Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: tubenit on November 15, 2010, 10:39:25 am
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Guys,
Despite this issue, this is one of the sweetest sounding and transparent touch sensitive amps that I've played.
I think this is the first amp that had hum in yrs that I couldn't resolve.
I had quadruple checked the grounds and reflowed solder joints multiple times.
I'm using shielded wiring on long runs.
I tried tying the heater 100R resistors to the power tube cathode. No change.
I've replaced a cap that I thought might be a problem. No change.
I replaced the reverb 1M pot.
Here are the symptoms;
I can remove the reverb tube and disconnect the reverb tank and the reverb pot still makes the amp have less hum when the reverb is full on. Dialed to 10.
IF I crank all the pots to about 7-8, the amp will start to vibrate and have a slight oscillation. When I say vibrate, I mean with my hand on the chassis or cab ........all of it physically vibrates & there is a slight oscillation and hum.
I can bypass the OD stage and it still does the same thing on the clean channel.
I've tried replacing all the tubes with others and no difference. Another speaker makes no difference.
At volumes that I normally play the amp, the amp hum is noticeable but only slight. Turning up the volumes on the clean and master volume increase the hum/vibration.
I've noticed turning the treble down on the clean tone knob lessens the hum. Increasing the treble on the OD tone lessens the hum.
I'm stumped. What would you guys suggest trying at this point?
Could the OT wires being reversed do this when there is no NFB?
With respect, Tubenit
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I'd check the PT. What you describe could be a defective assy, with shorted lams, and/or a poor mechanical alignment.
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When grounding problems get tough, here is what I do:
(1) is it really hum? or is it a noise? If its hum, is it 60Hz or 120Hz ? This will help to narrow it down.
(2) Try connecting a DC power supply or a bunch of batteries to the heaters - does it go away? This will help to narrow it down.
(3) I have had huge problems with hum getting into the sensitive reverb grounds. Make sure the cathode ground for the reverb retrun is well away from other noiser grounds. I usually have the preamp chassis ground seperate from the PI/power stage chassis ground.
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As FYL said, hum with physical vibration is usually a short or manufacturing defect in a PT or choke.
Assuming you can confirm/deny that cause first, does the hum stop when any 1 tube is pulled in the signal chain?
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Pull the rectifier tube and flip everything on. Does the chassis still dance? Maybe the PT is vibrating only when under a load
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I had a BUZZY Pt shake V1 and the whole thing hummed. Tube wasn't microphonic in the classic sense, but shaking it 60 x's a second it was a bit noisy.
Try loosening the PT screws, I'm sure you got that thing screwed on there good and tight. Loosening the screws will not allow as much vibration to be passed into the chassis.
Don't think like I did and rubber grommets are the cure. a sick PT only gets worse till it dies, promise you that!
I opened up the mounting holes to accept a grommet in each corner which worked till the PT died (few hour later) and had BIG holes where I wanted to mount the replacement :angel
BTW, was 60 hz. to tell if it's 60 or 120 (I can't tell without comparison) I plug a cable into the input and thumb the exposed tip. thats always a 60 hz hum.
good luck Jeff, You'll beat this and we all might learn somethin.
Ray
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Guys, I really do appreciate the responses! Thanks! I can't remember for sure, but I think when I put my hand on the PT and the OT that it actually felt like the OT was vibrating more strongly. I'll try that again. And I'll try it without the rectifier tube in and report back on that on both of those.
I like the idea of loosening the PT nuts slightly to see what happens. I'll definitely give that a shot. And yeah, I have them cranked down pretty tight.
The thing is that PT and OT aren't that old. :undecided: :sad:
With respect, Tubenit
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Wow! I didn't remember this correctly, so I apologize for taking us down a rabbit trail.
The amp chassis does NOT vibrate using a remote cab & speaker. But the remote cab and speaker vibrates but not nearly as volatile. In other words, I can NOT feel anything vibrating on the amp chassis using a remote speaker.
The reverb pot turned all the way up is quieter. Turned down is noisier.
The preamp tube socket area seems to be VERY microphonic. I tried a 12AU7, 12AT7, 5751 and 12AX7 and all of them are microphonic in that position. In fact, the heater wires going to that tube are microphonic as well as everything on the chassis around that tube.
Having said that with NO tube in that socket, nothing there is microphonic.
The vibration is from oscillation and I don't think it's from the PT or OT.
Even with no NFB, could the OT wires being reversed make a difference? I'm somewhat grasping for other things to try.
