jack today and the massive hum left, but when I was testing that out the two power tubes that were alwasy working were massively redplating, I had to shut it off quickly. I had made a light bulb tester but thought all was copacetic, and obviously its' not. I tested after that with the tester and it lights up instantly. More than just the heaters seems to be 'off' on the second half. I'll have to add solder to all the joints. I'm getting the thicker solder Sunday.
~Phil
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If your power tubes suddenly started red plating, and your light bulb tester now lights instantly, you "installed" a short where there wasn't one before. Be sure there's not a solder drip touching the chassis through a turret somewhere.... your ohm meter will be good for that. When I'm looking for something like that I measure ohms instead of just using the beep function.
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That's what I thought, I just spent a few hours with some new solder getting much better beads atop almost every joint, but some seemed to still be mildly dented inwards. I tested taht I have connectivity and good readings across the pins on the power tubes, etc, everything still seems fine, but when I power it on, again, still getting redplating etc. I'll keep playing with it, to see if I can figure out what's going on. Just wanted to update that I've made some changes but no real progress besides the massive hum being resolved via a fix on the input line grounding.
~Phil
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Just doing a little reading on my lunch break today, found this thread:
http://www.thegearpage.net/board/index.php?threads/tubes-red-plating.680379/ (http://www.thegearpage.net/board/index.php?threads/tubes-red-plating.680379/)
that pointed to this thread:
http://forum.metroamp.com/viewtopic.php?f=28&t=30036&start=0 (http://forum.metroamp.com/viewtopic.php?f=28&t=30036&start=0)
that then linked to something from Aiken amps, but specfically that EL34's at high voltages may need better grid resistors to reduce current, or just reduce overall current to the B+ rail. Does that jive with what you guys have seen in the past? I know the original schematic has a bit lower voltages than what I have in my current setup, because the old one seems to have had like 470V input and lowers it down from there, but my current PT puts it in more at the 490-500ish range to start... Would inserting some kind of voltage dropping resistor help here?
~Phil
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Double check your cathode resistors (if used) or if it's adjustable bias then check all connections there. (Captain Obvious says) something is making those tubes conduct way more than they want to. Measure from each cathode pin to ground, check from your grid pins to ground. You'll find it.
Oh, and I was wrong about my thicker solder, it's actually .062. I have an unopened roll of .050 which I think will be ideal for turret work. I need better eyes!
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The very first thing to do when a fixed bias amp is red plating is to check the negative bias voltage on the grid. That means pin 5 of all EL34s. You can pull the EL34s until you have proper negative voltage on pin 5 of all four sockets.
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I figured out the redplating issue. I had put in some 100Ohm resistors to ground on the heaters, but that seemed to be providing some mechanism to ground too quickly from the pins 1/8. I removed those, as they'd also been a bit browned due to too much current. Once I did so the redplating stopped.
I also checked the bias and without the tubes it is at about -29 to -31 on all 4 now, but I don't think that was an issue, if I sort out other issues, I may bias it back up to the higher levels, but for now I want to stay in a more 'cool' area.
I'm still seeming to pull a lot of current though, as I blew another fuse after doing this. I have checked the pins over and over and I can't see any path to ground other than that one. I've replaced the 3amp fuse with a 6 amp for now, since the newer one asks for it (the old '65 schematic wants 3, so I'll go back to that if everyone think's thats better, but I wanted it to stay up a bit while I tried to figure out what's up). I still see and hear significant cycling of some kind of sound, and the output after the very first input tube seems to go from 200mV to about 10V or so, which seems way wrong, but that was also something I'm not 100% sure of, because I kept having to shut it down because I got worried. I also tried to remove the first preamp tube as you requested sluckey and as I did on the 4th, ended up breaking the tabs on it trying to get the tube out. (the metal keeps flexing while I'm trying to get them out just slowly pulling while doing a circular motion) I think the breaks are because the stupid amp I have had the holes opened for the exit of the tubes, not for the sockets themselves, and therefore the holes I put into them are a bit too 'close' to the actual hole and it lets the screws tilt a bit. Basically a bad fit. I'm going to have to order 4 more preamp sockets and try that all again, after I do something to tighten up the holes in a good way (maybe those metal plates Doug sells for adapting an 8 hole to a 9 hole might do it?)
At any rate the nicer belton sockets seem to have a better quality metal as well, so maybe I can get a more solid seat against the metal with those also. At any rate, I don't want to keep trying so hard to get the tubes out of the sockets and end up breaking the screw tabs on all of them, its a royal PITA.
Why are they so hard to get in/out? Is that because the thinner solder gauge allowed it to flow down into the pin area making them mroe 'tight'?
Anyway, I'm going to defer more troubleshooting until I can get the sockets all replaced. ugh.
:cussing:
I don't see anything yet wrong with the power tubes, maybe somethings in the preamp phase that is sending too much to the power stage? at any rate, until I can replace the sockets, I don't want to keep fighting it.
~Phil
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Is there anything else I should get?
Yes. Get some responses to all the stuff I've asked for already. Your responses will likely determine which direction to go. Heck, they might even solve the issue.
Oh yeah sorry that's right I did have a few more things you asked to do that I've yet to do. I'll get on that.
Hmmmmm........ :think1:
Sluckey has been asking for these things for, what a few weeks now?
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I'm trying honestly, but I keep making one step forward, and two back. I'll try and summarize all he has asked for, and what I've done, so I know where I'm at
Phil
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I'm trying honestly, but I keep making one step forward, and two back.
This amp is a BIG amp to completely rebuild.
For the guys and Sluckey to be able to help you what Sluckey asked you for is very much needed.
If your "making one step forward, and two back" maybe it's because your going off in different directions than you should be. There is normally an order of attack in any amp to follow and not just I think I'll work on this now.
What I see you doing is what you think you should do before you take the time to organize the things Sluckey asked you to and post them. You go off on your own out of the normal order of attack and then make a mistake and take 2 steps back. Then you try to fix that and post questions about what the amp is now doing and looking for answers to fix it.
So you are ending up chasing your tail and don't have the time to organize and post what was asked of you because your distracted.
And round and round it goes.
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I figured out the redplating issue. I had put in some 100Ohm resistors to ground on the heaters, but that seemed to be providing some mechanism to ground too quickly from the pins 1/8. I removed those, as they'd also been a bit browned due to too much current. Once I did so the redplating stopped.
Those 100Ω resistors should have nothing to do with pins 1/8 or redplating. Maybe you had them connected to the wrong socket pins. One should be connected between pin 2 and ground and the other should be connected between pin 7 and ground. Recheck that the green/white filament wires connect to pins 2 and 7 of all four big sockets.
I've looked closely at the pic that shows the socket that you have the burnt 100Ω resistors connect. I can't figure out how the filament wires or those resistors are connected. That may just be the view angle of the pic. I see a green/white twisted pair of wires that are butt spliced to two 'white' wires and I assume those go to the PT. Then there is another green/white twisted pair going to the next EL34 socket. And finally, there is another light green/white twisted pair that goes to the little sockets. All three of those twisted pairs and the two 100Ω resistors should tie together at pins 2 and 7 of that socket (3 whites to pin 7, 3 greens to pin 2). They should not connect to anything else. I cannot tell what they actually connect to. And there is a solder blob at the top side of the socket. Some of the green wires seem to be connect to that mess. And there seems to be a yellow wire connected to that blob. If so that's a big problem and would burn up those 100Ω resistors.
There is also a resistor just to the left of that solder blob. It has a brown blob on the top and a red wire connected to it. What R# is that? There should be a 100Ω resistor (probably 2 or 3 watts) connected to pin 3 of every EL34 socket. This would be R26, R27, R28, and R29. Where are they? R47, R48, R49, and R50 (1K/5W) screen resistors are mounted very dangerously. Those resistors should not have one lead flying freely in the air and connected to those red wires! Most people would connect those screen resistors between pin 4 and pin 6 and the red wire would connect to pin 6 also.
I also checked the bias and without the tubes it is at about -29 to -31 on all 4 now, but I don't think that was an issue, if I sort out other issues, I may bias it back up to the higher levels, but for now I want to stay in a more 'cool' area.
You 'may' misunderstand the relationship between that negative bias voltage and 'cool' running tubes. _29 to -31 is probably pretty hot. -40v would be cooler. I suggest you set the bias pot for maximum negative voltage on pin 5 of all EL34s for now. That will keep the tubes running cool. BTW, what is the maximum negative voltage on pin 5?
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So to explain the heaters, I've got the heater splitting at a point above the first tube, one side going to the preamp, the other down to the first socket of the power amp tubes. I have tested them for continuity and they're all connected correctly on pins 2/7 on all power amp tubes (and 4/5 and 9 of preamps, and all heaters are heating correctly, reflowing and adding solder to the last two tubes heater pins seems to have solved a dry soldered connection that was being intermittent).
to remove the confusion on the other resistor, that's going off to my LED, its the current limiting resistor for the LED, that's all. I've tied the LED into the heaters there as well, but down at the pins of the first power tube.
As for them causing redplating, I was able to figure it out becuase with them in circuit, I was getting continuity between pins 1/8 and 2/7, with them out, that stopped. I don't know why, but probably because they weren't creating enough resistance to stop looping between 1/8 and 2/7 of the power. That may also be wrong in the 'why' but I clearly saw continuity between the two stop when I removed those. I may restore them or link them to a different ground line at some point later, I'm not sure, but for now it removes the redplating. I suspect they may have burned on an earlier error when I had connected the wrong lines for the heater to them, and instead of 6v they were running at 70 or so volts. I just didn't realize they were that browned until I figured out the issue. I think one measured only 45 ohms after the burn, and the other was like 30 ish so they were definitely not in good shape.
If you think it is wiser to just move the connection of the heater wire down to tube 1, and then connect off to tube 2, led and preamp tubes from those pins, I definitely can. it is just tight at that tube and I was trying to create a bit of space by sending off the preamp lines a bit earlier. Is this generally considered bad form? Or maybe I should even just remove that entirely and send the power lines from the last power amp tube on the left down to the first preamp tube?
I'm open to anything that way.
Willabe, I don't disagree, I'm sure you're right. I tend to be bad about that kind of mental flights off on tangents. It sometimes can be very helpful for me, and others it can be very detrimental. In this case I think I agree with you that its what is causing the issues. I need to, as stated in my last comment, sit down, review all of sluckey's suggestions, write them all out and go at them one at a time (I've done many of them but not all, and its been such a silly mess since I don't even recall which I did and didn't do.) I can then repost the list here, and then I'll go tackle them serially.
I really do appreciate the help, you guys' input has already been invaluable, and I'm learing tons (mostly due to my own mistakes, but that's sometimes the best/only way to learn I guess heh).
I ordered the sockets from Doug, and some 8 to 9 pin adapters to see if I can get these to fit better. but I can still keep testing with the sockets as is, and leave the preamp tubes in since I can't seem to get them out. (any tests that required removing them can wait until I fix the sockets, but I did do some of them for sure).
~Phil
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OK, I understand how the filaments are wired. You still have a big mess around that socket. At least the camera angle view makes it look messy. Maybe when you replace the sockets it will look neater.
As for them causing redplating, I was able to figure it out becuase with them in circuit, I was getting continuity between pins 1/8 and 2/7, with them out, that stopped. I don't know why, but probably because they weren't creating enough resistance to stop looping between 1/8 and 2/7 of the power. That may also be wrong in the 'why' but I clearly saw continuity between the two stop when I removed those. I may restore them or link them to a different ground line at some point later, I'm not sure, but for now it removes the redplating.
Of course you will measure continuity between pins 1/8 and 2/7. That's perfectly normal. If you look at resistance rather than continuity beeps, you'll see that continuity is actually about 50Ω.That's because you have a 100Ω from pin 7 to ground. And another 100Ω from pin 2 to ground. Those resistors are practically in parallel and give a total resistance of 50Ω. Now you also have a 1Ω resistor from pin 1/8 to ground. So, pins 1/8 are only about 50Ω away from pins 2/7. If those resistors were causing redplating, they must have been touching something they should not have.
What about all the other issues I mentioned in that same post? Especially the questions about the negative bias??? Those answers may lead to a solution to your red plating and fuse blowing issues.
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I can't set the bias from work, so I didn't mention that, I'll take a look, but again, the redplating is gone, but I can make the bias colder by pushing it up to whatever is max (If I recall in my testing it got up to about -65V?) Sorry I was even reading the valve wizards tube stuff and read the same thing this morning on the bus. That the higher the voltage the colder the bias, so I'm glad I got it from two separate sources on the same day, its a counterintuitive concept but reading it and seeing you remind me of it as well is good. I'll test that out when I get home, and have had time to write down all the things you've asked. I'm going to try to ensure I'm going at this methodically this time :)
~Phil
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> the higher the voltage the colder the bias,....its a counterintuitive concept
When you want to strangle somebody, you "press harder" to reduce his air-flow.
Zero grid-cathode voltage allows MAXIMUM current. Humans work well this way, less-well when strangled. In tubes, we never want to idle wide-open this way. We "strangle" it to a lower current, half or less of the peak current we want on the loudest signal peaks. In a push-pull amp, we can strangle down (press harder) to a very small current and still get full power: the signal voltage on top of the bias voltage can push current to the max if desired.
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Oh groovy, that uses words that make even more sense! Thanks :)
~Phil
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First up a question, sluckey, could you clarify number 10 below? I've put my question there after the request.
TL;DR post coming :) Here's the complete list of what sluckey's asked for, in 'done first and then those I'll be doing' format, so if you don't want to read, that's fine, but I wanted it here for my sake :)
1. Replace 18k 2W with 18k 25W - Done
2. replace full wave bridged rectifier with full wave rectifier for PT from AC100cph - Done
3. use different bias circuit for the above rectification - done
4. What is voltage level of input - Done, Showed that a bit later as well as all input voltages across the A-B-C-D-E
5. Use 200mV peak to peak on input for testing the audio circuit - done
6. Recheck all solder joints - Done - Did this over the weekend, I got thicker solder and re-soldered everything on the boards, and redid the power tubes as well. I also looked for any issues on the preamp tube sockets but they're all looking good.
7. Measure and post voltage readings for all filter cap nodes and every tube pin - Done
8. Post some high rez pics, post layout - Done
9. please put all pertinent layout and schematic in this thread - done
And TO DO:
10. Swap Tubes - I've swapped tubes around, but I don't have extras, do you mean to try some other power tubes? They were new when I got them, and the issues were occurring, but it is possible that Iv'e done something to toast them?
11. Pull the B+ fuse and measure bias voltage at pin 5 with the bias pot cranked at each end - will do
12. Divide and conquer. If you have added reverb, disconnect it, any better? - will do, I attempted to do this by pulling the reverb recovery tube, but i guess it may be possible that there's still something bleeding out after it gets in from the input tube side?
13. Pull V1 and V2, - Will do I've got to fix the tube sockets first but I will
14. Temporarily connect an input jack to the treble control wiper and plug a guitar in. will do
15. Recheck your drawings, make sure schematic agrees with the original (except for the mods used for the PT), make sure the layout agrees with schematic, make sure you amp is wired IAW schematic and layout. - I've done this, but I think i may do it again just as a sanity check
16. Plug in speaker and guitar, adjust bias - Done initially, I'l be doing the later recommendation to bias cold until I've resolved the issues (max of about -65V as I recall)
17. Take a look at my signature - that links to the how to cover that you've done it all right, as mentioned I've done this, but do think I need to do it again to ensure I've not missed something, because something IS still wrong
18. Use denatured alcohol to clean the flux - Started and got partly done before I re-soldered everything, so I'll repeat this too, is this part of the troubleshooting or just to see clearly that the solder looks good and post pics post clean up?
19. After everything else is balanced out, it may be worth removing the 18k 25w resistor - Will do (its now 22.5k because I didn't find one locally, but I may replace that with said 18 or remove depending on post fix voltages)
20. bias the tubes colder until issues are resolved by pushing the pin 5 to the maximum negative voltage possible (-65V I think) - Will do
21. restore the 100 Ohm resistors as they should not cause red-plating, just ensure they're not near anything that could cause issues by touching other connections etc. - Will do
22. Adjust the heater wires to be less ugly at the power amp first socket - Will do, I intend to just remove the preamp split, and go directly to the pins on the power amp, and then pull of the end of the last power socket straight down to the preamp tubes.
So I'll report back with my findings to the above second half and more of the list.
Thanks again for all the help, I hope to have a fix or more data soon :)
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You may find that now that the tubes have been red plating, that they are permanently altered and will not work correctly anymore. It depends on how badly and for how long they were red plating before you shut the amp off. I was using some JJ KT77's in my AC100 project and had a problem with the screen grid resistors not being big enough and before I noticed that one tube on each side had started to red plate. Once I had fixed the issue, that set of tubes wouldn't work correctly anymore and sounded very weak. A new quad of tubes resolved the issue. Generally speaking, once you notice an issue with the tubes red plating, after you have shut it down, take the tubes out and connect it up through your current limiter with no power tubes in it initially and see if you can spot the issue. If you can't then put the tubes back in and run it on the current limiter with the power tubes in there. The tubes will likely try to red plate but being current limited they can't get to that point and the light bulb on your current limiter will light up brightly. Then perhaps you can spot the issue. Once you have found it, then test it without the current limiter and verify your bias first. Assuming you have adequate bias, then you shouldn't see any more red plating, though again I would be suspicious of those tubes now and use another set. It can get expensive quickly if you are always burning through tubes so make sure to use your current limiter as it is intended so you don't burn stuff up.
Greg
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SoundmasterG, thanks, I'll keep an eye out for that. So far when I've powered it up, I don't have lack of sound, the hum coming in right now due to whatever's wrong is so loud at almost no volume, I'm pretty sure the output tubes are doing well right now. I'm considering getting another set but I don't want to replace them until I'm done troubleshooting (unless people thing the tubes may have been damaged enough to exacerbate the problem).
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Well I've gotten through a few more, I'm going to post what I have, and I'll update after I get the sockets fixed, From the other list, I completed
2. 11. Pull the B+ fuse and measure bias voltage at pin 5 with the bias pot cranked at each end - will do
it goes from -28V to -52V
11. 20. bias the tubes colder until issues are resolved by pushing the pin 5 to the maximum negative voltage possible (-65V I think) - Will do
I was working (and spent a LOT of time) trying things out with the reverb return (V3) tube out, and things started seeming 'ok' but there is now absolutely 0 output. I tested both sides of the OT and that comes within what seems like acceptable ranges. See this site: http://www.etronic-parts.com/product_info.php/info/p1090_VOX--Output-Transformer-AC100CPH.html (http://www.etronic-parts.com/product_info.php/info/p1090_VOX--Output-Transformer-AC100CPH.html) they show what they get for impedance readings on that OT, and mine are almost identical (the power output side is a bit mismatched, the above image shows 18 and 18 for a total of 36 ohms, I think I was seeing closer to like 15 and 19 or something similar, I'm not sure if that is within tolerances? On the other side I saw 0.5 Ohms and 0.2 between the 8 Ohm wire and 0.1 between 16 ohm.