With respect, Tubenit
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A simple way to find out if reversing the OT wires would do anything is to remove the power tubes and use your DVOM and measure the resistance from the OT center tap to the plate of each tube.If there as a big difference then yes,reversing the plates might help.If it's the same it probably won't do anything.
Also measure for DC voltage backing up the input grid.You may have a bad coupling cap if that's the case.I had an issue similar to this and I isolated it by tacking in a .1uf cap in series with the input grid(pin 2 or 7,whichever you use).That stopped it completely.It did not hurt the sound of the amp in the least to have that cap in there either.
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OK, here is a shot in the dark, but it has caused me problems twice in the past making tubes extremely microphonic. Input jack wired incorrectly or one of the input jacks not shorting properly (this has even happened on new jacks). Totally makes the whole amp microphonic. Tapping pots, wires, tubes, sockets, resistors, can totally hear it through the amp.
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Hey Tubenit,
I caught some grief over this method a while back. I think it was HBP that didn't think it stood up to logic, but I insist that I have had excellent luck with it.
Get yourself a test lead with alligator clips on both ends. Turn your amp on and set it to whatever setting gets you the most hum.
Connect one end of the test lead to someplace on the chassis where there is nothing else grounded. Systematically connect the other end to the grounded end of everything that is grounded.
If you connect to something that causes a noticeable difference in the hum, disconnect that ground and find a different place to ground that component.
This is how I have successfully combatted ground loop hum in many amps including commercial amps as well as scratch builds.
Dave
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Happened to me once. Have you tried to tight the contacts on the offending socket?
Hope this helps.
Best Regards,
Rzenc
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Guys, thank you for more ideas. I will try eliminating more stuff based on some of the responses.
The shortening on the input jack is working just fine & the input jack has worked for quite some time thru various revisions of this amp chassis. I don't think that's it.
The socket is making good contact with the chassis and the bolts are on tight.
I'll try physconoodler's and Dave's suggestion within the next couple of days and report back.
Thanks, Tubenit
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Jeff,
I would think you know about placements for things already but w/ oscillations in a high gain amp, problems go up exponentially and these are a result. High gain amps cause me to go the extra mile to keep signal ground and chassis grounds seperate for as long as possible. I put my filter caps as near as possible to the tubes they supply and their associated cathode's ground also. All grid wiring has to be kept real short and grid stop resistors mounted on the tube's sockets. Etc, etc, etc... All of these little kinds of things help and putting every component on a tag board isn't the best method w/ high gain amps. I know you've aready built the amp but I'm throwing stuff out there for general info. purposes. (I feel like I'm giving the instructor info he already knows - sorry) Many have already offered you great advice, I'm just chiming in trying to help or give moral support.
It could really help to see how and what the chassis looks like as for parts placements, wiring and grounding.
The OT secondary and the main filter cap areas are the two highest current areas to beware of avoiding w/ anything carrying a small signal and/or it's ground which is ripe for inducing oscillations & noise.
Your early description definitely sounds like you're dealing w/ oscillations. They aren't always heard and can be easily out of our hearing range. They get much worse usually when turning up the gain/vol dials as you've said when you feel/notice it and not always when on "10". They can find their very own sweet spot on "8" or another setting. If your amp is having any lack of sustain in the slightest, this is another dead giveaway for this problem. I wish I could help you further but I would only be guessing at things at this point. If you do find this specific vol thing out, I have an easy little trick to try that's worked for me once. Good luck my friend!
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tubenit, what does the filament chain routing look like? e.g. v1 to v2 to v3, etc...
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Did pulling V1 kill the hum?
Here are a couple easy things to try...
Pull the OD tube and the reverb tube and leave the OD switch off. Put a ground clip lead directly on the top end of the .01 cap on the board. It's labeled 'E2'. Does this kill the hum? If not, the hum is from the PA and/or PS. If it does kill the hum, move the ground clip closer to the input jack, first grounding the tip of the FX send jack, then the top side of the 1MA volume pot (the one after V1B), then the grid of V1B, then the top side of the 1MA volume pot (the one after V1A), then the grid of V1A.
Bridge a 20 to 40µF cap across each of your filter caps (one at a time).
If the above doesn't reveal where the hum is coming from, try powering the filaments with a 6V lantern battery.
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The filament chain looks like the layout I posted (if I am understanding your question correctly).