I'll keep working at it, I've gone completely though the drawings and didn't find anything massively bad except that the reverb pot ground wasn't wired down. I've completed that. I started thinking I'd messed up on the connections for the V2 and rewired that but I'm going to have to revert it. My Visio is wrong, because the schematic shows that the second half of the tube's Anode connects directly to D, but somehow I thought it was supposed to share R8, I rewired it to come in at R8 and the weird cyclical sounds seem gone, but I don't honestly think that's related, because its now 'wrong' instead of right. At any rate, I'm still just not sure why I'm getting absolutely no output now.
I've retested the voltage readings on pins 4 of all power tubes and they're still looking right...
Any suggestions on why the OT seems silent now when it was working before? Would that indicate the OT is fried? my measurements of impedance should confirm its still good right? The same number of wirings, its not shorted nor is it significantly smaller or higher resistances?
I'll work on the remaining ones, but I'm getting closer. I think the quieting down of things may have been because I wired the reverb pot to ground, but not sure? Would that be it?
~Phil
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OT's rarely fail...I am sure it is fine.
Do you have a signal generator and an oscilloscope? If so they are very useful in tracing the signal through your amp to see where it is going and what the signal looks like from stage to stage. If you don't have one, it makes it a bit harder to troubleshoot when you lose the sound.
If the amp will be stable with those power tubes at an acceptable bias level without the tubes red plating or orange plating, then they will be fine most likely. You aren't at a point yet where you can ascertain that though....try them and see over time how they work....but be suspicious about them until they prove they are fine.
Greg
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Divide and conquer.
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Sluckey, I have been, I've got the oscilloscope up and am tracing the output all the way to the side just after the phase inverter and it is fine. This means the signal is getting to the power tubes but then nothing. (the other work I've done has stopped that oscillation, but maybe that is being caused when the output is going to the speaker?)
I was in the middle of the divide and conquer process but realized I was getting no output (I was before, and I don't know wht else has happened besides me blowing that fuse, but obviously something went with it).
I can basically say now for sure that I have output from the input all the way to the tubes, I guess I need to check for output at pin 3 of the Power Tubes? The other parts of it weren't tested becasue the entire input stage now seems stable after soldering the reverb pot to ground. So either the lack of output indicates my original issue is tied to the output working OR the grounding fixed the cyclic issue.
I'll see what else may be amiss. I even tested the speakers by plugging them into a different amp and they're workign fine there, so its not the speakers.
I'll mess with it more tonight, I'd run out of time last night when I'd gotten to this point.
~Phil
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I should qualify what I mean by "I was".
I have listed on your "divide and conquer" part was to disconnect reverb, my first step in that, (for the second time), was to remove the reverb recovery tube, that way its not able to feed back back 'in' and things seemed stable, no cyclic issues, but I also had no output, thus I started tracing the signal and found it went all the way through the preamp to the power amp stage.
I hadn't yet gotten to desoldering the input and output parts of the reverb so that it just went directly from preamp stages bypassing reverb to phase inverter yet, and realized it seemed stable after my soldering of the things 'wrong' with the schematic/layout, and then found no output, so I'd done very little yet.
I should have more time to tinker this weekend, and I'll measure what's coming out of the power tubes as well. Pin 3 to the OT that is.
If I get output working again, and the previous steps have resolved the other issues, then we're golden, if not, I'll resume the divide and conquer plan.
~Phil
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I have listed on your "divide and conquer" part was to disconnect reverb
Go back an read my divide and conquer statement. You left out the most important part of what I tried to tell you.
Now that you have a decent bias voltage range, set the bias to -40V on pin 5 of all EL34s. Make sure there is no red plating. Got sound now?
BTW, pull one pair of EL34s so you are only using one tube on each side of the OT. No need to risk blowing all 4 EL34s at this point.
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Recheck your drawings. Make sure your schematic agrees with the original (except for the mods you've had to make to use that PT). Make sure your layout drawing agrees with your schematic. Make sure your amp is wired IAW your schematic and layout.
Recheck all solder joints.
Swap tubes.
Measure (and post) voltage readings for all filter cap nodes and every tube pin (even the pins that should be zero volts). What voltage is on pin 5 of each EL34? Pull the B+ fuse and measure bias voltage at pin 5 with the bias pot cranked to each end. What voltages do you get?
Divide and conquer. If you have added reverb, disconnect it. Any better? Pull V1 and V2. Temporarily connect an input jack to the treble control wiper and plug a guitar in. This connects the guitar directly to the input of the power amp. It won't be nearly as loud but should be hum free and very clean sounding. Is it? If so, the problem is in the preamp. If not, the problem is in the power amp.
Post some hi rez pics of your amp. Post your layout drawing.
PS... Please put all pertinent layout and schematic drawing in this thread so we don't have to go chasing them down in other threads.
Reposting your divide and conquer post, because I want to understand what I'm missing.
First off, thanks for the help, it is really making a difference, I don't want the tone of this post to come off wrong, I'm not complaining, just honestly trying to understand what I've missed. If there's one thing I've learned in my life, its that email/texts/forum posts can be misread for tone/intention, and this is completely "I'm ecstatic you're helping, please know your help is appreciated, I feel like a dumbass for missing something" and nothing else. So to continue:
In your above post/list, I've done
:recheck drawings and schematics
:recheck all solder joints
:swap tubes (I asked for clarification on this in a post a few posts ago, but I have swapped tubes around quite a bit, from each half of the push pull setup, not sure if that's what you meant or if you meant to swap in different tubes, but I don't have extra's right now)
:Measure and post voltage readings
:pull B+ fuse and measure pin5
:post hi rez photos
:post latest schematics and layouts
all above are done, (with that one caveat of the swap tubes)
that being said the rest was what I considered the "divide and conquer" part of what you asked but maybe you meant all (but all BUT this is done, and is what I was working on). From above:
Divide and conquer. If you have added reverb, disconnect it. Any better? Pull V1 and V2. Temporarily connect an input jack to the treble control wiper and plug a guitar in. This connects the guitar directly to the input of the power amp. It won't be nearly as loud but should be hum free and very clean sounding. Is it? If so, the problem is in the preamp. If not, the problem is in the power amp.
In that I had only done the first part, reverb, so to be sure I understand what I'd missed, are you meaning the fact that I seem to have good sound right up to the power amp that the problem is in the power amp?
If so, I guess I just didn't read that far, as I was trying the first part of that paragraph (I've reread it in the past too, but sometimes forest trees, you get the idea)
thanks so much for the help, if I gather what you're saying now, I may have the tubes biased too cold so that they're in that state where they can't produce output? I've read something about that if you push the bias too far, then it can't produce output, but it didn't click until now, (and even now I may not be right lol)
~Phil
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Ok so I'm working from home so I figured I'd go down and try removing two power tubes, and bias to -40V and guess what, its working great, I think one of the two tubes was bad... (I biased to -40V first and it still was oscillating, but I realized I'd forgotten to remove the two tubes). I was getting clean tone w/ no issues.
I then plugged in my guitar instead of the oscilloscope, and bam, its sounding great. At low volumes it is clean with a bit of crunch when pushed, but it goes into distortion pretty early on for what I'm used to, is that due to using the 12AX7s instead of the 12AU7's for the V1 and V4? I don't know which of the tubes I've pulled is bad, but I guess I can put the amp to low volume, and swap one tube at a time until I find the one oscillating and then replace that one right? Or should I get a balanced pair?
Thanks so much! :worthy1:
I'll have to do some more general checks, I have to desolder/restore that one output from the V2 that I put up to the resistor in error, but it doesn't seem to have caused major issues, I also need to restore the 2 100ohm resistors to ground on the heaters and then I think I'm pretty good. (I will get it all good, and then check if I can remove the 22.5k 25watt resistor after all is good and done and tested etc.)
One thing of note, the amp is SUPER quiet, I'm only hearing minor hum at high volumes with P90 pickups pretty close to the speakers.
Edit: clarification, I meant removed the sine input not the oscilloscope, its still connected so I could monitor, but basically all is looking good other than distortion at about I think 11 to 12 o'clock which sounds a bit nice but with some harmonics that seem 'off' a tad, I'll have to get all 4 power tubes in, and what not and see where it starts distorting in the chain (I have a 4 input scope, so i can set it in 4 locations and see what's what.)
~Phil
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At low volumes it is clean with a bit of crunch when pushed, but it goes into distortion pretty early on for what I'm used to, is that due to using the 12AX7s instead of the 12AU7's for the V1 and V4?
Correct. 12AX7 has a gain of 100. 12AU7 has a gain of approx. 20. I would put 12AU7s in the sockets that call for 12AU7s if you want it to sound more like an AC50.
I have to desolder/restore that one output from the V2 that I put up to the resistor in error, but it doesn't seem to have caused major issues
I still don't know if you know how that should be connected. The schematic is right. Pin 6 must connect directly to power supply point "D". Your layout is wrong. You show pin 6 connected to pin 1. Fix that. And fix your layout too.
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sounds good, will do. I thought the 12AX7's might just make it overdrive a bit better, instead its a lot earlier and maybe too much.
For the second note, yeah, I basically somehow did the layout wrong, and then when I first did it I think I was looking at the schematic and had it right, but on second review of the layout, doing the 'check it all' all over again, I found that error and duplicated it. It didn't cause too much of an issue, but it may be causing some of the odd sounding distortion because now both stages have the wrong setup, pin 3 is supposed to have one resistor and 6 none, now they share it, so I'm sure that changes things.
Thanks again for the help!
~Phil
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pin 3 is supposed to have one resistor and 6 none, now they share it, so I'm sure that changes things.
We ain't talking about pin 3! We're talking about pins 1 and 6.
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Yes sir, sorry was going from memory, the anode pins, someone needs to slap me silly. Lol
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Yes sir, sorry was going from memory, the anode pins, someone needs to slap me silly. Lol
OK. I just had to be sure we were communicating properly. We've spent a lot of time talking about this little detail, which BTW, is a BIG DEAL.
I looked closely at the pics you posted and it appears that you had the actual circuit wired correctly to begin with. So, the error was really just on the layout. There is a much simpler way to wire those pins. Move the 100K from the board and connect it directly between pins 1 and 6. Then put a short jumper between pin 1 and pin 7. Finally, connect pin 6 directly to point "D". (See attached V2 pic)
And this brings up a point I wanted to mention. Look at the attached danger! pic. You have 3 turret lugs laced together and connected to ground. And just below those 3 turret lugs are 2 blue resistors that connect to high voltage. The grounded lacing is dangerously close to the turrets that the blue resistors are connected to. I would remove that giant lacing wire and redo using 22AWG (about the size of the resistor leads). Keep the lacing well away from those turrets that have high voltage on them.
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Oh yeah, never noted how close that was, thanks! I'll get that fixed asap.
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Oh and another note of something I'd realized that many times is used like that change to the pinouts, I forget that pin 6 is not used and can be used basically as a terminal strip, so that's what's happening there in the picture you have, you set them both to that pin 6 and then to D, its being used like that. Very smart, something I've seen on many others but need to remember more often.
~Phil
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Oh and another note of something I'd realized that many times is used like that change to the pinouts, I forget that pin 6 is not used and can be used basically as a terminal strip, so that's what's happening there in the picture you have, you set them both to that pin 6 and then to D, its being used like that. Very smart, something I've seen on many others but need to remember more often.
~Phil
I don't know where you got that bit of mis-information but please try to forget it. Pin 6 is the plate in that tube and is absolutely being used in that circuit.
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Maybe he's thinking of 6L6's and 6V6's?
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I know. And it hasn't been too long ago that I mentioned using pin 6 on his EL34s to mount those dangling screen resistors in this thread. But we're talking about a 12AX7 cathode follower now. Just trying to stay on track. :wink:
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Sorry my head is still in the El34 pin 6 not connected mode, ignore my insanity :P
I've been thinking about the power tube side too much and my brain is stuck there.
And now that my brain is on the right page, I see what you mean, connecting the resistor between 1 and 6 means that the anode in the first side gets the resistor but shares the route from there to D with 6, I've already got 1 to 7 jumpered per the schematic and layout, (there's a light yellow line between the red line and pin 7 there).
Forgive me for being addle brained lol.
~Phil
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my head is still in the El34 pin 6 not connected mode
Speaking of which, did you take my advice about fixing those flying 5 watt resistors?
Take a look at a V2 socket that I wired. Notice that the jumper between pin 1 and pin 7 is actually the resistor lead just bent back around. Looks kinda neat, don't you think? The Fender 5F6A Bassman was the earliest amp I've seen that done. Marshall, Vox, and everybody else has been copying that ever since 1959.
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Very nice!
Is that somethign I could pin from 4 to 6 and then from 6 out to B so they're over the tube socket and the wires are less 'open' or flying? Is it also generally better to do a single run for each tube to the B connector or is it fine to bring them all into one line like I have it now?
Do you mean flying just because they don't have strong anchor point for both connecting sides of the resistor?
~Phil
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Do you mean flying just because they don't have strong anchor point for both connecting sides of the resistor?
Yes. One end of those resistors is totally unsupported. That's dangerous! There's very high dc voltage on those resistors. What's to keep them from flopping around and touching chassis, or something worse. This pic shows a very safe method for mounting those resistors...
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Outstanding, thanks. I've learned so much with this build it's insane. I can't say again how much I appreciate your help sluckey, also everyone elses. Willabe as well I do know your guidance pulled me back into the right space so I could be more methodical about doing what sluckey asked. You all rock :)
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Ok, I made all those changes, and its now working again. I put the 1k5W resistors through to pin6 and then out to the next tube and then off to the B point. Then I changed V2 to use the method you showed by removing R8 and putting it right on the tube with its one lead bent over to pin 7 and the resistor between 1 and 6 off to D. I also removed the ugly wiring around the power tube for the heaters, and went straight into it and then took off of the tail end of the last power tube down to the first preamp tube and ran them that way. I also then put back the two 100ohm resistors to ground on the heaters at the last preamp tube. Now I just need to get a few more tubes, (two power tubes and 2 ECC82/12AU7's for the original design). I should be in good shape at that point I think.
One question, right now the reverb section is using half of the 12AX7 from V1, is it going to work best with the 12AU7, or should I move it to sharing the same tube as the output stage so that its still in a 12AX7?
The orginal circuit I took for it is here, http://el34world.com/Forum/index.php?topic=7957.0 (http://el34world.com/Forum/index.php?topic=7957.0) and its a 12AX7 for input and recovery. I'm thinking I should modify this so that I use the 12AX7 and leave half of the input tube unused instead (the 12AU7)
Thoughts?
Otherwise, though it is working great, still quite clean sounding other than the not so nice sounding distortion from the 12AX7's when pushed over about 2 on the volume dial.
~Phil
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I put the 1k5W resistors through to pin6 and then out to the next tube and then off to the B point.
I don't understand what you're saying? I'd like to see an updated pic.
You should have a wire from point "B" to pin 6 of the first EL34. Then another wire from pin 6 of the first EL34 to pin 6 of the second EL34. Then another wire from pin 6 of the second EL34 to pin 6 of the third EL34. And finally, another wire from pin 6 of the third EL34 to pin 6 of the fourth EL34. Then the resistors simply connect between pin 4 and 6 of each EL34. Exactly like the picture I posted.
One question, right now the reverb section is using half of the 12AX7 from V1, is it going to work best with the 12AU7, or should I move it to sharing the same tube as the output stage so that its still in a 12AX7?
Theoretically, a 12AU7 should make a better reverb driver than a 12AX7. I would not change anything yet until you put a 12AU7 in the V1 socket. Then let your ears tell you what to do.
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Sorry Yes you've described what I did, I did exactly what your picture above shows. the 1k5W is connected between pins 4 and 6 on each tube. I take the B line directly to pin 6 of the first tube and run a wire between pins 6 on each of the other tubes to the end.
Okay sounds good, I'll give that a try for the reverb section.
One other thing I've read and may want to mess with, is that the reverb works but is pretty mild/soft. If I want more reverb I either need to change the 1M pot to a 2.2M pot OR add another inline resistor with higher value in the output section before or after the reverb pot. Why does increasing the resistance increase the reverb? Does it mean that the amount of time the signal spends there is slowed more due to resisting the current flow and therefore gets more overall color to the tone? or is it something else?
~Phil
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I've got the last of the schematic changes, and the visio update. I hope this is right, I posted them back to the schematics page as well.
I'm also about to upload the first video of the project, its going to end up as multiple videos, this one is over 10 mins long so I didn't want to go too long. It covers the planning and schematic setup.
I've also got pics of the updates to the build, as requested by sluckey. https://goo.gl/photos/tkdWfVc9nQ43hn339 Look at the last few photo's the first 5 were from before.
~Phil
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Very frustrated at this point. Any input suggested.
I got the new tubes in last week end, and tried them out, and everything blew up. The 2 100 ohm resistors to ground from the 6.3 volt rails blew up and burned up instantly. That is at 6.3 volts AC, so it should be at under half a watt and I have 1 watt resistors in it. I took them back out. I power on now and I think that first power on with the new 12AU7's either blew them up, or they were bad and blew things up, because the same tube that had the resistors that connected from it to ground (V4) still has glowing heaters and a DMM Test of the pins 9 to 5 work just fine, but the other one seems to have burnt up the heaters. I can't get continuity between those pins at all on the tube now. The new power tubes are in with the others and they seem okay, I had to dial back the bias a bit because they were redplating at -39 I think it was, so I took it down to about -42 (I guess the new tubes must have adjusted the bias voltages because they were at -41 or -42 before I swapped in the new ones).
The 1.5k resistor from the PI blew up and I had to replace it (Also from that first power on with the new tubes).
Any thoughts here about why an amp that I had working but that just sounded like I needed a few new 12AU7's would fry like that? Did I get a couple bad 12AU7's?
I also put back in some 12AX7s and can see them pushing some signal out, but it doesn't make it far through the circuit before it seems dead.
One other oddity, maybe this is just my measurement that was wrong, but I had my DMM set to VDC and connected to main ground and whne I measured the heaters I get the same voltage as B+, like 510 or whatever it is (I think its 510, didn't write it down). Is that normal or is that why things are going berserk? Where could I be leaking B+ into my heaters? I've checked every pin on the tube sockets and there is a clear gap between the heater pins and the anodes.
So frustrating that I had it working and now I feel like I'm back to square 1 again! :(
:cussing:
~Phil
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I'm betting there is a short between pin 2 and 3 on one or more of the EL34s. Use your ohm meter to check.
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When you're right, you're right... pins 2 and 3 have continuity on one of the new tubes... I think it may have ended up burning up the heaters on one or both of the new 12AU7's as well, would passing that many dc volts through the heaters end up burning the heaters? What else should I check that may ahve been damaged? I would guess I need to go back to the seller and see if I can get replacements? This is the first time I've bought tubes that weren't from doug. Guess that's what I get... /sigh.
~Phil
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would passing that many dc volts through the heaters end up burning the heaters?
they should be ohmable on the tube. And ya, look at a tube data sheet, it gives a max rating for heater to cathode volts, typically around 100v
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pins 2 and 3 have continuity on one of the new tubes
Are you measuring that directly across the tube pins with the tube unplugged? If so, the tube is bad. If you are measuring across the pins on the socket, then it could be a bad tube or a bad socket.
Unplug all the tubes and measure between pins 2 and 3 ON THE SOCKET. Don't use the continuity check. Check for actual ohms. If you get any reading other than infinity or open, replace the socket.
I'm suspecting a socket because this is not the first time those 100Ω resistors have popped.
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Ahh, I tested the tube, I'll have to check the sockets as well. So then it is posible that the issue the whole time has been a tube socket? could that have ruined the new power tube as well?
~Phil
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A short between pins 2 and 3 on the socket should not cause a short between pins 2 and 3 on the tube. But, a short between pins 2 and 3 whether actually on the tube or the socket , puts B+ voltage on the entire filament string and that may cause havoc with other tubes, maybe even the EL34s.
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I'll still definitely check the tube, but it was new to the environment, so it was likely what caused the explosive behavior. I'm still not sure what heated up the resistors the first time but it wasn't this drastic for sure. It may have been due to something arcing that shouldn't have before I rewired things the way you showed me.
~Phil
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Confirmed, no issues with the sockets between pins 2 and 3.
Phil
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puts B+ voltage on the entire filament string
did you repair the smoked parts? IF NOT, leave them out for a test, ohm (meaning amp unplugged) from pin 4 to 5 of each socket (all tubes out), whatcha get?
Now ohm EITHER 4 or 5 to ground?
Ohm either 4 or 5 to the last B+ tap?
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Shooter, I'll give that a go. I did replace the 1.5k resistor that burnt from the last preamp tube, but not the 100ohm to gnd from the heater rails. (no point if they'll just burn up eh) I'll do that though, I will see what pins 4 and 5 have with the ohm meter, and report back.
~Phil
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they'll just burn up
They are besides reducing noise, a safety *part* if things are bad, so keep power OFF and tubes out til you can sort this out.
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so from 4-5 on each socket I get 1.38M resistance on all 4
from 4 to gnd I have a really odd behavior, it starts (depending on randomness) either at about 2M and then just starts going down as I watch it, or sometimes it seems to start at about 44k I think and then go down. It doesn't make sense to me that while I'm watching it, the resistance can just count downwards. Any idea on that?
From 5 to gd I get about 1.3M on all 4 sockets
from 4 to last b+ rail I get the same behavior where it starts at a high resistance like 2M or 44k at random depending, (I can remove, and retouch it and get different results on the same pin?) and then slowly it just goes down in resistance.
From 5 to last B+ rail I get 1.37M on all but one I think it had 1.34M.
As Sluckey showed, I'm pretty sure we found the culprit was that tube. But then again the random fuse blowing while I'm taking test measurements with all tubes out is definitely odd as well.
I can also use the variac and run it at lower voltages to test if you think it would be good to go there.
~Phil
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but not the 100ohm to gnd from the heater rails. (no point if they'll just burn up eh)
They won't burn up when you get rid of the problem that puts B+ on the filament string.
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Agreed absolutely sluckey, and I think we found that. The tube with pins 2/3 shorted. 500V through 100 Ohm is a lot of watts. 500/100 = 5 Amps... * 500 V = 2500 Watts right? They smoked and popped instantly, which makes sense heh. I've asked the seller to replace the tube and they agreed to, but I asked them about the damage to the other tubes and they said the tube fault shouldn't cause any other damage to the other tubes, I reiterated that HT voltages in heaters WILL damage the heater in the tube but am waiting for a response.
~Phil
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:think1:
Did I get a couple bad 12AU7's
I shoulda stayed home, sorry for the mess-up, didn't read past that statement :BangHead:
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No worries, heh. The one thing I've been confused about, though, is if it is normal behavior for resistance to be extremely high like that on pin 4 that slowly seems to recede down to a much lower resistance? Is that a sign something is failing, or is it just some inline cap charging that acts as a high resistor until fully charged? And since the DMM has very low voltage/current it takes a while?
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If you need a reason to never shop at tubedepot.com again, here it is. I got a reply that they'd replace the tube that had the short on 2/3. They insisted though that a short from HT to heaters could not damage any other tubes and refused to let me explain why and ended up hanging up on me. When I tried calling back, they refused to answer the phone afterwards. I'll be writing up detailed complaints on every public forum and site I can about ensuring people do not use them.
~Phil
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been confused about
My post about pins 4 & 5 were for the preamp tubes, 12AX, not power amp tubes! I ASSumed :cussing:
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So to my disgust, I just went through all my tubes and I think most got fried on the heaters.
On one power tube the heaters have continuity, but they also have continuity to pin 8...
The tube depot power tubes, one had the 2/3 continuity issue, the other has no continuity on the heaters
one of the tube depot au7's has all 3 heaters no continuity (4,5,9 dead)
The other has no continuity on pin 4 but between 5,9 it does.
two other 12AX7's have pin 4 gone as well
so I think I've got two good 12AX7's and maybe 2 good EL34's. That means I can't even test the amp until I get more, because I need 3 preamp tubes at a minimum to push signal. /Sigh
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I can't even test the amp
you CAN test, and be 90% sure it's good with NO tubes.
make the filaments produce 6.3vac with NO B+
verify NO shorts, probe EVERY socket pin to it's normal return, say V1A pin 8, it's *normal* path is to ground, and it should be about 1.5K. Pin 6 has a *normal* path to B+ and it should be about 100K.
with the exception of filament, all that can be done with no power, no tubes, just an ohm meter a schematic and quality time :icon_biggrin:
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ahh true, shooter, and I had done some of that last weekend. I'll keep at it until I get new tubes.
thanks.
~Phil
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Any joy yet?
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Not yet, tubes are due to arrive on Tuesday. Once I get them, and test it out, its going in the final video (I have one more where I go over everything in detail, but we have to edit that one still). and then one where I show it all working and give a demo of the sound/tone etc.
~Phil
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Tubes all in, all seems moderately good. If I keep volume down around 11 o'clock, its okay, sounds pretty good (I think the speakers I'm using suck and are not well connected, so they're crackling). I did a video and will pull some audio for everyone to hear. I did get a bit of that odd chirping a few times, not sure what that was, but it seems to have only happened when volume exceeds maybe 1 or 2 o'clock.
I have a working amp, but it seems to need a bit of 'dialing in' I also have a lot of noise with my P90's if they're too close to it. Once I moved away (after I stopped the video), the hum went almost totally away.
I also had some pretty odd come and go volume and distortion issues, but again, that may be a badly connected speaker. I need to go over the connections there again and make sure that's not related. I'd be happy for input once I get the audio sample here in a bit (maybe tomorrow evening).
I am pretty stoked that now taht I have a new full set of tubes, its working fine (with all 4 EL34's to boot.)
~Phil
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Decided to upload it now:
https://soundcloud.com/pompeiisneaks/my-ac-1002-first-play-after-first-power-up-and-new-tubes
I've got that popping squeal noise at about :45 to :50 seconds in. Let me know if you hear anything else really 'off' I'm kind of wondering if the awful sounds aren't just a bad connection in my amp, but I don't think the squeal/pop noise is...
Anyone heard that before? I've also noted that after I stopped recording, I was pretty close to the speakers with my P90's and they were causing most of the hum, when I stepped back maybe 3-5 feet the hum stopped.
~Phil
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I've got that popping squeal noise at about :45 to :50 seconds in.
Is that snap/pop coming through the speaker? Or from inside the amp? Sounds like a high voltage arc to me. I also hear a lot of hum. Does the amp hum if no guitar is plugged in?
Hopefully some of the guys that can still hear good will chime in.
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Its coming through the speaker. When I had the dummy load up, you'd still see the oscilloscope bounce like something 'popped' but there's no audible sound that I can think of. The audio doesn't catch part of the sound it has a bit of a squeal like sound right as it pops like a cricket chirp or something. I think there's no hum with no guitar plugged in, but I'll check. Like I mentioned I realized I had some single coil pickups really near the amp and the speakers, when I walked about 3 feet away the hum almost disappeared.
Edit: Change hum with 'no' guitar plugged in hehe.
~Phil
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There's something, maybe a few things wrong, way too much distortion too early and not a good sounding distortion, it's also muddy sounding.
And when you turn on the reverb somethings really wrong. :w2:
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That popping worries me a LOT. :w2:
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I think its muddy sounding because I've got the tone controls all mucked up, I was playing with them during the audio and at a few points you hear I somewhat had it dialed in pretty well, then I went and turned it back to muddy. I think I was paying more attention to the noise and distortion.
So if that popping is arcing, what would be arcing and why?
~Phil
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Turn the lights off and watch for a spark inside your amp when you hear the pop. Sometimes you can see the culprit, sometimes not.
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I'll give that a go. What usually can be done to resolve arcing? I'm guessing it means something is a bit too close to a grounding bus right?
~Phil
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What usually can be done to resolve arcing?
It depends on what exactly it is that's arcing.
You have to find it first (if something is arcing).
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Ok I did more messing around today. I turned out all the lights near it, it was pretty dark, and I played around and could get the popping to happen off and on but never once saw a visible arc of any kind. I thought that maybe its distorting because it was biased a bit too cold so it was distorting early so I adjusted that a bit, looking at different bias levels between maybe -50v and about -45V and I think I even went shortly to -38 but then it seemed to stop outputting almost any sound ( was i hitting the quiescent point where the tube no longer can conduct?) During different points of the biasing, I was getting between 8 and 11mV off each tube depending. I believe I got about the best sound at about 8.1mV on all tubes, but I didn't write it down as much as just tinker. If it was 8.1 mV
Which, of course, is way low for that tube.
Then... :cussing: I also accidentally was monkeying with trying to get voltages to calculate plate dissipation and accidentally shorted through one of the 1 ohm 1w resistors and blew it up. /sigh. I knew there was a reason they tell you to connect, power up, read, power off, repeat etc, I'll have to get another 1 Ohm so I can get that tube working again so that one isn't taking half the load. It still plays well. I think part of the hum has been found in two locations.
1. I realized that the reverb transformer is the same direction as the output transformer and touching it. I removed one screw, and rotated it away from the OT and a decent amount of hum went away.
2. I also noted when I was probing the heaters that when I did so, and created a good path to ground, a lot of the hum went away. (I'd removed the 100 ohms that blew earlier due to the 2/3 pin burnout of heaters debacle).
I'll restore those, and hope that helps a lot with hum. (I'll have to redrill the second hole for the reverb transformer in a slightly different location and tighten it up in that new spot. That way it has an alternate rotation off 90deg and is also no longer touching against it )
/sigh, tomorrow I'll run to my local shop for the resistors and get those in, and then give er a whirl again.
Side note, I don't see a big change in hum with or without the guitar cable now that I tested that either.
Edit: clarity and missed my '2' even though I wrote it out :)
~Phil
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That amp has too much overdrive/distortion and a lot of those weird sounds you are getting are likely oscillations. A stock JMI circuit Vox AC100 is a very clean amp and will only start to go into overdrive when it is all the way up. The rhythm guitar and lead guitar (and bass) parts from the Beatles "I Feel Fine" were done with a Vox AC100, so you can hear Lennon's rhythm part. That is with the amp all the way up....that is all the overdrive/distortion it should put out. That is way less than your recording showed.
What I would do is hook the amp up to a resistor dummy load, hook it up to a signal generator outputting about 100 mV, and hook up your oscilloscope and check the various stages to see how the signal goes through the amp and how it distorts at each stage. Despite what I said above regarding overdrive/distortion levels, you will see overdrive and distortion on the scope, but it will sound mostly clean when you listen to it. You should be able to ascertain the function of all the controls on the amp with the scope and sig gen before you even plug it in to play it. Get it distorting at the output of the amp and then back off a bit and check each stage and see if one distorts before the other., etc. Check the voltages at each stage also. Sometimes you can have too much gain from one stage and it will overdrive the next badly causing grid current distortion and blocking. You can also get oscillations due to poor layout or poor soldering or a lack of shielding in certain places. That can also be caused by bad grounding. I forget if you started with a stock AC100 circuit, or if you modified it for more gain, but I would suggest to go with the stock circuit, get that working, and then modify it from there. The use of the 12AU7's where they used them in the Vox is key to that sound also. If you use 12AX7's instead, then it doesn't get that same harmonically rich sound and is too overdriven.
Greg
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Greg, thanks, I've got a from scratch build of a Vox AC100/2 based on the chassis of a Vox AC100cph. Therefore it is a bit of a mild frankenstein. I'll give that a go. I have been playing a bit with the scope to see where it seems to go berserk, but I'll try it as you've said and see if I can find where the distortion is originating. I've got the right tubes in now. The 12AU7 at V1 and V4 per my schematic. I may end up completely removing the reverb circuit and seeing if everything else gets better, and if so, try and figure out why the tried and true 1 tube reverb is the issue (it may not be but I'll see).
~Phil
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Okay so tonight I replaced that resistor that I blew, and also returned the two 100 Ohm to ground on the heater line. The hum seems a lot quieter now, with it biased a bit better. I realized, though, that the bias is a bit too hot. The current is only about 20mV. The tubes expect about 40mV at 70% dissipation, if I'm correct.
Here comes the 'something is wrong' moment. I slowly bring up the bias pot until I get near 40 and about 36 or 37mV it starts humming loudly and the voltage, without me touching it, starts climbing to about 44 or so and I hear that click/pop and I've got the volume down to 0 on the speaker. after that the mv drops to something like 0 or even -5mV and then climbs back up to about 36 or so. If I dial it back just a tad, to about 34, it doesn't seem to do it. I'm guessing it will if I play because that will cause the current to climb (its not class A so it will right?)
Does that mean somethings being overdone in the tubes themselves? I'm wondering if this circuit needs lower voltages? The schematic for the original states the power tubes should be about 450, and mine are about 480, the schematic also calls for much lower voltages on the preamp tubes than what I'm getting, I'll copy/paste from my previous measurements on this thread. Am I just running at a too high of a range for the tubes?
One other thing I'm thinking could also be it. The voltages for the reverb are really hot too, like 400 ish, and I'm guessing that circuit, with the preamp tubes, would do better with something like 150 to 200 V? no?
Do I need to adjust my voltages across the board with a dropping resistor?
Here's what I get on my A-E points:
A: 504V, B: 484V, C: 481V C': 476V, D: 320V, E: 293V
but the amp asks for:
A: 470 B: 450, C: 430, D: 310, E: 270
That means the currently setup 22.5k resistor in R23 (the big 25 Watter Steve found was way too lacking).
may need to be higher to bring voltages down to what's expected. Or do I want to put one inline before A to drop A down to 470 and the rest should just fall in line?
Do I need to put the reverb connections from B and D to something a bit lower? (after adjustments mentioned above). so that they're a lot cooler for that reverb transformer and the circuit in general?
I'm still confused about how I can drop the voltage. I know I need to put in a resistor in series before the A point and that will help drop the Voltage, but I think you need to know the current, is that the sum of currents of the plates for all tubes?
ugh, still a noob at power supply design!
I've been playing with PSU designer but I don't know how to setup a bridged diode like mine has (two of them) so its also odd in that way.
Any input on what's going on? Or are my voltages all 30V too high nothing to worry about?
~Phil
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I would disconnect the reverb circuit for now and get the stock amp working first. Reverb circuits are notorious for causing hum and noise if they are not correct. Go to the exact original circuit and get that working. After you have the amp stable then you can start adding changes one at a time.
I don't recall what the voltages were in my build and my notes aren't handy right now so I can't help you too much with that, although your voltages do seem a little high, though they should work fine as long as you have the bias set correctly. The AC100/2 schematic calls for raw B+ of 470 V and 466 V at the tubes themselves so you aren't that far off. What are the voltages at the tubes? Pins 1 and 6 are the plates and pins 3 and 8 are the cathodes for the preamp tubes. There is the supply voltage to be concerned with, and also the actual voltage that the tube is seeing. The power tube plates and screens matter, as does the preamp voltages. The way to control those voltages are the dropping resistors which are between the nodes and caps in the power supply. Changing one early in the string such as the ones between A and B will change all the voltages downstream also. The exact voltages aren't super critical, but they are important as they determine how much headroom each stage has.
Before you start in with any adjustment to the voltages downstream though, get the bias set correctly so the amp will be stable. When you talk about current, you should be talking about amps, yet you are mentioning mV. I am assuming you are using EL34's? Those are a 25 watt dissipation tube. 70% of 25 watts is 17.5 watts and 60% is 15 watts. The bias should be set somewhere in that range for the amp to be happy. So if the B+ is 504V, then around 34 mA at that voltage gets you close to 17.5 watts, and 29.8 mA gets you close to 15 watts. (504 * .034 = 17.136, 504 * .0298 = 15.0192) The B+ will change however as you adjust the bias up and down, so you have to remeasure as you are making each adjustment and do the calculation again. (higher B+ with lower bias current, conversely lower B+ with higher bias current) You can figure out the bias current if you use a Bias Rite, or a 1 ohm resistor from pin 8 to ground and measure voltage across the resistor. Since it is 1 ohm then whatever current is going through the resistor will convert straight over to the voltage that you measure. Notice I haven't mentioned the bias voltage yet....that will be whatever is necessary to get the tube idling at the current you want...that is all the bias voltage matters...the value of it isn't important otherwise...the current through the tube is what matters. Keep in mind that that the cathode bias current also includes the screen current in addition to the plate. (which is what you are measuring with the 1 ohm resistor to ground from the cathode of the power tube) The error due to the addition of the screen grid current is in a safe direction so if you set it for 70% of max dissipation, it is actually a little less than that.