The layout could be a factor? In a small combo amp, I have tended to place the reverb on the far end prior to the preamp tube. It's always worked for me in the past. This is in a pretty small tweed combo type chassis. The amp has been rebuilt several times since but the photo of the original is here:
http://s28.photobucket.com/albums/c216/tubenit/?action=view¤t=chassisexteriorpotside.jpg
The amp has incredible sustain. Typically I play it with the volume and each of the OD controls at around "5". It has minimal hum at that level. No one has ever commented about the hum, but me. The band and the sound board guy haven't said a word. I'd say the hum noise is different but around the same level of hiss that I had on the vintage Princeton Reverb I had. It's just that I am used to very quiet amps at idle. I played the amp for a number of months before ever turning it up and hearing the oscillation.
soundclips here: http://www.soundclick.com/player/single_player.cfm?songid=9534655&q=hi&newref=1
http://www.soundclick.com/player/single_player.cfm?songid=9575220&q=hi&newref=1
However, IF I dial the volume to 10" and the OD controls to about "8" then it has the oscillation/vibration issue. At when that kicks in, it's obnoxiously loud. I'll never play it that loud. I just want an amp as quiet as possible.
Sluckey, I will walk thru your suggestions step by step as soon as possible and report back.
Pulling V1 totally kills the hum.
THANKS guys! All of you are incredibly generous with your information and help. This is a fabulous forum to be a part of.
With respect, Tubenit
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Sluckey,
Once again, your knowledge and generousity has helped me out! While I have had great help from dozens of guys on the forum ........ I think you still hold the lead in helping me narrow down the issues & resolve them.
I did pull the reverb and 5879 OD tubes. It was still noisey before grounding anything out.
I marked on the schematic where the issue is. It's between the V1a section and the volume pot. It is like whisper quiet every place else like I would want it to be.
Grounding V1-2 grid did nothing to eliminate the noise. But grounding the volume wiper does eliminate noise.
Maybe a bad cap? or a bad pot? or a bad solder joint? I can easily rebuild that entire section. I paralleled a .01 & .001 cap into the volume/tone section.
With respect, Tubenit
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OK, I thought to try something else and narrowed it down even more. I repeated this several times to be sure.
I'm thinking one of the silver mica caps is bad/leaky? Or maybe a bad solder joint or something? Perhaps the tone
pot is bad?
I clipped off the switchable 390p & the noise is still there despite grounding that pot lug where the other silver mica
attachs.
Tubenit
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I clipped off the switchable 390p & the noise is still there despite grounding that pot lug where the other silver mica
attachs.
I'd suspect wiring, layout, or connections before suspecting the silver mica. I would still try bridging that filter cap for node E. I'd love to see some gut shots showing how the control panel connects to the rest of the amp and where the pots/switches are physically located.
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Here's what I'll do. I am 1st going to disconnect the tone stack from the volume pot and see what happens?
Yeah, after thinking about it, I don't think it's the 390p cap since grounding the volume wiper eliminates noise and signal would be going thru that cap.
Then I'll clip another filter cap across node E and report back. I've never had a filter cap go bad before, so that will be a first.
Thanks again for the help! It sure is appreciated!
Best regards, Tubenit
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The cap may not actually be bad. But 10uF may just not be enough. Bridging with a 20-40uF will let you know either way. And the filter cap may not be an issue at all. Just want to eliminate it from the suspect list.
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OK, I removed the tone pot from the volume pot. Noise is still there.
And I did jumper a 47uf/500 across node E that feeds both plates on V1. Noise is still there. I did the same thing on node D & the noise is still there. The plate voltages on V1-1 & V1-6 look good to me.
I went ahead and replaced V1-1 resistor & cap. Noise is still there.
I tried jumpering from the non grounded lug to the wiper of the volume pot. Noise is still there.
Grounding V1-7 eliminates noise. Grounding V1-2 has the noise still there.
I am really puzzled with this one. Even with the 5879/OD & reverb tube out, the noise is very obnoxious when volume and master volume are on full.
Now what??!! Is it possible to just have a bad tube socket? I don't see any signs ever of arching. The socket looks good.
:sad:
With respect, Tubenit
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Put a .1uf cap in series with pin 7.In other words,put a cap in line right on pin 7.If it stops the noise then you do indeed have a leaky cap.
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Put a .1uf cap in series with pin 7.In other words,put a cap in line right on pin 7.If it stops the noise then you do indeed have a leaky cap.
What? Don't do that. Pin 7 must have a dc resistance path to ground (more correctly to the cathode). If you break that dc path with a cap, the tube bias will assume some unknown state and fail to control the tube properly.
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Really? The grid has AC.The cap will block the DC to the pot or input.I've done this before and it worked.Then I found the leaky cap and replaced it.
Actually it was the input grid,not the other side.