Greg
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I know it is frustrating with the problems you are encountering, and you want to play the amp, but get the amp back to stock, and get the basics right first and then start to tackle the problems ones at a time in the method I mentioned previously in the last post. It is the quickest way to solve all the issues. You can't begin to troubleshoot anything until the amp will sit there for hours without complaint while you troubleshoot the live circuit. A lot of the weird pops and distortions can be due to oscillations, bad grounding, and also bad soldering or parts accidentally touching if you didn't do the layout very well so it is worth it to check everything with a fine tooth comb if you still have issues after getting back to stock.I did my AC 100 build in a Sovtek Mig 100U chassis. I used the Sovtek power transformer and used a Heyboer OT and choke. The voltages were higher in mine than stock also...I think about 470 V B+ instead of 450 V. I was aiming for a combination of the early cathode biased ones and the later fixed bias ones so my preamp voltages were tweaked to be in the middle of both. The early cathode biased ones ran VERY hot in a small chassis and liked to burst into flames sometimes. The later fixed bias ones still run hot though. Mine is using switchable cathode bias and fixed bias though if I had to do it again I would just stick with the fixed bias as they sound very similar to each other and the amp runs VERY hot when in cathode bias mode. I had mine biased about 65% of max dissipation when in fixed bias and it was still running quite hot compared to the typical Fender design. I also kept blowing screen resistors until I got them sized large enough. ( I think 7.5 watt is what are in there now.) I ran into an issue with mine with the layout and my second channel I added and was going to have to gut it and start over and I haven't done that yet so mine isn't finished but it is stable aside from that oscillation when I get up to about 8 on the higher gain channel, and the stock AC100 channel works perfectly and sounds great and is also clean until it is almost all the way up. That is what yours should do too in a stock configuration.If your voltages are climbing like that then you have the bias current too high most likely (bias voltage injected into the grid too low). The amp will hum a lot when the bias is incorrect. You should be setting the bias when all controls are at zero and you are not playing the amp. It should also be going into a resistive load. Get the bias correct so you don't blow up any more tubes and then start addressing the other issues after that.
Greg
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Thanks for the info. I'm mentioning mV because I've got 1ohm resistors on pins 1/8. I should have made that clear. This means I was near 70% for the tube of 40mv or 40mA which at 480v is 19.2 watts, or 70% (or near to it). That is when things start climbing like mad and getting near 50 before the pop and then dropping to near 0 before climbing again. I definitely think I will remove the reverb circuit to see what that provides, I think it may be part or all of the problem, then I need to figure out why it is so off.
~Phil
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Ok, so you are reading the current as mV but it is really mA that you are indirectly reading. Good that is clear, thanks.
If I do the math, 70% of 25 watts dissipation is 17.5 watts and 60% is 15 watts. 19.2 watts is more like 76.8% of the max dissipation for an EL34....these amps run hot so you should be running it down around the mid 60's rather than the mid 70's, especially if you want it to be stable so you can do testing. It doesn't hurt things to run it at 60% of the max if it allows the amp to behave. You can get everything right and then turn the bias up later if you want to experiment but I'd advise to cool the bias off. The reason things start climbing is because the tubes are likely going into thermal runaway. Are you sure you aren't getting orange or red plating? You could also have a socket that has arced and if that is the case nothing you do will work until you replace the socket. What kind of sockets did you use? If you aren't careful you will blow out a set of power tubes. As I said before, once they have a bias failure and get too hot, they are never the same and are susceptible to thermal runaway again.
I once had a Bassman 100 that was fine on the bench but would blow a fuse and a set of tubes when played live. The problem ended up being the socket, but it took three times for the problem to show itself on the bench, and when it finally did, it behaved the same as what you describe...it would start to bias creep and then go out of control.
Greg
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A: 504V, B: 484V, C: 481V C': 476V, D: 320V, E: 293V
That is a little high but EL34s work well with those voltages. What voltage are your filter caps rated for?
With the issues you've had, especially the plate to heater short in the EL34, it's possible that the OT has been damaged from over current. Let's hope not, but keep that thought in your head.
Pull two of the EL34s and see if you can adjust the bias current up to about 35-40mA and keep it stable. You must still have one tube on each side of the OT primary, so don't pull the wrong tubes.
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The caps are all either 350 or 450 and are in quads of parallel and series in the A and B stages, the rest are 475V 16uF FT's (the amp had 6x 100uF 450V and I bought two more from doug that were 100uF 350V (which means that the max power on that quad would be 700V instead of 900V but since I'm at 484 on the B point, I'm good there too).
I'll try pulling the two tubes.
Greg,
I've got an app that is called Tube Bias Calculator that says for an EL34 70% dissipation for the tube is 36.5 mA for a AB fixed bias. I was trying to get it near that ballpark without going over, and that is when it started taking off. The tube sockets are brand new, so I'm hoping they aren't arced. I did a test of the pins to make sure they weren't shorted out to any other pin at one point in this long, long thread :)
~Phil
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Haven't had a ton of time yet today, but during a short work break, (WFH today) I went down to the garage, and pulled two tubes and did a bit of biasing.
One thing I realized was that my assumption (made an ass outta me and umption) of 1ohm was wrong. The resistor on the one I was biasing from is .8 ohms. Which makes it a lot hotter than I planned.
At any rate with two in any of the sockets I can't seem to get any problems at all. When I put all 4 back in, they definitely start going off again. I did make the realization of the above at that point, and realized I'm at 85% dissipation /doh. I dialed it back to about 30mV on that one, and it puts me more like 68% and 35.4 mA.
Basically, I should have been wary of that and forgot. Also I have others that sit more like 1.1. ohms as well. I think the bias was too hot at my '35-40' mV range and now its much better. I'll also want to play around with removing the reverb circuit, but I may very well have a better sounding amp after just that.
I'll report back a bit later if I can get some more down time working to monkey with it a bit. I think time to mess with the reverb circuit removal may have to wait until the weekend.
~Phil
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Okay, here's something quite odd. I've just put the scope on, and see this. I'm attaching a screenshot of the scope.
The yellow trace is my input connected directly to the jack. The blue is at the 47k resistors right before it hits the power tubes.
The bigger deal here is that when I turn the volume up a bit, the INPUT gets all kinds of odd distortion.... is that a ground hum coming into the jack itself?
Let me know what that kind of noise means?
Edit: the input is normally very clean until I turn up the volume a tad.
~Phil
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I'm sorta lost in the weeds on this build, but if I seen that at my input, i'd scope my input signal 1st, NOT plugged into the amp, if that's good, i'd pull all tubes except V1, then put trace 1 on the input jack, trace 2 on the output of V1A, AFTER the coupling cap.
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Maybe I'm not explaining it very well. (I have done what you said and it is compeletely clean there on the signal input not connected to the amp).
The signal is super clean if the amp is off or at 0 volume. Once I up the volume a little, that noise shows up on the input connected probe.
I don't need to go further than that.
~Phil
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Oh I just reread that and now I get what you mean, figure out if the interference is gone with only V1, then add V2, etc until it shows up, and then figure out what stage is the cause
I'll give that a try.
~Phil
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:icon_biggrin:
figure out what stage is the cause
:icon_biggrin:
circle gets the square!
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Well just tried that and found 'where' it is happening, V1 and V2 (input and tone stack) have no issue, as soon as I added V4 (skipped V3 related to reverb) it came back. I'm guessing, though, that the issue may be in the reverb part of the circuit. I'll take some time tomorrow to desolder the parts related to the reverb circuit and jumper past the 100k resistor between the tone stack and the PI that was where the reverb circuit lives so it goes strait to the end... I'll see what's up. I've got to go see Deadpool with my daughters tonight :)
~Phil
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Greg,
I've got an app that is called Tube Bias Calculator that says for an EL34 70% dissipation for the tube is 36.5 mA for a AB fixed bias. I was trying to get it near that ballpark without going over, and that is when it started taking off. The tube sockets are brand new, so I'm hoping they aren't arced. I did a test of the pins to make sure they weren't shorted out to any other pin at one point in this long, long thread :)
~Phil
Phil,
You should stop using that Tube Bias Calculator and just figure out how to actually do the math yourself. For a 25 watt tube like an EL34, you multiply 25 times 0.7 for 70% and times 0.6 for 60% and that tells you the range you have to work with. If you were using 6L6GC's, then since those are a 30 watt tube you multiply 30 times 0.6 or times 0.7 and you can figure out your safe bias range with that one too. Then you measure your plate voltage and multiply by what you read across your 1 ohm resistor. (say you measure 37 mV and 480 V, so you take 480 times .037 which = 17.76 watts) Its that easy, but remember that every time you adjust your bias voltage, then plate voltage will change as will the bias current so you have to re-measure and recalculate, and different tubes can pull different current from each other too, even when paired with each other. If you have them too far out from each other with their current draw, then the amp can hum excessively.
For the sockets, look closely at them and see if you see any evidence of arcing. Ceramic sockets are immune to arcing, but the plastic types and micalex etc., can and will arc sometimes. There is something going on....either a arced socket or a bad tube in your quad that causes the amp to misbehave when you try to get stable bias.
The scope shot you posted is showing high frequency oscillations at the top of the waveform there, but they seem to be there from your signal generator source unless you have that scope shot somehow of something else? As a comparison, on my AC100 build on the clean Vox channel rather than the high gain channel that I added, I could turn the amp almost all the way up with the signal coming in from the signal generator and see the signal at the power tubes much larger but still a clean sine wave. As I turned it all the way up then I just started to get some overdrive and distorting of the waveform, but no oscillations like that.
One big problem with your amp that may be causing some of these issues is the layout. The sockets for the preamp are over on the left and your board is over on the right and every sensitive grid wire (grid wires are like antennas) from there has to run over to the sockets in parallel with other stages and also with plate wires, cathode wires, and whatever else you have in there....that is a good recipe for oscillations and weird noises. Take a look at a typical Fender layout....the input comes in, goes across the board, and then to the sockets which are right behind the board. Then it comes back up to the board and goes up to the tone controls, back to the board, back to the sockets etc. The stage wires don't cross each other, or go parallel with each other in long wire runs like what you have going on. The Fender layouts aren't perfect but they are good and work with most amps for the most part.
Greg
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Greg,
Thanks, Sadly the chassis is as it was for the old Vox AC100CPH, so I don't have the ease of switching it around much, I do understand what you're saying, though. after grounding the heaters the hum has gone down significantly.
As for the bias, I already knew that method, and have used it in the past, but I like calculators that do the math for me. Ultimately, though, if you see one of my last posts, it was due to my 1 ohm resistor actually being .8 ohms that I was so far off. I promise, the input signal is clean. really clean. If I have the amp off, or remove the cord and plug directly into it, there is a perfect sine wave. I just showed the shot of what happens as soon as I turn the volume up from 0 to about 2 of 10 and that shows up immediately. Per the troubleshooting I did yesterday, I found that it goes away until I put the PI Tube back in, but I suspect its the reverb part of the circuit causing that. the popping is the tubes going into runaway due to me biasing them at 85% or so because of estimating wrong with that .8 ohm resistor. I've dialed that back so that its near 65% I think and will get the reverb issue resolved if not removed and move on from there.
~Phil
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Oh and the power tube sockets are ceramic, so per what you mentioned, they should be solid.
~Phil
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Okay I've pulled the reverb part out completely, and have been doing some testing. I've found a few things.
1. it is still distorting quite early.
2. It is still getting into runaway mode even at hotter bias settings.
3. I did some voltage measurements with the oscilloscope at the 4x47k resistors that split out to the 4 tubes as well as some bias measurements on either side of the tubes and found that they're off quite a bit. Example:
left half of PP about 34mV and right half about 9mV
left half of output from PI at a fairly low volume 62V right half at same volume, 40V
4. The output voltages from the PI PtP seem pretty high. With the sine wave input going into it, and with volume up to max, I was getting about 500V input. (I kept thinking I had my 10x probe set to 1x or something, but it wasn't, on the scope it was set to 10x and on the probe it was at 10x. That seems pretty hot for input into a EL34 no? Also I'm seeing plate voltages climb to something like 100mV when I get the volume up there so I turned it back down pretty quickly. (maybe that's due to it being a sine wave?)
I see two potential things that need sorting.
1. Fix that imbalance, I'm not sure why its so off, but I measured all the resistors in the region of that circuit and they all seem right, the ground ones are at 1M the R19/R20 level, R21/22 add up to like 174k in circuit (2x330 is about right, a touch high but seems fine)
2. figure out why it seems to be so hot coming out of the PI.
So in looking at the EL34 datasheet, it seems to imply that it should be 50V RMS input to the grid, thus my worries that at even nominal volumes input its so high... what would be causing that? Is there something wrong with the bias on one or several of the preamp tubes pushing it that high?
I'm going to do some re-measurements to ensure I'm not going insane, but I definitely saw the imbalance.
~Phil
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One other thing to note, pulling the reverb didn't seem to clean up that odd static, but then I kept wondering why my probe was so dirty compared to the other (the blue one on the screenshot). It ended up being messed up, I couldn't even get a clean line on it on the on scope reference square wave, so I switched them and the noise seems to have mostly gone away. I also resolderd the connections to my speaker and that also seems to have removed some of the worst of the crackling.
As for the hum, its only really bad if I've got my guitar close to it and when I walk about 5 feet away the hum is pretty low and at 10 feet it's almost gone, so I think its just noise from the p90's.
~Phil
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Nvm on that odd static, it's back. I did more testing, probing etc, Here's the latest voltages of all tubes and HT rails:
Preamp, Rows down are V1-V4, columns across are Pins 1, 2, 3, 6, 7, 8 (note V1 and V3 have no second half in use, its the reverb for V1, V3 first half is reverb return, but nothing is coming into that tube right now)
| 88.7V | 69mV | 2.8V | - | - | - |
| 196.6V | 28.8mV | 1.5V | 323.2V | 198.7V | 199.2V |
| 319.9V | 95mV | 79mV | - | - | - |
| 278.5V | 310mV | 11.9V | 280V | 159mV | 11.9V |
Power, rows down are V5-V8, columns across are Pins : 1/8,3,4/6,5
| 26.8mV | 477.5V | 463.8V | -38.90mV |
| 29.5mV | 478.9V | 464.9V | -38.93mV |
| 20.7mV | 482.6V | 469.4V | -42.19mV |
| 21.0mV | 482.3V | 469.2V | -42.01mV |
The Power rail (HT/B+
A: 487.7V, B: 467.9V, C: 468.9V, C': 450.0V, D: 321.9V, E: 279.5V
I'm still boggled...
Like I said, when I did test with the tubes out that 'static'/noise disappeared until I got the PI in, but I did have the power amp tubes in, do I want all of them out so I see if the PI isn't really the cause? (I though I was just pulling and replacing the preamp tubes to see.)
~Phil
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Good the power tube sockets are ceramic....that should rule out arcing at the socket. Since you had the bias way too hot previously, some of the tubes in the quad may be defective now. You could try to run just a pair as you did before but swap tubes around so you can see if one or more in the quad is having issues...but before you do that, try this.....take the power tubes out. Leave the preamp tubes in. Run a clean sine wave about 100 mV into the input of the amp. Trace the signal through each stage at various volume levels and see if you have any issues. You should be able to measure the clean signal on the grid, and then measure it on the plate and see a much larger signal that is inverted and without weird anomalies at each stage. You should be able to see the effect when you adjust the tone controls on the signal if you measure after the tone controls with the scope probe. When you get to the phase inverter, you can measure the voltage output when the rest of the preamp is a clean sine wave and see what you get. The voltage output there will be a bit less when you put the load of the power tubes back in, but this will give you an idea if the preamp of the amp is ok or not. If not then you can fix until you have that sorted and then move on to adding the power tubes. As Sluckey said before, divide and conquer. Simplify the situation and get one section sorted and then move on to the next. Removing the reverb is good even if it is not the cause of the issues as it simplifies what you are trying to do to find the problems.
With the layout, it looked to me like you should be able to put a small board in between the preamp sockets and the power tube sockets...but if you can make it work as it is now it might be easier than changing it up. I would try to run your wires between the board and socket a bit more organized so you don't have different stages running between the sockets and the board right next to each other, and perhaps use shielded cable for the grid connections. I would also think about adding grid stopper resistors right at the sockets as Merlin mentions on his site and in his book. That may help if you are having trouble getting it to behave.
I forget about how close the two sides are in that phase inverter design. It is a floating paraphase circuit....you can see that on Merlin's site. It doesn't balance perfectly, but it should balance pretty well. You're still using the 12AU7 in the PI slot right?
Greg
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I am using the 12AU7's in the V1 and V4 PI stages, yes.
I just did some more troubleshooting, and I'm suspicious the issue is with the OT. I had the power tubes out, and can't get the weird issue to happen at all. I put in Two tubes, and ti works fine as well on the scope,
As soon as I put in all 4 tubes it starts freaking out. Additionally I tried swapping tubes, not only in and out only two at a time, but also the socket the tubes were in (keeping them to their own half of course) so that if I had left side tube 1 and 2, first time I did 1,2 then 2,1 and with just 0 1, 1,0, 0,2, 2,0 for each case (i.e. 0 meant socket empty) and mirrored the same combo on the right side as well, I never once got the issue with just two tubes in, but eveyr way I try with 4 it does that stupid noise issue.
I am also suspicious per something that sluckey said about the OT, so I tested the impedance of each half, and I get 19.8 ohms on one side and 16.7 on the other. I checked and found someone selling them and lo and behold I see this:
http://www.etronic-parts.com/product_info.php/info/p1090_VOX--Output-Transformer-AC100CPH.html (http://www.etronic-parts.com/product_info.php/info/p1090_VOX--Output-Transformer-AC100CPH.html)
they're supposed to be 18 ohms a side, is a variance of 2 ohms enough to matter? Is the OT what's wrong here?
~Phil
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I took a look at my notes from my build. B+ on mine was 467 V. Power tube plates were 460 V and power tube screens were 452 V. The negative voltage going into the power tube grids for the bias was -35.9 V. I don't know what the mA were that the power tubes were biased at as I didn't write it down on these notes. The phase inverter was all mucked up as I had been playing around with different circuits there and so I don't think the voltages that I had on these notes are going to help at all and I can't find my notes from when I had the stock PI circuit. I had a 440 V supply to the PI, and 352.3 V, 344.4 V, and 328.8 V to the other nodes. My nodes were also supplying the high gain stage in my amp though so they would be different than a stock circuit, plus I had adjusted dropping resistor values to get the supply a happy medium between the fixed bias and cathode bias versions of this amp so my values are all different than stock. The first gain stage in my amp had 91.5 V on the plate and 3.25 V on the cathode. This is with the stock 12AU7. The second gain stage which is the 12AX7 is at 214 plate and 1.58 cathode. Third stage is the cathode follower and it is at 352.3 V plate, 214.3 V cathode.
I also attached a picture of mine so you can compare. Keep in mind that mine was a work in progress that will have to be gutted and started over. My board was split in two due to the Z mount of the OT. I got some attachments for the OT so I can mount it above the chassis and will be doing so when I get back to this project. This will allow a full length board, which will put the circuitry for the PI right next to the socket instead of several inches away on the second board. I was getting oscillations on my high gain channel at the PI due to the long runs to the circuitry from the PI socket, and the fact it was passing right under an unshielded OT. I also had some experiments going on and had tacked some parts in so it is sloppy right now around the PI and the power tube screen grid resistors too. I also had paralleled film caps with the e-caps in the power supply to check that out. I won't be adding that to the next version so that will simplify some things. Another difference was I am using DC filaments on the first gain stage in each preamp so that is what the small board is for. Also I am using cathode biasing and fixed biasing and have switching in there with relays and some solid state components to do channel switching too, so it is pretty involved. With the DC filaments and the ground buss on the board only touching the chassis at one point, and everything soldered to it in order like a galactic ground system, this amp is the quietest one I own, and I've made quite a few projects and have quite a few commercial products like an AC30, Sunn 2000S, Silvertone 1484, etc.
Whenever I get back to this amp it will look quite a bit different on the inside but you can get an idea of what I was talking about as far as wire runs and not running wires from the board to the sockets all jumbled right next to each other. Aside from the PI section in this amp, the grid runs from the board to the sockets are maybe a couple inches, whereas in your amp they are like 6 inches or more...hard to tell from the pic though. In your amp you may be able to get everything working well with that layout, but down the road you can do some thinking about how you want to lay stuff out and use some of the Hoffman examples or refer to Fenders and come up with something that will reduce the chance of having problems.
Greg
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I am using the 12AU7's in the V1 and V4 PI stages, yes.
I just did some more troubleshooting, and I'm suspicious the issue is with the OT. I had the power tubes out, and can't get the weird issue to happen at all. I put in Two tubes, and ti works fine as well on the scope,
As soon as I put in all 4 tubes it starts freaking out. Additionally I tried swapping tubes, not only in and out only two at a time, but also the socket the tubes were in (keeping them to their own half of course) so that if I had left side tube 1 and 2, first time I did 1,2 then 2,1 and with just 0 1, 1,0, 0,2, 2,0 for each case (i.e. 0 meant socket empty) and mirrored the same combo on the right side as well, I never once got the issue with just two tubes in, but eveyr way I try with 4 it does that stupid noise issue.
I am also suspicious per something that sluckey said about the OT, so I tested the impedance of each half, and I get 19.8 ohms on one side and 16.7 on the other. I checked and found someone selling them and lo and behold I see this:
http://www.etronic-parts.com/product_info.php/info/p1090_VOX--Output-Transformer-AC100CPH.html (http://www.etronic-parts.com/product_info.php/info/p1090_VOX--Output-Transformer-AC100CPH.html)
they're supposed to be 18 ohms a side, is a variance of 2 ohms enough to matter? Is the OT what's wrong here?
~Phil
You're just checking resistance with your meter? It is normal for the OT to have a mis-balance like that between sides....almost every one I have ever measured is off a bit. 18 ohms a side is what they aim for but barely any difference in the actual amount of wire on each side will make for a difference in your readings...it doesn't take much wire to give 2 ohms difference.
Hmm, so swapping around with the power tubes has no issue with two no matter the pair? I was figuring you would have a problem with one of the tubes in the quad. I am still suspicious of those tubes. Do you have a tube tester? Sometimes it will help to bring them up on a tester and see if one is way out from the others or if it has a short or something. Do you have any other EL34's or KT77's? You could try 6L6GC's also if the socket is wired appropriately if you had any of those laying around.....but really, try it without the power tubes and go through each stage with the scope and signal and make sure you can verify that there are no weird distortions or wave forms on the scope and you have clear amplification from stage to stage. Don't worry about playing it...and in fact if you have a resistor load, do your troubleshooting while connected to that instead of a speaker. It will give more predictable behavior.
Greg
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These tubes are brand new and the second full set I've had in it (The first set seemed like soemthing was off so I swapped two out, got two more from tube depot.com and they had pins 2/3 shorted on one side, so I burnt out the entire set of tubes heaters... so I just put in a new set this week).
Since I can literally try any two of the tubes at a time and not get the issue and ONLY when all 4 are in, it makes me think it is not the tube. The only other thing I'm slightly suspicious of is that one of the sockets seems a tad loose to put the tube in, but I don't think that would cause this, it would be intermittent and fail at random causing some odd behavior. As for the new power tubes, they were apex matched all 4. I don't think it is the tubes.
Maybe I should try and tighten up the pins in the socket?
I'm just at a loss. It definitely seems to confirm that the issue is either in the power stage or the OT. I can't find anything specific to a socket or a tube so far, so I suspect the OT.
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I was getting about 500V input
I'd verify this, 50v would be more *inline*
I never once got the issue with just two tubes
So ALL tubes in ANY 2 sockets, works?
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Correct, I kept trying every variation I could of tube and socket, but only in pairs, I.e. Tubes 1 and 2 on the left side always, and tubes 3 and 4 right side always. I didn't specifically see any reason to swap tube pairs to the other side.
Effectively, I did, on a half (and repeated this on the other half) where I tried tube one in socket one and two, and tube two in sockets one and two, then repeated this same for tube swap on the right side. Nothing. I pot in four and bam, it re appears
Phil
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Definitely make sure the sockets are adjusted correctly on all pins. If the pin 5 is not making contact, then that tube would want to run away since it wouldn't receive its negative bias voltage, or receive it intermittently. That could very well be the problem.
Greg
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I had carefully gone through and re-soldered all the connections at a previous point in this thread. I also, as a sanity check, did a continuity check from the wires before the turret right before the pins, to the base of the pin itself and get 0 ohm resistance. The connections are all good there.
Phil
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I'm not talking about the connections on the solder terminal side of the sockets. I am talking about the part that contacts the tube pins when you insert the tube into the socket. Make sure all the sockets are adjusted correctly so all the tube pins contact the contacts inside the sockets. When you insert the tubes into the sockets, there should be some resistance and all pins should contact the socket. If any pins are not adequately touching the socket, but especially pin 5 where you negative bias voltage comes in, then you will have LOTS of problems getting the amp to run stable as it will not get its bias and will run away. You can use a dental tool or very small screwdriver to adjust the socket terminals so they will grip the tube well when you insert the tube. Ceramic sockets are harder to adjust than micalex or other plastic sockets, but it can be done.
Greg
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On that may be an issue with the first tube. It seems really loose and easy to pull the tube out. I'll give that a try.
Phil
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That may have eliminated the really bad noise, but the mV readings on the cathode are still all over the place.
Maybe I'm doing it wrong?
Do you check the bias there with nothing going through the amp? Or under sine wave load? Maybe that is why I'm getting of readings?
Phil
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More of that weird one sidedness. Back to just two tubes in the amp. No popping like I've had (I pulled the other two because it was so bad almost every second now). It's gone, but the bigger problem is that I can now measure the voltage on pin 8 and I get about 50mV on one (It has a 1.5 ohm resistor so that's about the ballpark for 70% .050/1.5 = .033mA at 480V = 16 Watts) the other tube on the right half is at 13mV! (it is at 1.1 ohms, so .013 / 1.1 = .012 mA and X480 = 5.67 watts. )
That seems like a pretty extreme imbalance on those two tubes no?
What would cause that imbalance?
The imbalance follows the socket, not the tube, I switched the two of them places and I get the identical same values on the same sockets.
Edit: Side note I tightened the pins on the first socket and it now is quite hard to get in and out like the others.
~Phil
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Swap positions with the two tubes. Does the low reading follow the same tube?
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The imbalance follows the socket, not the tube, I switched the two of them places and I get the identical same values on the same sockets.
From my last post...
it stays with the socket, does not follow the tube. I realize the wording isn't right... I should have said the imbalance "STAYS" with the SOCKET :).. I did swap them and it the tubes don't matter.
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OK, so now swap the OT plate leads. Does the low current reading move to the other socket?
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So I just desoldered the two plate leads from the OT. (I left the jumper to the second socket as it was, because the pairs should still be left two right two) and re-soldered them to the opposite side of the tube pairs. I started it back up, and yet again, the sockets contained the imbalance, not the tubes and now that I've done that NOT the OT. Its something coming from the PI right? I'll go over the circuit and see if I can figure out why that would be imbalanced.
~Phil
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To be clear, the low current reading moved to the other side/other 2 power tube sockets, when you swapped the OT primary wires?
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No, the low current reading stayed with the socket. Meaning its always been low on the right half of the tube pair. Swapping tubes didn't change the location of the low reading. Swapping the sides of the OT didn't change anything (significant. Before it was 50mV left tube , 12 mV right side tube , after swapping the OT to opposite halves of hte tubes, I did it again and get 50mV left tube 15mV right tube).
I hope that makes sense. If I did it with say just two tubes, two sockets and two sides of the OT, then let me call it LT/RT for tubes, LO,RO for output transformer and then LS/RS for sockets. Here's what I got:
LT/LO/LS
50mv
RT/RO/RS
11mV
Then
RT/LO/LS
50mV
LT/RO/RS
11mV
Finally
RT/RO/LS
50mV
LT/LO/RS
15mV
so it stays with the side/socket, not the transformer or the tube
I just went over the layout, and the readings on each one and I don't see anything at all that seems off/wrong.
Looking at the schematic, (reattaching here, with a close up of the layout around the PI).
Does anyone see anything wrong there? I don't. I've gone over it again and again. I also can't see any components that don't read right. The R19, R20, R21, R22 all read correct. C11 and C12 have consistent decent looking ESR ratings on my ESR meter. I also checked C13 and R15 and see nothing wrong there....
~Phil
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Here's an older photo of the PI section. It's in identical shape still, but I've resoldered all of it at one point to get the connections better, etc.
The yellow wire on the left middle, near the green wire goes off to the grid of the pin 2 of the first half of PI. Then the two red wires just directly above it next to it and one over are the output from pins 1 and 6 of the anode on the tubes. That goes off to B+. the same lines, though also have dual wires coming from pins 1/6 to the left and right halves of the PI mirrored section. Then the blue lines leave to go up to the power tubes. I've also measured the voltages at those blue wires and see a difference there as well.
I have the input to the grid going off the yellow wire at the top right most section where the resistor and capacitor connect in via a jumper wire to the lower section of the blue wire output through those two 1M resistors (R19/R20). Nothing seems wrong, but something must not be right. (BTW there is a jumper underneath that red wire on the bottom between R19/R20 , you just can't see it because it is covered).
~Phil
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In your drawing pin 8 is not connected.
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Oh yeah that's just an error on the layout drawing, I've jumpered at the socket pins 3 and 8.
I'll fix that now.
~Phil
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Here's the best picture I have of the PI, you can see the black wire jumpering the two.
~Phil
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That may have eliminated the really bad noise, but the mV readings on the cathode are still all over the place.
Maybe I'm doing it wrong?
Do you check the bias there with nothing going through the amp? Or under sine wave load? Maybe that is why I'm getting of readings?
Phil
You should know that on the particular socket that you adjusted, any tube that was in there while you were doing your testing likely may not have been receiving its negative bias voltage on pin 5 that would have kept the tube in check, so that tube may have been cooking itself the whole time. Something to be aware of.
When you check the bias, all controls should be at zero and there should be no signal going into the amp. You should also have it on a resistive load instead of a speaker as a speaker causes differences at different frequencies and the resistor load allows all frequencies to perform the same. If you try to check the bias when you have a signal going into the amp, the cathode reading that you get across your 1 ohm resistors will change. Setting the bias is done when the amp is in an idle condition, which means no input to the amp. Try it again with no input signal and all controls at zero and see if the readings are the same.
Greg
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In the pic you just posted, the board mounting screw looks very close to a turret, might just be the pic angle?
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I went and looked and it is a bit close, so I removed the screw (5 screws is likely fine) but it didnt change anything, still imbalanced. Also when I check the voltage of the lines coming out from the PI one side is -40V the other is -37 V so theres a 3 V difference from that as well, (not sure if that makes any difference)
~Phil
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If you are concerned that the pi is somehow causing the problems with your output tubes, just disconnect the coupling caps from the pi plates. Then you will be dealing only with the output tubes, OT, B+ supply, and bias circuit.
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By disconnect you mean remove and jumper over right? Because they're what connects into the inputs of the Power amp right? Does that make me lose the dc rejection though? Or should it be pretty nominal at this point of the circuit?
~Phil
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Just unsolder the end of the cap that connects to the pi plate and stand the cap up so that lead does not touch anything. Do this for both caps. Don't read anything else into what I'm saying.
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Okay, sorry It didn't make sense, I did it. And is still doesn't make sense. Now that my coupling caps are gone, there is no route to the power stage, and therefore I get no readings in the power stage, coming in... Or do you want me to measure the plate current on the power tubes now that the PI is disconnected?
~Phil
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Or do you want me to measure the plate current on the power tubes now that the PI is disconnected?
Yes. If you are concerned that the pi is somehow causing the problems with your output tubes, just disconnect the coupling caps from the pi plates. Then you will be dealing only with the output tubes, OT, B+ supply, and bias circuit.
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Doh, it dawned on me as I hit send. I retested, and with that disconnected, I get the same imbalance
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Then it's not coming from the PI.
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So what could it be?
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If it's not the PI, not the OT and it's not the power tubes, what's left?
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I don't know? Sockets? The 1k resistors all tested as good, the 1 ohm resistors in the power section tested good. I doubt two of the four sockets I just bought form Doug a few months ago are bad. If it is one, what do I do to find out?
~Phil
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You have 1 socket that is drawing low current, why?
What you have left is;
1. socket.
2. the wires that go to that socket.
3. the solder joints on that socket and on the other end of the wires/parts that go to that socket.
4. cathode ground connection.
5. -bias and the wires that deliver it to the power tube grid return R's.
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Which socket is the 1 drawing low current?
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I just put all 4 back in to see what was what, and I think I may have the one that is the bad socket or at least bad place.
The second from your left pic, what I'd call V6 (V5-8 are power tubes) Here's what I just read with all 4 back in:
V5, V6, V7, V8
26.4mV, 45.0mV, 12.5mV, 12.5mV
That may indicate either something isn't soldered well on V6, or that V6's socket has something wrong.
What else could make that one pull so much current?
~Phil
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Oh sorry I actually didn't answer your question right, the right two sockets are low, the left two, higher, but it seems that, as you can see, one is a lot higher than the other.
~Phil
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What are the voltage readings on pin 5 of each tube?
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From the same below:
V5, V6, V7, V8
26.4mV, 45.0mV, 12.5mV, 12.5mV
-37.0V, -37.2V, -40.5V, -40.6V
~Phil
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Oh and side note, not sure if it matters, but touching those makes a lot of static noise and scares the crap outta me each time :) bad hum type thing that's pretty loud in my speakers that have volume set to 0.
~Phil
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You have 1 socket that is drawing low current, why?
What you have left is;
1. socket.
2. the wires that go to that socket.
3. the solder joints on that socket and on the other end of the wires/parts that go to that socket.
4. cathode ground connection.
5. -bias and the wires that deliver it to the power tube grid return R's.
I forgot;
6. power tube grid return R's/value/wires to & from/wired incorrectly.
7. power tube grid stopper R's/value/wires to & from/wired incorrectly.
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So to recap all 7... 1. I am suspicious that one socket may be bad, but lets go over the rest
2. the wires that go to the socket. I've been testing them with the continuity tester to ensure things are going where they should, by touching the base of the pin, not the wires themselves, and then connecting to the previous connection or even just past a turret and always have gotten good continuity.
3. I've resoldered them all over again ensuring I got good continuity from 2 above.
4. I can test the ohms resistance for all cathodes to ground with no issues (as well as continuity because low ohms under like 20 count as 'continuity' on my meter)
5. I also can touch the bottoms of the pins and get the -grid values I reported already. Each pin is getting that voltage
6. Sorry, I'm not familiar with the 'return' name? Is that the 1k, from the B+ resistor or the 1 ohm to ground? the only other resistor is the grid stopper which is the 1 47k resistors. I'll report on those.
7. Same as 6 above, the report on all resistors will be below. I have tested that the wires give me the connections I expect, as for 'incorrectly' I can describe them to you:.
For the 1 ohm, I go out of the pin 8 which is jumpered to pin 1, and go directly to a grounding bus on each one. I can measure their resistance by connecting my ground to the far end of the ground rail away from all three and get the same reading as if I put the ground just under it. I test the ohms by touching the base of the pin itself, not the wires
for the 47k I have tested that I get the voltage of each on the pin, and I've tested each of them before and after the resistors and the voltages seem to make sense and indicate they're all flowing as expected.
for the 1k resistors, I've tested voltages on them and they all are showing correct. I've tested them by touching the pin 4 base and then read resistance on the other end of the line and I get the readings I'll report. As for their wiring, I've got one end of the 1k resistor in pin4, the other in pin 6 and then I've got jumper wires between all 4 pin 6's that then leaves the V8 tube off to the B point.
I hope that describes them, I can hunt down my pictures too if that will help, they're on this thread but I don't mind avoiding you going hunting for them.
Here are the measurements:
V5, V6, V7, V8
26.4mV, 45.0mV, 12.5mV, 12.5mV
-37.0V, -37.2V, -40.5V, -40.6V
0.6ohm, 1.4ohm, 0.9ohm, 1.3ohm
0.976k, 0.978k, 1.001k, 0.981k
Does that help?
~Phil
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Here's a pic of the first three output tubes, missing the 4th in the picture. https://goo.gl/photos/Z2av6nsFeQF2cxv26
~Phil
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In the Vox schemo, R21 & R22, 330K/350K?, I can't read it, are the grid return R's.
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Oh gotcha, since they're in parallel, I can't measure them independently, but either side measures 172.7 Ohms (which would indicate about 345.3 each). They're supposed to be 330k so that would be = 165 is expected. its a bit over. If they're actually out of balance in some way, would that cause the sides to diverge? Should I break them out of circuit and measure?
~Phil
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No, I don't think that's needed. Wait for Sluckey or 1 of the other guy's to confirm.
One other thing, most meters can't measure a 1 ohm R accurately. And there's been a few threads where it's come up that because of this and that modern 1 ohm/1% R's are made well enough to not even worry about them being off enough to throw off the K current measurement.
(I've been sick for a week and I'm going to bed. :sleepy2: )
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Oh I may have 1 ohm 5% resistors! That definitely would be part of why they seem so off. I definitely don't think my Fluke 17 can read them super well, it seems to have only .1 accuracy for ohms.
I just got over being sick, thanks for your help, get some rest!
Should I go buy some 1% ones and replace them? (I replaced a few I popped being stupid, the ones I got from Dough are 1% I believe.)
~Phil
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Here's his last power tube readings;
Here are the measurements:
V5, V6, V7, V8
26.4mV, 45.0mV, 12.5mV, 12.5mV
-37.0V, -37.2V, -40.5V, -40.6V
0.6ohm, 1.4ohm, 0.9ohm, 1.3ohm
0.976k, 0.978k, 1.001k, 0.981k
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Oh I may have 1 ohm 5% resistors!