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At this point, I am going to replace the volume pot and if that doesn't solve it, then I'll redo the heater wires to and past that socket. If that doesn't work, then I'll replace the socket.
Not sure what else to try. The problem seems to be from that V1a gain stage going into the V1b gain stage if I am understanding what I am doing correctly.
I did replace the coupling cap from V1-1 to the volume pot using a substitution box, but it made no difference at all.
With respect, Tubenit
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Did you do the alligator clip dance? I'm telling you, it has helped me many times.
Dave
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Dave,
I actually did the alligator clip dance with the reverb grounding and the heater center tap prior to you mentioning it.
Having said that, the issue seems to be dramatically connected to the V1a area going into V1b. So there isn't much grounding there to sort out. Two cathode resistor/caps and the ground on the volume pot (the tone stack is removed and it's still doing it). I will try your suggestion with those however and report back.
Thanks for the input. This really seems to be an odd problem but I am resolved that I will get it corrected.
With respect, Tubenit
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I had the same problem with a 5E3 and I found it by using a cap in series with the input.Iy=t did not make the tube runaway and it stayed biased,but the noise and microphonics were gone.Like sluckey says,don't try it on pin 7,but it worked for me on the input grid.
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Are you sure there is no tiny bit of solder on the preamp socket that may be touching another pin? Or even chassis? Maybe a socket is bad and one of the pins is broke inside the socket making an intermitant connection. It really sounds ground related. Cant wait to see what it turns out to be
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Looking at V1a ............ I have replaced:
- the plate resistor and cap
- the cathode resistor and cap
- the tube socket & I tried another tube there also
- the volume pot
& I rewired the heater wiring and reversed the green and white incase the 5879 heater wiring to the 12AX7 wiring would be creating a hum.
I completed disconnected everything relating to the reverb.
NOTHING has changed!
Sluckey or someone ........ can you confirm if I have a problem with V1a with where the alligator clipped spots to ground are?
Am I looking in the right area? Is V1a into V1b the issue?
Thanks, Tubenit
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Out of curiosity, where are your ground return paths grounded tubenit? (Sorry if I missed this in an earlier post)
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I used Hoffman's grounding scheme like I do in all my amps. Preamp filter caps and pots and preamp grounds all on a buss wire. It worked in the Tweed Overdrive just fine which is even higher gain.
I did try Dave's alligator clip dance but it made no difference.
With respect, Tubenit
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Do me a quick favor: on v1a, unsolder the 33k and ground the input grid. I realize you have already tried grounding the grid, but unsolder that 33k - just a hunch.
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Have you tried feeding the V3b from node D or C?
Not sure if this is a problem, but usually one PS node feeds no more than two stages, in your case node E would feed V1a and V1b.
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Do me a quick favor: on v1a, unsolder the 33k and ground the input grid. I realize you have already tried grounding the grid, but unsolder that 33k - just a hunch.
Thats all i can think of thats left to question. That input jack might be the demon
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Unsoldering the 33k from the input grid still creates alot of noise.
With the 33k unsoldered off of V1-2 and V1-2 grounded, it is still noisey.
With the 33k unsoldered off of V1-2 and V1-7 grounded, it is quiet.
I did try powering V1 off a different node last night. No change ........ still noisey.
Man, I am at a real loss on this one? :cry: :embarrassed:
Am I correct thinking the problem is between V1a & V1b? Have I accurately isolated it to that position?
PLEASE offer some more ideas if you have any. I'm really stuck on this one.
I still have ALL the reverb unsoldered from the circuit & the tone stack disconnected. That actually seems to have made things worse. The hum is worse with all that out of the circuit.
With respect, Tubenit
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Jeff, you're doing all the right stuff. At this point we need to see some hi rez pics. And make a sound clip of what you're hearing.
What are the voltage readings for pins 1, 2, 3, 6, 7, and 8? And take some resistance readings for pins 2, 3, 7, and 8. All resistance readings to ground (HT CT) and other probe directly on the socket pins.
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Could the reverb pot be too close to the input jack or the volume?
On a side note:
Which way the 47pF cap is on the amp? Layout shows it parallel to plate resistor and in the schem it is between cathode and plate.
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The amp looks a MESS because I have the reverb and tone pot unsoldered. And the amp has been thru numerous incarnations. So, PLEASE cut me some slack on how messy it looks. It looked neat in the first build. In case you're wondering the grounds to the cathodes of V1 are good. I did run/swipe a wire under the tagboard to make sure there wasn't something there and it did OK.