Should I go buy some 1% ones and replace them? (I replaced a few I popped being stupid, the ones I got from Dough are 1% I believe.)
I would.
Yes Doug's 1 ohm's are 1%.
Heck, for the price ($0.15 from Doug) buy at least a dozen or 2 dozen and be done with it for a while. :icon_biggrin:
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Which socket is the 1 drawing low current?
Willabe,
This pic you linked here is of my amp that I showed for example, not of pompeiisneaks's amp.
Greg
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Heh I thought he'd just pulled in some random one to show 4 tubes for me to specify :) Yeah I posted a pic of three of them (The 4th is the one that mirrors the other lower output tube, so since it seems identical to 3 I think its good, or I hope so).
~Phil
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This pic you linked here is of my amp that I showed for example, not of pompeiisneaks's amp.
Doh, my mistake. :BangHead:
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Here's a pic of the first three output tubes, missing the 4th in the picture. https://goo.gl/photos/Z2av6nsFeQF2cxv26 (https://goo.gl/photos/Z2av6nsFeQF2cxv26)
~Phil
Phil,
In this picture your 1 ohm resistors coming off the sockets and wrapping around a buss with solder to tack the connection into place isn't the best approach. That buss can easily contact the chassis at any point along its length which could compromise grounding. A better approach is to either use terminal strips near each socket that you connect the resistor to from the socket and then connect your buss or wires to those. Another approach is to use tip pin jacks along the back of the amp as in the Sunn 200S example that I attached to this post. Years ago I modified this Sunn and added the tip pin jacks. The wire goes from the socket to the tip pin jack, then the resistors go between the red tips to the black tip pin jack which is then connected to ground. With either of these approaches that I mentioned, there is no way that the wires or resistors can move and then have your connection points change as with the approach that you used. You can see the tip pin jacks at the back of the chassis in the Sunn. I believe Hoffman sells those tip pin jacks too.
This pic you linked here is of my amp that I showed for example, not of pompeiisneaks's amp.
Doh, my mistake. :BangHead:
Its cool Willabe! :icon_biggrin: Just wanted to head off any more confusion...haha :l2:
Greg
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Not sure I follow, if the tubes ground sooner that wouldn't be a problem would it? The problem you often have is that the preamp side can ground before the power stage leading to hum, I recall being taught and reading that it is often best to get the power section and output sections to chassis ground before the preamp side. So if somehow the power section grounds to chassis earlier than they already are, isn't that a 'win win'? There is also a really thick paint coating over the chassis anyway which reduces the chance of a ground that I shouldn't need to worry about anyway. (a lot of older amps used to ground pins 8 directly to the chassis right next to them, instead of running a bus to a ground somewhere else). This is why I wasn't concerned about the grounding for the output tubes. Is that a misunderstanding?
~Phil
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Not sure I follow, if the tubes ground sooner that wouldn't be a problem would it? The problem you often have is that the preamp side can ground before the power stage leading to hum, I recall being taught and reading that it is often best to get the power section and output sections to chassis ground before the preamp side. So if somehow the power section grounds to chassis earlier than they already are, isn't that a 'win win'? There is also a really thick paint coating over the chassis anyway which reduces the chance of a ground that I shouldn't need to worry about anyway. (a lot of older amps used to ground pins 8 directly to the chassis right next to them, instead of running a bus to a ground somewhere else). This is why I wasn't concerned about the grounding for the output tubes. Is that a misunderstanding?
~Phil
Phil,
It isn't about grounding sooner or later....it is that you want to control your chassis grounding because poor grounding can and does lead to hum and noise. What you have there is a random ground occurring randomly. :icon_biggrin: It is best to control all of your grounds in either a galactic grounding scheme like O' Conner and Merlin recommend, or use specific ground points like Hoffman recommends. Powder coated chassis' will not stop parts from grounding btw. I found out that the hard way on a previous project some years ago. In my example amp I posted above, everything grounds to a buss that is on the boards. They all ground in order from preamp, phase inverter, and power amp, and finally that buss and any other grounds anywhere else in the amp all ground to a terminal attached to a bolt that is secured to the chassis by itself. it is best practice to put that bolt near the input of the amp, as any noise on the input would then ground out there before it gets into the amp, but in the case of my example amp it was quieter bolted near the power supply end of the chassis so that is where it was connected. With that setup there is only one point on the chassis that is connected to ground and it works very well if done correctly. With Hoffman's approach, the grounds are controlled so that the noisy power amp stuff grounds to one point and the quieter preamp stuff grounds to a different point. With either system though the ground connections to the chassis are controlled and any ground issues then are easy to solve if they occur as there are only a couple places to look. With the setup you used, it is possible to randomly have that buss grounding, or not grounding at various points along its length. This can create issues depending on where that buss runs in relation to other noisy areas of the amp. It is also not best practice to connect wires and resistors that way and there is a reason that no commercial amp ever made used that approach. It would never pass any regulatory bodies anywhere in the world either. It is your business if you want to build something with questionable practices. I'm just offering a suggestion....you can use the advice or not. For myself I always try to build the best and safest amp that I can.
Greg
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First off, I take all input and suggestions very seriously, I'm just trying to understand the 'whys' of things. I'm still unsure I understand, because it 'seems' to me that you've just re-confirmed what I'm saying, but also stated against it as well, so I'm trying to understand.
There isn't an 'optimal' way to ground per se, right, many that work.
1 is that you ground the noisiest stuff on a bus to the near chassis ground, and then ground the preamp connections to a bus that is slightly farther away, less potential for noise. I am using this right now with my bus bar, I ahve two of them, one that connects the power stage and output tubes directly to a bus that is grounded near to a PT bolt. The second bus grounds a few inches away from the PT and is on the other side of the narrow side of the chassis from the PT.
2. Ground the noisiest stuff directly to chassis. I.e. power and output tubes go to ground asap so they get out of the grounding plane the fastest, and defer the grounding of the preamp side still similarly to 1 above.
so in my current grounding scheme, what you're saying if you ground via 1 or 2 randomly it can add noise? Why would the two optimal grounding options, at random, cause a problem?
I.e. right now it is grounding well to the PT... oops it just grounded directly to chassis... but that's the same high quality connection as the previous?
What am I misunderstanding? What does a good ground that accidentally gets a different good ground do to contribute noise?
If I were to put some kind of grounding anchor on the left hand side, which means that I have a dual ground for that bus, is that good, or bad?
Is your suggestion to simply put a 3 hole Terminal strip and connect the first tube on the left to it so the rest of that ground is lifted off the chassis, or do I put several of them along the length?
Ultimately with this build, I'm trying to finish it in a sane way, since I've put in about 1000 into it and its driving me nuts not getting it working and I'm trying really hard not to sink even another penny in unless it will definitely help.
The hum is not gone, but it is significantly cleaner since I restored the 2 100meg resistors to ground for the heaters. I don't necessarily like my layout or how the heaters are wired, but I don't have a lot of room to improve that due to the layout that I'm using that mirrored exactly how Vox did their previous one. The only major difference right now is that they also had about 2 more grounding connections (They were grounding each board to chassis, the main power one twice).
Also a lot of my hum, as stated, was due to me standing between my speakers and the chassis with single coil pickups. When I wandered a few feet back it was almost gone. (I still want to fix that last annoying little bit, though, I think its my heater dress... I may rip it all out again, and give it one more shot, but maybe after I put coax cable from each connection of board to grid of the preamp tubes, to reduce input noise. )
Thanks for your help. I'll implement whatever is the right way to do this, just trying to understand it.
Edit: Clarity, one statement didn't make sense.
~Phil
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..........I think its my heater dress... I may rip it all out again, and give it one more shot, but maybe after I put coax cable from each connection of board to grid of the preamp tubes, to reduce input noise. )
I would not do that. Do the shielded cable on the grid wires 1st.
You *might* have a ground loop or 2 in that amp, but you do have some ~long grid wire runs that are running parallel to other wires that could be a problem. The longer they run together, the more 'hash' they can/will pick up. Given your layout, the overall size of the chassis and # of tubes/grids, ie, several ~long grid wire runs, each possibly adding/picking up hum/noise, I would do this 1st before messing with the grounds.
You kinda have use chasing our tail here. And you still have more than 1 thing going on that your trying to fix.
Sluckey told you on the 1st page of this thread; Divide and conquer.
The hum issue started on page 1 of your thread, which is fine. You found out that 1 or both 100 ohm heater faux CT R's blew and Sluckey told you to leave them out until you get the B+ on the heaters fixed.
Then you say you didn't put the 2 x 100 ohm heater R's back in after you straitened out the shorted tube(s) that blew them. :w2: So that helped a lot, good. Then you mention that the hum was again reduced by moving the guitar (single coil PUP's/P90's) away from the amp. Single coil PUP's are noisy that was the reason humbucker PUP's were invented, to buck the hum that can be picked up.
With your 100w amp, you have a bigger PT with more current than a smaller amp, so, your amp PT will put out/radiate more 'noise' to be picked up. And because your amp is a 100w it will be amplified a lot more than with a 20w amp. Yes there still is noise to signal ratio no matter what the amps power but still a 100w amp's hum will be noticed a lot when turned up. 100w & bigger PT + single coil PUP's = hum/noise.
....I'm just trying to understand the 'whys' of things.
I think I posted this link for you before;
http://www.valvewizard.co.uk/Grounding.html (http://www.valvewizard.co.uk/Grounding.html)
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Willabe,
Promise, I didn't bring up the hum issue, that was soundmasterg. I'm fine with discussing it, but my primary problem is that of the imbalance in the power section, it has to be why I'm getting such horrid sound out of the amp and the runaway pops that start happening as well. As I see it, if I really am understanding what's happening, the two right power tubes are set at a point that they're probably fine, but the left two have one that is always biased near runaway if not already in runaway, it then climbs any time too much input signal comes in, and pops as it overloads and is arcing in the tube itself? Or from one pin to another in the chassis becase it was never meant to do so.
I think if I can find out why socket 2/V6 is so high compared to the other three, I think that will resolve the majority of my problem. I just can't seem to track that down.
To reiterate, last measurements for anyone that could maybe help (staying on the primary issue):
V5, V6, V7, V8
26.4mV, 45.0mV, 12.5mV, 12.5mV
-37.0V, -37.2V, -40.5V, -40.6V
0.6ohm, 1.4ohm, 0.9ohm, 1.3ohm
0.976k, 0.978k, 1.001k, 0.981k
This is with no input into the amp, volume at 0.
why oh why would the V6 be at 45.0mV?? If I use the associated ohm readings thats the following in mA
44mA, 32mA, 13.9mA, 9.6mA
(Of course, as was stated earlier, the measurements I'm getting on those ohm's could be off due to measureing that small being hard. If not, then we just estimate they're at 1ohm and get the same mA as mV)
I also seem to remember the V5 resistor being at .8 ohms the other day, maybe those have been worked too hard with something else I did wrong in the past and are slowly dying?
Edit: SoundmasterG, that in no way is a negative comment to you, just that I've definitely led this thread all over the map, and I'm trying really hard to stay focused on what I need to do to resolve the major issue facing me. I may even start a separate thread on hum removal once I clear up the major problems here.
~Phil
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Try this, leave the PI caps un-soldered, completely unwire everything from the 4 PA tube sockets, re-wire with new wires, new R's, THEN recheck bias volts at each tube socket(at the tube side NOT under side).
leave tubes OUT till all 4 have *close* bias
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I was going to try something similar tonight, I don't have time to do it all tonight, but if what I was going to do doesn't work, I'll try that. I was going to disconnect the socket that is significantly higher, pull it out completely and try to remove any solder on the socket. Get it really clean, and test every pin with my DMM. Ensuring that every pin has no continuity to the other pins by plugging one side in with my DMM probe like a tube pin and touching everything else. Then once I"m sure that's good, I'll resolder it back in, and give it a try. I'm suspicious of that one tube that is so significantly off, and want to really give it a once over.
~Phil
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Phil,
No worries. it is good that you are trying to learn and don't take what I said too harshly. There are many problems with this amp including layout, poor connection methods at various points, etc., but as you said, you need to get the amp stable before you can do anything else. Willabe's suggestions about using shielded cable on the grid connections is very good....I made that point earlier also. Having those grid wires running next to plate and cathode wires and in parallel with other grids from other tubes is a recipe for oscillations and crosstalk. The grounding article Willabe posted is very good reading and you should really read up on that whole site as there is much useful info there. The 1 ohm resistor connections to the bus is another area that you should address throughout the amp. Any connections like that are not a good way to connect things, as Sluckey mentioned earlier about your screen grid connections. You want things connected up in a such a way that if the amp gets tossed around or dropped or thrown into a van or something that no connections or parts will move so that they could short out etc. There are books and websites out there that talk about the proper way to connect things and it may also be useful to research some of the UL requirements so you can see what regulatory agencies around the world consider appropriate construction and change your methods in the future so you can make stuff that flows with those recommendations. The hum sounds like it is mostly your guitar so I wouldn't worry about that per se. First rule of troubleshooting hum is to eliminate the guitar from the equation by unplugging it and seeing if the hum goes away. But, address the bias issues before you do anything else. You need the amp to be stable before you can continue troubleshooting.
On the power tube bias, when you are checking it, do you have a signal running through the amp? Do you have it connected to a resistor load or a speaker? All controls at zero? There should be no signal coming into the amp, all controls should be at zero, and a resistor load attached to the amp instead of a speaker. (In the absence of a resistor load you can use a speaker but the reading will change with frequency.) Then check your bias and set it to say 65% of max dissipation so the tubes will be happy. No need to run them hotter than that until you are doing the final dial in for tone after everything else is fixed on the amp. So 65% of the 25 watt max is 16.25 watts. At 470 V B+, that would be between 34mA and 35mA. The tubes should be stable and happy at that point. you should be able to fire up the amp going into a speaker with no signal going into it and have it sit there and work without the tubes running away or anything else happening. If the tubes start to get unstable on their bias, and start to runaway, then either there is something wrong with that tube, the socket, the connections inside the socket where it contacts the tube, the bias supply, etc. It may be worthwhile to take the power tubes out and adjust every socket so they all the pins tightly grip every pin on the tube when you insert the tube, just to be sure.
Greg
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Greg,
Thanks, I've already taken the sockets and tightened them up, they all feel nice and tight when inserting. (some didn't at first). When I'm checking it, I have no input into the amp and volume at 0. I've had a speaker connected for some of the biasing, but I can go back to the dummy load after I clean up the one socket that seems suspicious.
~Phil
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........ suggestions about using shielded cable on the grid connections is very good....I made that point earlier also.
Yes you did. :icon_biggrin:
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I also seem to remember the V5 resistor being at .8 ohms the other day, maybe those have been worked too hard with something else I did wrong in the past and are slowly dying?
I'd get some 1 ohm/1% R's from Doug instead of those 5%/1 ohms you have.
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They're on order now, got 20, and a tag board for the other end of that power bus, as well as a chassis for my gibson eh-185 build that's coming up soon :) (gotta keep it under the monthly budget allocated by swmbo)
~Phil
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Okay I ended up doing V6, cleaning everything up really well, removing excess solder, testing continuity on all pins and found nothing shorting and good connectivity on the pin itself, resoldering them all very carefully, and restarting it, and still the exact same imbalance issue 25mV, 50mV, 12mV, 12mV.
I then went and repeated the process for V5, removing excess solder, cleaning it up testing continuity etc, and this time it started going into runaway badly again, I couldn't get consistent measurements so I dialed down the bias more until it stopped. I then measured and it is now balanced per side, but not for both sides. The reading now is: 24mV, 24mV, 5mV, 5mV. I now think if I can take the time to clean the other two out as well, and get them setup, things may be fine. I'll report back on it when I can get it done.
I think ultimately this may yet be another side effect of me using too thin of solder at the onset. I did 'resolder' the connections with new solder, but I didn't clean up big blobs at the bottom. They may be causing some kind of bad arcing or bad connectivity, who knows. Ultimately we'll see if the same cleanup will now balance out both sides of the power tubes side.
Hope to get to that tomorrow or thurs.
Thanks to all for the help, crossing fingers this will do it.
~Phil
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Well, good news, and bad news.
I've completely redone all 4 power tubes and now I have very consistent readings across the board on the power section. I just read the mV on them and got
30.0mV, 29.8mV, 30.1mV, 29.9mV
Voltages on the other pins all seem in line.
I then put in the speaker and I've got horrid noise and distortion. Lots of clicks and pops. I'm going to have to look over the areas I desoldered for the testing, I may not have gotten those resoldered in very well, but I'm at a loss as to how I had clean tone before at low volumes, now it sounds horrible at low volume. I'm already done for now, nursing a sore back, I'll take a look at it tomorrow again, and do some checking. I tried the tube removal from V1 - V4 and the horrible static that is back disappears until the PI is back, but again, that could be between the PI and the Output side. At least I now have a stable power side.
When I play it has a lot of static and pops as well. hopefully tomorrow I can get a look at it with fresh eyes.
~Phil
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Well, good news, and bad news.
I've completely redone all 4 power tubes and now I have very consistent readings across the board on the power section. I just read the mV on them and got
30.0mV, 29.8mV, 30.1mV, 29.9mV
Voltages on the other pins all seem in line.
I then put in the speaker and I've got horrid noise and distortion. Lots of clicks and pops. I'm going to have to look over the areas I desoldered for the testing, I may not have gotten those resoldered in very well, but I'm at a loss as to how I had clean tone before at low volumes, now it sounds horrible at low volume. I'm already done for now, nursing a sore back, I'll take a look at it tomorrow again, and do some checking. I tried the tube removal from V1 - V4 and the horrible static that is back disappears until the PI is back, but again, that could be between the PI and the Output side. At least I now have a stable power side.
When I play it has a lot of static and pops as well. hopefully tomorrow I can get a look at it with fresh eyes.
~Phil
Thats good news Phil. Now that the power section is behaving, you can try more troubleshooting to see if you can pinpoint where the problems are. If you pull the PI tube and the noises disappear then the problem is before the PI circuit somewhere, not after. It could be caused by oscillations due to poor layout, or poor soldering connections. If you have any shielded cable, then now might be a good time to connect that on the grids in the preamp. If you do this, remember to only connect the shield on one side to ground....the other end of the wire should have the shield floating. You may also see if you can route the wiring going from the board to each preamp tube differently so stuff doesn't run all in parallel. Perhaps use zip ties to secure stuff in places so it will have some space between each stage and cross at 90 degree angles if you have to cross?
Greg
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Sounds like a plan, I'll give that a go.
~Phil
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Okay so tonight I just had time to resolder the areas I'd removed previously and go over a few other parts on the main board that seemed suspicious, I then turned it back on and got things going. I had to dial the bias down to about 25mV just to keep it stable, or it kept going into the clicking state (is that runaway?). Once I did that, though, it seemed to really settle down.
I could then play pretty damn well, it was clean a bit muddy/dark sounding, but all in all it actually sounded like an amp. It did do one thing that was odd. When I connected the scope probes to one half of the 47k resistors into the grids it gave me perfect output of the input, but the other side, as soon as I'd connect the scope would start screeching like just introducing the scope caused some massive feedback.
What would cause that? Is it yet another soldered connection somewhere that's not working right?
I think the bulk of the problems on this build may be chalked up to just using too thin of solder honestly! /sigh. If anyone has any ideas I'd be happy to hear them. If not, I'll poke at it more tomorrow or Sat, got caught up in other things after only an hour or so of work on it tonight.
~Phil
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These amps can be a bit muddy sounding sometimes...they don't have the crisp upper frequency clean like an AC30 or a blackface Fender...that is just the nature of the beast. They do give a very touch responsive and harmonically rich sound though which many other amps do not. I added a cut control to mine to be able to crisp up the top end and I like the addition.
It is strange to me that you have to keep dialing back on the bias to get the amp to behave....I am still suspicious of those power tubes myself due to what was happening to them previously when they would run away.
I think the scope probe thing could be the fact that they can act as an antenna of sorts when you connect them up to a grid. I didn't have that problem when I was messing around with mine specifically but every amp and layout is unique.
Greg
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Yeah I realized its basically the normal channel on the vox, which is also not so clean. I may just add a 'top boost' switch in it so that it alters that cap from the .047uF to the 500pf one in the top boost channel on the AC30. (edit: Seems like I missed the additional 100pF cap on the volume pot itself... does that being on the volume pot make the extra 100pF dynamic where whatever portion of the sound goes through that works like a variable tone control?, to make that switchable, do I need to also use a dpdt switch and add a second line for that cap in the path with the volume?)
That's pretty much the majority of the difference that I can see, and then its switchable.
Sound like a logical option?
As for the tubes, are you saying you think they've been damaged due to previous runaway conditions and therefore go into runaway sooner?
~Phil
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Yeah I realized its basically the normal channel on the vox, which is also not so clean. I may just add a 'top boost' switch in it so that it alters that cap from the .047uF to the 500pf one in the top boost channel on the AC30. (edit: Seems like I missed the additional 100pF cap on the volume pot itself... does that being on the volume pot make the extra 100pF dynamic where whatever portion of the sound goes through that works like a variable tone control?, to make that switchable, do I need to also use a dpdt switch and add a second line for that cap in the path with the volume?)
That's pretty much the majority of the difference that I can see, and then its switchable.
Sound like a logical option?
As for the tubes, are you saying you think they've been damaged due to previous runaway conditions and therefore go into runaway sooner?
~Phil
If you forgot the cap across the volume control then that would do a lot to affect the tone at any volumes lower than max and adding it in there will make the amp have better top end for sure. I would just add it rather than making it switchable as it sounds a lot better with it in place. The stock AC100 only has one channel and it isn't really the same as the normal channel from an AC30 as it has more gain stages and more tone controls and uses different tubes.
Yes on the tubes. Since you were running them in a condition where they kept going into runaway, it couldn't have been good for them. I had that happen on my AC100 project with JJ KT77's because the screen grid resistors I was using were undersized and they were cooking themselves and then the tubes would start to act funny. I also had trouble with the fixed bias and my switching and getting the fixed bias mode to work correctly. So basically the tubes got really cooked once and after that the whole quad didn't want to behave. I could try one of the pairs and it would be ok but as soon as I tried the quad it would want to go out of control eventually. My tube tester showed that they were still good, but their performance parameters had changed from when they were new by quite a bit. I had to run the negative bias voltage a lot higher (current a lot lower) to get them to behave and they weren't happy at all in the cathode bias mode even with larger cathode bias resistors that made the tubes cooler running. I replaced them with a new quad and fixed the issues that were causing the tubes to have problems and it worked fine after that. So if it was me I would be suspicious of those tubes myself.
Greg
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Ok I had a buddy over today to fix his amp and in going to buy a few caps I got myself the 500 and 100pf and replaced the one, and put the 100 on the volume pot. Major difference in tone, and I really like it.
So I didn't have much time to do more than that, being that I was helping my friend get his amps tuned up (peavey and an electar, both resounding successes, filter caps going out on the peavey and I just changed a 3.3k cathode resistor on an 12AX7 on the electar to a 1.5 k to bias it for more tone overdrive, it sounded great)
I'll work on it some more tomorrow. I did get one more bit of testing done, though. The still annoying clicks and pops happen every time with 4 tubes. As soon as I get down to two it sounds amazing. This has happened though, with EVERY set of tubes I've ever put in it, even these brand new ones do it. Why? If I bias them really hot, like 25mV or lower, they do okay but can still climb out of control if I'm not careful. If I bias them too cold, nearing the full 70% it can barely go over 3 or 4 out of 10 without going nuts. Why does the amp sound so great with only 2 tubes? It can literally be any two tubes, inside two, outside two, and any of two of the four. I just can't get the full 4 to settle down.
Any pointers are greatly appreciated. I can't see myself buying another 4 power tubes after now going through a total of 10. (first four were doing the same, so I pulled two and it seemed good, we figure two tubes were dead so I bought two more, the other two, one had a short on pins 2/3 and blew the heaters on all the other tubes.)
~Phil
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can still climb out of control
I'm not well versed in P-P but my understanding is the bias supply, if correct should be quite stable. Maybe do the same thing there you did with the sockets, rebuild it?
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The clicks and pops to me sounds like loose connections somewhere, or oscillations, or both. Oscillations can be due to poor layout...things like running lots of wires in parallel for example. Essentially you end up with an invisible circuit as the signal might go through the first stage, get amplified, then go to the second stage and get amplified, but it also gets picked up by the first stage and amplified again which sends something out of control. That is just an example but you get the idea. Doing your layout in such a way so you avoid any chance of this is important. The AC100 stock circuit is pretty low gain so you can get away with more errors than on a higher gain amp, but the layout could still be contributing to some of the problems you are having. Isolating the grid connections with shielded cable might help, but you don't know until you try it. Same with re-routing wires o there aren't so many in parallel with each other between the board and preamp tube sockets. For loose connections you just need to look at every single solder joint and connection. The problem could be as simple as one connection that looks ok but isn't. Using a chopstick to poke around at connections while the amp is idling with no signal into a speaker can help to try to find where a poor connection is.
I am not sure what is going on with your tubes not wanting to be stable. I thought though that you were able to back off on the bias after resoldering the sockets and it was stable at a lower bias now? There is no reason to run the bias hot if it sounds good at a cooler bias. The cooler the bias the longer the tubes will last. Mine sounded pretty good at about 65% of max dissipation when in fixed bias mode. Cathode bias mode runs it more like 80%, and that is with cooler running cathode bias resistors than stock, but mine was stable there also once I got the screen resistor wattage sorted out.
Greg
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Greg,
Thanks, that makes sense, I'm hoping to have some time to use the shielded wires for the remaining preamp grid connections. I hope that helps.
I did have it pretty stable the other day, and then turning it on yesterday it started doing it again. That's another part of it that seems the most frustrating, it can be very periodic.
I'll post back results once I've done the shielded grids and have tried to separate the leads per section a bit more.
Shooter. Sounds like a good idea, I've gone over a lot in the other areas, but don't know that I've really gone over the bias section.
Question. When I setup the pot for the bias, the left side is just hanging because as I understood it, it wasn't needed, I'm using it like a variable resistor. Should I tie that loose end down to something? Could that be related?
~Phil
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Okay, so today I did two major changes:
1. I ran sheilded wire from every grid on every preamp tube.
2. I resoldered the connections around the bias circuit.
While running in two tube mode, the tone is still outstanding, no problems, everything just works great. It is also now almost completely dead quiet.
When I put in the other two tubes, it is a LOT better, it seems to be able to handle more volume before the pops and they come much less often. Nevertheless, it still happens. I think I can get the volume up to about 4 or 5 of 10 and play softly without any problems, if my pick attack is just a tad too hard it causes a pop and the same behavior. If I play nothing, and get the volume all the way up to near 10, it starts going nuts with the pops and i have to back off really fast.
Any thoughts on next steps?
I've also noted one other weird thing. The main volume pot seems to have a weird situation at 0 where it actually has some volume coming through, and when I dial it just a tiny bit towards maybe 0.4 it goes almost silent and then starts climbing again... is the pot off a bit or something? Anyone seen that behavior before?
~Phil
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Is this a head and speaker cab? Do you have the head on top of the speaker cab?
If so take it off the cab so the speakers don't shake the head, better?
The main volume pot seems to have a weird situation at 0 where it actually has some volume coming through, and when I dial it just a tiny bit towards maybe 0.4 it goes almost silent and then starts climbing again... is the pot off a bit or something? Anyone seen that behavior before?
Yes, guys have had that happen, it's the pot's wiper/R track. Least of your worries, you have much bigger fish to fry.
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No the amp is still up on my workbench, the speakers are next to it a foot or so away. no direct physical contact.
I'm pretty sure the issue isn't with the speakers, I'd get this popping visible in the scope with a dummy load connected as well.
~Phil
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get this popping visible in the scope with a dummy load
so where does it start, before V1, after PI?
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I don't know to find the exact source. I remove any tubes in the preamp side and it is gone. I think whatever it is, it needs the complete input signal to work. (I'll test this again, but so far as I recall I can't get it to pop unless I've got at least all 4 power tubes in, and all preamp tubes in. I'm not 100% sure that means 'its the power section' but it seems to indicate something like that's related, but not sure.
~Phil
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now that your bias is stable (it is stable with all tubes?) just hook up a dummyload, stick a signal at the input, I'd start at the PA tubes grids and walk back, I'd also be chopsticking everything as you go
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I'll give that a try again, I swear I've done this and I can get the click/pop everywhere? Not sure I can recall now, it's been a week since I did that last. I did tap everywhere with a chopstick attached and got no real noise of any kind.
It almost seems like an issue with the whole circuit overloading and causing the pop. The only thing I can always see that is indicative of the issue is that if I keep my DMM connected over the 1ohm resistor measuring the tube current it always does this when it pops. The reading will vary maybe between 10mV and maybe 40 or 50mV when it is working fine, when the 'pop' happens I always note the mV reading climbs rapidly up to about 100 or 110mV and pops, and then goes down to 0mV and climbs back up to the stable point again.
And yes the 4 power tubes are still now very well balanced and giving me almost equal current readings across the board.
One other thing a coworker (who is going off of electronics knowledge, not tube amp knowledge) said was to check and see if it could be the input filtering. All but two of the input stage filter caps are originals from the amp and its an early nineties model if I recall, but I used my esr meter and they all showed as good. I also tested their capacitance before putting them in, and that came out fine. Could the filter caps be doing this if they're dried out and being pushed a bit hard?
~Phil
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With the issues you've had, especially the plate to heater short in the EL34, it's possible that the OT has been damaged from over current. Let's hope not, but keep that thought in your head.
You still have not done anything to eliminate the OT as a suspect. There is a method to check transformers for shorted windings (see attachment) but I don't trust it to find all problems. The only sure way to test an OT that only shows problems under high current conditions (ie, 4 output tubes v. 2 output tubes) is substitution. I'm not gonna tell you to buy an OT, and I'm not gonna guarantee that's the problem, but it has to be eliminated. I will say that if it were my amp and I had your determination, I would. I know this project has already dipped deeply into your pockets, so it's your call. This is all based on the notion that you have wired the amp correctly and have made safe/solid connections.
I also think that whenever the power problem is resolved, you'll likely be fighting a different group of gremlins associated with that preamp layout.
For the other thread, just put a slow blow fuse in the mains fuseholder. Quit trying to figure out why Vox used a 3 amp fuse in the original and a 6 amp fuse in the newer amp. These two amps are very different and have different power requirements, not to mention 50 years difference in the age of the engineers. Think about all the changes you had to make just to be able to adapt the new PT to the old power supply.
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You have oscillations or loose connections somewhere. The loose connections could be the OT also in that internally it could be arcing. I would check all connections in the amp with a chopstick while the amp is running and connected to a speaker so you can hear what it does as you hit and move parts. Also run it while connected to a dummy load and on the scope with a signal and make sure the signal can go from stage to stage showing amplification without any weird artifacts on the scope. You should see a clean sine wave that starts to square out on the tops and bottoms, but not small mini wave forms in weird shapes on top and bottom or sides of the sine wave. You should be able to see the tone controls modify the signal also as you move the controls as long as your scope probe is connected after the controls anyway. The fact that the shielded cables reduced some of the issues hints to layout issues and possible oscillations. If after you have checked all this stuff and you are still having issues, then you may look into the OT. If you get a replacement Heyboer makes a real AC100 clone output transformer that is very good, though for testing you could use any 100 watt transformer that works with El34's. Come to think of it I have the original OT from my Sovtek Mig 100 U still around. It works perfectly....and I was going to sell it but hadn't gotten around to it yet. I could make you a deal if you wanted it though. PM me for details.
Greg
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I may do so if I can't get this sorted.
I've done the chopstick test yesterday, per your recommendation, I didn't find any noises at all.
So for the scope test, I'm guessing I want a pretty low output to ensure I'm not seeing any of the pops and then look for oddities? Is that what you mean? I am guessing I don't want it to keep popping as that may end up doing some damage. Every time I do get a few pops I've always turned down, stopped etc to limit risk.
I may do a short video of me doing the scope part and link here so you can give me ideas on what is being seen.
~Phil
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Sluckey,
Sorry I didn't see your response. Yeah, I'm going to try anything else I can before I try the transformer route, but I may just buy the one SoundmasterG has to elimitate that as an option if nothing else works.
So if there was a bad part of the OT, why would 2 tubes not trigger the short? are shorts like that sometimes based upon hitting the right amount of current to make them arc and if you're under that, they don't?
~Phil
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I may do so if I can't get this sorted.
I've done the chopstick test yesterday, per your recommendation, I didn't find any noises at all.
So for the scope test, I'm guessing I want a pretty low output to ensure I'm not seeing any of the pops and then look for oddities? Is that what you mean? I am guessing I don't want it to keep popping as that may end up doing some damage. Every time I do get a few pops I've always turned down, stopped etc to limit risk.
I may do a short video of me doing the scope part and link here so you can give me ideas on what is being seen.
~Phil
You can see oscillations and pops on a scope most of the time so you can run the amp on a dummy load to do troubleshooting rather than running it on a speaker all the time. Then you can put it at whatever volume you need to duplicate the problem. Does the popping happen at lower volume? With signal on the input of the amp? Without signal? Never with two power tubes but it does with four? Never with the phase inverter tube out but it does it with that tube in? Ask yourself some of these questions. The whole idea is to weed out the possibilities until you end up with what is causing the problems.
When I build an amp, I bring it up on a dummy load and a variac and current limiter with no power tubes from the beginning. If it is stable then I go to full power on the variac and set the negative bias voltage into the grid. Then I add the power tubes and bring it up on the variac to make sure the amp behaves with tubes in it. If so, then I run it at full power again and set the bias current. At this point the amp will behave so I can do more setup, so I will put a signal in and make sure each stage passes signal the way it should and all controls work. Once I have determined all of that, then and only then do I plug it into a speaker and try to play it. Usually there is some more adjustment needed to get each stage to behave the way I want as far as how each stage overdrives, etc., but then most of the amps I build are my own designs so I can't necessarily count on some pre-existing schematic that is known to work. In this case since you are building a known circuit, you should be able to get it to behave as it worked in the past with the Vox circuit on those original amps. It sounds like in your case you bypassed some of those steps and went right into trying to play the amp and when it didn't work right, you have been chasing your tail to try to sort out all the issues. The approach I mentioned is more meticulous but in the end it takes less time to set up a new amp. I think you are learning that is the case also. :)
Greg
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Well, actually, no, I did do what you are saying, but I am learning due to some of this process anyway.
I found only on the scope test phase, and things seemed odd. At that time it was recommended to me to put a guitar in and put the speaker and see how it sounds.
Before that, I was following all of those steps. i.e. brought up voltage on the variac, found an under specced resistor that started smoking early, then fixed that, brought up the amp again on variac, voltages all seemed stable, added the tubes, seemed stable, hooked up sine wave to the input, and at that point I saw odd behavior.
I was asking at that time what would be best next steps, and was told to plug in a guitar and verify that it is indeed doing so. That's what I did.
There has been a massive amount of back and forth by multiple people all helping. (which is greatly appreciated), But with each one it seems to help me narrow it down a bit further, but also adds to the overall confusion of the thread for others to understand what's been done.
I've just had to repeat some steps a lot to confirm things because this thread has been so hairy and long running. I forget what I did and what I got. (I really need to start keeping a journal with notes on everything so I can go back and say "Yes I did this, and here was the results" etc.
I'm learning a massive amount with it, and am happy this forum exists to teach me this kind of thing, but I know this thread is becoming epically painful for everyone that's trying to help... :cussing:
~Phil
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Its ok Phil. It is interesting for many of us to try to help solve the issues you are having. Hopefully eventually you will be able to get it working the way it should. Have you made any more progress in the last several days?
Greg
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Nah, its been a pretty busy week with other things going on. I've been mostly responding to this thread during down time at work, not at home. Today I'm going to film a video of the amp working with two tubes, and then not working well with all 4 and use a higher quality mic so I can get it sounding right for people here, and to put on my video series. I hope to do some of the tinkering this weekend as well, though to try some of what's been suggested here and see if I can isolate it. I'm tempted also to restore the reverb with the 2 output tubes working and see if that's fine. If not, maybe, even though it's not 'in circuit' per-se, I may still be allowing something to leak through that circuit that is causing the issues, not sure. I think the only way I could really remove it would be to desolder all the components or at least a few of them that are directly next to the existing one. I've not gone to that level of troubleshooting, I just bypassed the 100k with a soldered jumper, and removed the transformer connections and tube connections. (I don't know enough yet to say if that is even remotely related, but it is worth bringing it back into the working circuit and see if it sounds fine then. But I don't want to do that until I've tried the other things here as well.)
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Okay, I'm going to be doing some testing this afternoon, but I recorded some audio. I don't know if you'll be able to hear my voice, but about 1:37 into the video I switch from 2 tubes 50W to 4 tubes 100W and you'll hear the difference.
Not sure what's going on exactly, but the horrible sound will be obvious.
https://soundcloud.com/pompeiisneaks/test2-4tubes (https://soundcloud.com/pompeiisneaks/test2-4tubes)
~Phil
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Found out something quite telling. Maybe someone can help me figure out what it means. I think I mentioned that if I put my oscilloscope probe on one side of the PI output (the first stage) it is clean and everything looks nice. As soon as I clip it on the second half of the PI it causes massive distortion and freakout of the scope signal. I thought I should step backwards and see what happens. If I clip it to the other side of the capacitor, right after the tube anode but before the cap, it is super clean again. That is to say, if you look at the schematic image I'm attaching Point A is clean Point B goes nuts. Is this something leaking in from either the bias or the other side? Or is the cap itself shot?
~Phil
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Point A is clean Point B goes nuts
Point A and everything left is probably good, which leaves the PA circuit, which has been built n re-built except like Sluckey suggested, the OT.
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Listening to your sound clips, even with two tubes in the power amp, and when you turn it up, all that hash (ugly clip and fuzz, sounding like a ring modulator) on the notes are oscillations. You can use a signal generator with the amp connected to a speaker and listen to what level and frequency it causes some distortion to appear, and then connect up to a scope and signal generator, connected up to a dummy load so you don't have to hear everything, and look at where in the circuits the weird grit is adding into the sound. You will see it on top of the waveform as a little artifact or squiggle that makes the wave be something other than a sine wave or slightly squaring off the edges. An AC100 should be clean all the way up the volume with just a slight bit of overdrive near the end of the range of the volume control...maybe like from 7-8 and up. Everything below that should be really clean.
I assume you are still using the stock 12AU7's? Verify that you do indeed have the correct value cathode bias resistor and plate resistor values for everything in the preamp and phase inverter. It sounds like some of those stages are clipping before they should....in other words the signal coming from one stage is at a certain level, but the following stage is biased into cutoff or grid current limiting so part of the sound coming out of that stage is distorted during the waveform. It is either a biasing issue in the preamp/phase inverter, or due to the layout some of these stages are picking up signal that they shouldn't and are amplifying things they shouldn't. The fact that you changed to shielded grid wires and it helped is an indication that you could have some oscillations going on. The popping.....I'm not sure about that. I don't think it is an OT issue though because it would do it with just two power tubes also if the OT was bad.
I've had project amps make the ring modulator gritty hash noises like yours is doing and it was always oscillations of some sort. You should be able to see them on the scope. Turn the volume up and down while monitoring in different places and you should be able to see it on the scope.
Greg
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Point A is clean Point B goes nuts
Point A and everything left is probably good, which leaves the PA circuit, which has been built n re-built except like Sluckey suggested, the OT.
But how would the OT impact the amp directly AFTER the capacitor, wouldn't it cause the problem to happen pretty much at the OT itself? It seems to me like either:
1. the cap itself has something wrong, so that when the audio passes through it, it gets really messed up
2. there is something between the PI and the PA section that is bleeding in something nasty.
Why else would the scope show such clean signal until I clip it past that capacitor and then it sends ALL the signals before it into haywire? it makes the entire scope go berserk.
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Listening to your sound clips, even with two tubes in the power amp, and when you turn it up, all that hash (ugly clip and fuzz, sounding like a ring modulator) on the notes are oscillations. You can use a signal generator with the amp connected to a speaker and listen to what level and frequency it causes some distortion to appear, and then connect up to a scope and signal generator, connected up to a dummy load so you don't have to hear everything, and look at where in the circuits the weird grit is adding into the sound. You will see it on top of the waveform as a little artifact or squiggle that makes the wave be something other than a sine wave or slightly squaring off the edges. An AC100 should be clean all the way up the volume with just a slight bit of overdrive near the end of the range of the volume control...maybe like from 7-8 and up. Everything below that should be really clean.
I assume you are still using the stock 12AU7's? Verify that you do indeed have the correct value cathode bias resistor and plate resistor values for everything in the preamp and phase inverter. It sounds like some of those stages are clipping before they should....in other words the signal coming from one stage is at a certain level, but the following stage is biased into cutoff or grid current limiting so part of the sound coming out of that stage is distorted during the waveform. It is either a biasing issue in the preamp/phase inverter, or due to the layout some of these stages are picking up signal that they shouldn't and are amplifying things they shouldn't. The fact that you changed to shielded grid wires and it helped is an indication that you could have some oscillations going on. The popping.....I'm not sure about that. I don't think it is an OT issue though because it would do it with just two power tubes also if the OT was bad.
I've had project amps make the ring modulator gritty hash noises like yours is doing and it was always oscillations of some sort. You should be able to see them on the scope. Turn the volume up and down while monitoring in different places and you should be able to see it on the scope.
Greg
Well what I don't get, including what I just said to Shooter's point, though, is that I AM using my scope and when I probe anywhere right up to that cap, it is a very clean sine wave. As soon as I clip beyond it, it goes haywire. I'm going to upload a full video of this tonight, and I'll link it here as well as the video section thread I have on it. That way maybe people will understand what I mean by the before/after the actual cap. I'm not even past the turret points it is connected to, just one side of the connection, clean, other it goes to hell.
~Phil
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Okay here's the video. I'll post it over to the video section like normal as well, but it specifically relates to what's my latest problem here:
https://youtu.be/gzv4JObPVoA (https://youtu.be/gzv4JObPVoA)
I hope this helps clarify what I mean.
Please do let me know if you see anything obvious.
I really appreciate everyone's help!
~Phil
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I really, really, really hope this isn't it. I just was reviewing the schemating for the umpteenth time, and realized I don't have the 4x 100 ohm resistors on the Power tube anodes.
I seem maybe to think someone told me they're not necessary, and I can't find another schematic with them, but I'm also having a hell of a time figuring out what wattage they'd need to be? I am trying via two ways:
1. In theory the max wattage they should push out is 25 watts (for 100W output). But the biggest resistors I can find locally are only 25 watts and I think you always want to oversize them right? Or is that wrong?
2. If I use Ohms law, I'm in serious trouble. 470VDC through a 100 ohm resistor means I need a 470 watt one???? That seems nuts. (ac volts are only about 3).
so A: could these missing resistors be causing all these issues?
and B: what wattage should they be?
finally C: Do I really need them?
If that's unrelated, let me know, not trying to derail the focus on the odd behavior here, but maybe this IS the cause of the odd behavior?
~Phil
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A 3 watt resistor will be fine.
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Okay I'm going to go get those put in.
So the most confusing thing on that, for me, is how 3 watt resistors can work there? Its at 470V ... Ohms law for current is V/R which is .47A Multiply Amps times Volts again for watts... equals 470 watts?
How did you calculate 3 watts?
I'll go buy some locally really quickly, get them in, and see what happens.
Why does this amp use them when so many I look at don't? Is the internal resistance of a transformer just not enough in this case due to the higher operating voltages?
Thanks,
~Phil
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A 3 watt resistor will be fine.
On my build I ended up having to use 5 watt resistors as the 3 watt ones were burning up over time. I used some metal oxide or metal film ones.
I'll take a look at your video in a bit Phil and let you know what I think after watching it. I am trying to remember what effect having those plate resistors on the power tubes does, but at the moment I don't remember. You might as well add them though and see if there is a difference. It could reduce the problems you are having and stabilize things with the bias perhaps.
Greg
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You're using the wrong info. There ain't 470V across that resistor. We don't even need to know how much voltage is across it. We want to know how much current is flowing thru it. That current will be slightly less than the current flowing thru your 1Ω cathode resistor. That should be less than 50mA at idle, probably never more than 100mA when driven hard. So, using 100mafor the current the power will be .1 x .1 x 100 = 1W. (Power = I2R.) Double that for safety and a 2 watt resistor should work, but I opted for 3 watt in metal oxide because they are flame proof. A 5 watt cement resistor like you have on the screens would be another good choice.
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Cool, thanks sluckey. that actually makes sense. I think in other cases I'm looking where a resistor went to ground so 100% of the voltage was dropping over it. In this case that is the 'theoretical maximum' that is impossible because the tube only allows that much current. Sounds perfect.
Greg,
I'm hoping that maybe part of this is that the power stage is just being able to go too hard with that much voltage, and the resistors will throttle it a bit, and keep it out of Power Amp distortion. Not sure, that's just the theory that hit me when I thought about it.
If this is it, I'm such a noob. I swear I went over this like 5 times. BUT If I look back at my layout, it does NOT have those on it, only the schematic, so likely my error was in the translation from schematic to layout.
I went to vetco a local electronics place to get it quickly, and I found they only had 200 ohm resistors, so I'm doing two in parallel to get 100. That also means, though, that I'm getting 6 watts right? If you do two in parallel it halves the resistance, but sums up their wattage as they roughly take half of the current?
~Phil
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Phil,
Those plate resistors on the power tubes do serve to limit current that the power tubes see so putting some in place may help matters. You'll find out soon enough if that is the answer or not! I still think that some of the layout issues in the preamp are contributing to the oscillations you are experiencing.
If you put two 3 watt resistors in parallel then each resistor is still 3 watts but the total the circuit sees is 6 watts.
Greg
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OMG!!!!
it sounds INCREDIBLE. That was the last bit. I'm sure everything I've been told to do by everyone here has helped, but MAN that was the last piece of the puzzle. I've got video of the power on, and first play with crappy video mic, then we decided to put the nice mic in and give it a real go.
I'll probably be uploading that video in a few days to a week, but the only final steps are to restore the reverb circuit and make sure that sounds good. If so, I'm ready to put it in the chassis I'm building for it :)
YAY.
Thank you,
THANK YOU,
THANK YOU ALL :)
:worthy1:
:icon_biggrin:
:laugh:
:l2:
This group is so full of smarty amp gurus I'd never have done it without you
Did I say thanks?
~Phil
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I should add, that I'm still smacking myself for such an obvious miss. I feel like a complete moron at this point. Don't know how I missed it, but at least I eventually found it. :cussing: :cussing:
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Congratulations! It's about time for some joy. You have been very determined and it's finally paid off. I had noticed the resistors were missing but since the newer circuit did not use them, I dismissed the idea and never suspected them. You deserve an attaboy! Well done.
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Congratulations! It's about time for some joy. You have been very determined and it's finally paid off. I had noticed the resistors were missing but since the newer circuit did not use them, I dismissed the idea and never suspected them. You deserve an attaboy! Well done.
Coming from you, that is indeed, high praise. Thanks so much for your help (You too Willabe, PRR, Greg, shooter and anyone else I've forgotten.) I'll report back after the reverb circuit is back in.
Cheers!
~Phil
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Well son of a gun, plate stoppers fixed it. :think1: :icon_biggrin:
Those plate resistors on the power tubes do serve to limit current that the power tubes see so putting some in place may help matters.
They also act like grid stoppers/miller effect IF you put them very close to the tube pin. (Where did you mount those R's, how close/far away from the plate pin?)
Seems like Vox had a 'bug a boo' with that amp? Was it their layout? (You didn't use their layout.) Was it they had the power amp on the edge? Maybe a little of both?
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Well son of a gun, plate stoppers fixed it. :think1: :icon_biggrin:
Those plate resistors on the power tubes do serve to limit current that the power tubes see so putting some in place may help matters.
They also act like grid stoppers/miller effect IF you put them very close to the tube pin. (Where did you mount those R's, how close/far away from the plate pin?)
Seems like Vox had a 'bug a boo' with that amp? Was it their layout? (You didn't use their layout.) Was it they had the power amp on the edge? Maybe a little of both?
JMI Vox actually contracted out for that amp to one of their assemblers/suppliers and that company designed the amp to their direction that they wanted 100 clean watts and similar to the AC30. I think it might have been Triumph Elections, though I would have to look in the JMI Vox book by Jim Elyea to be sure. The amp came out in cathode bias version (which was 80 watts RMS) in 1963 so they were the first of the big loud amps among the big guitar amp companies. They also used a head box that was way too small with too little ventilation and the amps liked to burst into flames!
Congratulations Phil! Hopefully that is indeed the fix and you are on your way to glory! Give us some soundclips or videos when it is all done.
Greg
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I've connected them right on the pin itself, then to the wires that lead to the OT, and shrink-wrapped them.
Greg, yeah I've read they were designed initially for the Beatles, and they had a lot of problems with the Mark I, so the Mark II is the one I based it off of (not the ac80/100 but the AC100/2).
Edit: yes I did record a video of the discovery, and fix, and the resulting quality. It is clean up til the volume hurts, and only then starts to overdrive a bit. When I put it at max volume I was stunned at how loud it was, and it was really crunchy. Excellent tone.
~Phil
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I've connected them right on the pin itself, then to the wires that lead to the OT, and shrink-wrapped them.
Greg, yeah I've read they were designed initially for the Beatles, and they had a lot of problems with the Mark I, so the Mark II is the one I based it off of (not the ac80/100 but the AC100/2).
Edit: yes I did record a video of the discovery, and fix, and the resulting quality. It is clean up til the volume hurts, and only then starts to overdrive a bit. When I put it at max volume I was stunned at how loud it was, and it was really crunchy. Excellent tone.
~Phil
Phil, it sounds like you connected them well. It will be cool to hear the finished project.
Yeah the big bands like the Beatles and Stones used the AC100. The Troggs used them too. Beatles used them for bass for A Hard Day's Night, and for bass and guitars for Beatles For Sale, Help, and Rubber Soul. Beatles used the early cathode bias 80/100 version for most of their time, though I think there was one 100/2 floating around there that all three used at various points. With my project I made an amp that is switchable between cathode bias and fixed bias, and went for somewhere in the middle of the two for preamp and phase inverter voltages. There isn't a whole lot of difference in sound from what I can hear between the modes and the cathode bias version runs a LOT hotter. If I was to do my project again I would just build the fixed bias version. All the AC100's are clean and loud almost all the way up with a tad bit of crunch at the top, but overall very touch responsive and harmonically rich. They are very loud too as you have noticed! :) Mine will be getting power scaling, since I added a second higher gain channel to it and that channel can benefit from the power scaling.
Greg
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Cool, thanks for the info. Yeah I also did the top boost mod to mine because I wasn't a fan of the mid tone, (it sounds a bit muddy without it). Otherwise you're right, it barely starts over-driving until I get to almost max level and then its LOUD!!!
~Phil
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Glad you're a happy camper!
put some earplugs in and annoy the neighbors :l2:
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Yeah I think that first try I did, my wife is helping me film the video's and she has on a pair of noise cancelling headphones, and was like NOOOOOO :D.
~Phil
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Okay, got the reverb back in and it also sounds great, if not a 'tiny' bit soft. If I have it at max you can definitely hear it, but for the amount of reverb I like its just a touch light even then :) But I am a bit of a reverb head, so I think its fine for now.
I've got a video filmed and being edited with the final tone, minus reverb, I guess I'm going to have to put just a tiny bit of reverb on it as well for the audio clips.
~Phil
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Okay, got the reverb back in and it also sounds great, if not a 'tiny' bit soft. If I have it at max you can definitely hear it, but for the amount of reverb I like its just a touch light even then :) But I am a bit of a reverb head, so I think its fine for now.
I've got a video filmed and being edited with the final tone, minus reverb, I guess I'm going to have to put just a tiny bit of reverb on it as well for the audio clips.
~Phil
Is the last schematic you posted accurate of how the amp is now? It will be helpful to others if they want to see what you did and where you ended up at. Looking forward to seeing the video!
Greg
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It is very close, but I do need to upload my latest. (That has the top boost changes, but as far as I can recall that is the 'only' difference from my last upload...)
I'll post the updated schematic at the first post and remove the older ones so it reflects what's up. (Unless people like the historic ones for reference?)
~Phil
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It may cause confusion as you were working from an old schematic there and if someone reads the posts they may get confused. I'd post it here at the end but its up to you.
greg
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For completeness here, I'll also upload the latest schematic and layout. I added the mod to it that I did for top boost.
~Phil
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And the reason for the last update, is that the video is here with the audio of the finished amp.
~Phil
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Thanks for adding those at the end Phil.
The amp sounds good though when you turn it all the way up it sounds like there might be a bit more distortion than usual for this circuit, or perhaps a bit of fizz going on...but maybe that is just the microphone you used being overloaded?
Its been an interesting project I'd say and hopefully you learned some things that you can take away from it?
Greg
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Greg,
Absolutely, I learned a lot from the project, and am excited for the next! :)
As for the distortion at max volume, I had the microphone set too hot, I was trying to get it good enough for the lower volume stuff, and then when I played it at max volume it kept clipping, the noise is from the mic/recording. The tone has only a nice overdrive sort of crunch to it, no distortion that I can speak of. My wife was there helping me film and she also agreed it was nice and overdriven, but not distorted like in the video. Also the reverb sounds obvious but soft, but in the video/recording, I think the levels were too low, so it ended up a bit hard to hear.
~Phil
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Greg,
Absolutely, I learned a lot from the project, and am excited for the next! :)
As for the distortion at max volume, I had the microphone set too hot, I was trying to get it good enough for the lower volume stuff, and then when I played it at max volume it kept clipping, the noise is from the mic/recording. The tone has only a nice overdrive sort of crunch to it, no distortion that I can speak of. My wife was there helping me film and she also agreed it was nice and overdriven, but not distorted like in the video. Also the reverb sounds obvious but soft, but in the video/recording, I think the levels were too low, so it ended up a bit hard to hear.
~Phil
Hi Phil,
That makes sense and is what I thought might be happening. So now that you have it working correctly are you going to stick with the original 12AU7 input stage and phase inverter or will you use a 12AX7 or something else for more gain?
I like the sound of the original circuit myself, and just added a second higher gain channel in my amp and that approach works well. I don't have reverb added to mine....I think it sounds fine without, and I have a Fender reverb amp to use if I want reverb anyway. My project is sidelined for awhile still though but I hope to get back to it and finish it sooner rather than later.
Cheers!
Greg
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I actually didn't try going back to the 12AX7 for any of hte PI or first tube, but I don't necessarily need the OD, I am likely to sell this amp as I don't have need for a 100 watter. (I was trying to repair and re-sell the original AC100CPH but it was so badly damaged, I gave up :D)
I don't have space to start collecting all the amps I want to build :) Since I'm about to start another build I'm going to have to make space for it.
~Phil
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I actually didn't try going back to the 12AX7 for any of hte PI or first tube, but I don't necessarily need the OD, I am likely to sell this amp as I don't have need for a 100 watter. (I was trying to repair and re-sell the original AC100CPH but it was so badly damaged, I gave up :D)
I don't have space to start collecting all the amps I want to build :) Since I'm about to start another build I'm going to have to make space for it.
~Phil
Yeah 100 watts is a bit much for guitar unless you add power scaling, and with the AC100 being so clean there wouldn't be much point in adding power scaling to it, so it ends up being just a very loud, harmonically rich amp. Hopefully someone buys it when you go to sell it.
Greg