:wink:
The 5879 tube is to the left of the preamp tube in the photos. But please remember, the problem exists with the OD bypassed and the 5879 tube removed.
ALL of the reverb is disconnected. I completely removed it from the circuit. The 47p cap was removed earlier yesterday.
I checked all resistor values and they are correct.
V1-1 193v
V1-3 1.6v
V1-6 188v
V1-8 1.6v
resistance
V1-2 39k
V1-3 1.5k
V1-7 varies with volume pot to about 1M
V1-8 1.48k
I don't get a reading of anything on V1-2 or V1-7 but I am not sure how to measure for that?
With respect, Tubenit
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Tried the lantern battery to power the filaments yet?
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On V1,pin 2 you will get an AC voltage reading from your guitar.You need either a guitar plugged in and being played or a signal generator to produce the AC signal.
See what I think is that your microphonic noises are from DC backing up to the input jack.
A few years ago I had this issue on a Tweed Deluxe.I would get this microphonic ringing and noise even if I touched the input jack.Someone here on the forum suggested I put a .1uf cap in line with the input signal,effectively blocking DC voltage from reversing up to the input jack.
It worked like a charm.That's why I suggested it to you.
I understand Sluckeys concern about having the tube find a bias point,as he says,'unknown' and running wild.Well it didn't and this is simply a diagnostic aid to isolate your problem,not a permanent thing.
For the time it takes,it may be an idea.Lord knows you've tried everything else.
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Phsyco,
I'll give it a shot also. Couldn't hurt to try it for a few seconds and maybe that'll work?
With respect, Tubenit
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Tried the lantern battery to power the filaments yet?
I have not tried that yet, but I can. I'll have to go purchase a battery though. Not sure when I'll have time to do that, but I'll attempt it. It may be this wkend before I have time to go by a store and purchase one. I take it that I need to power all the filaments or just the preamp filaments when I try this?
Q1) With the 33k input resistor removed and V1-2 grounded, it is still noisey.
Does this effectively remove the input jack as a concern? I have an
extra switchcraft jack that I can easily install if needed.
Q2) The on/off switch seems to be more noisey clicking on and off. Any chance that
could induce hum that showed up more on the V1a 1st gain stage?
Q3) The other thing I pondered is I am wondering if the resistor between node
D & node E could either be defective OR that a solder joint on that B+
rail might be bad. I think I'll replace it & resolder. Could that be a
concern?
Q4) There is some very slight hum from the speaker grounding all the points that
Sluckey stated to try. Grounding those makes the amp either super whisper
quiet or on V2-7 ........ it sounds "normal" to how my amps typically sound at
idle. Grounding V2-2, it is VERY loud when I turn the vol & master vol to 10.
That is what I should be hearing correct? The slight hum is slightly louder
as you move towards V1-a but it sounds like I would expect the amp to.
The other thing I haven't tried is replacing the shielded wire from the volume pot wiper to the V2-7 grid. It did not test shorted internally or shorted to ground. Having said that, it seems like I had a shielded wire test OK once and prove to be a problem. So I'll replace that and see if anything else happens.
I know I am somewhat desperately grasping at straws here, but I am at a loss to logically explain why the volume of hum is so loud at the V1a to V1b area?
The hum with V1-2 shorted to ground is probably 3-4 times the hum with V1-7 shorted to ground. The hum with V1-7 shorted to ground is what I typically hear in my amp builds at idle.
With respect, Tubenit
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Q1 - yes this removes the input jack and resistors from the equation.
It is something around V1A. Here are some random thoughts:
- bad socket
- cracked component (like a resistor)
- cathode ground point is too close to a power stage ground point and hum is getting in that way
- bad tube
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I replaced the socket with a brand new one.
Replaced the resistors & caps
Tried 5 different tubes of various gain level
Using the same paralleled terminal layout and power supply hookup as I have in other amps successfully & even in this amp in previous incarnations of it.
Still makes the noise between V1a & V1b
Thanks for the ideas & please share any additional things to consider. Your post helps confirm that I'm at least considering some of the right areas to look at.
With respect, Tubenit
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Wow! Im starting to think that Casper may not be a friendly Ghost. Boy when you find this problem you will never forget it
Neither will i :cry: Have you ruled out any bad wires broke internaly under the insulation?
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Sorry about that - didn't mean to reiterate things you have already tried.
I had a reverb return noise that caused me lots of trouble. HBP helped me to track it down to the cathode ground of the reverb return. I am wondering if you might be having the same problem but with V1A.
Perhaps - just as a something to do until a good idea comes along - detach the V1A cathode ground, and temporarily jumper it to other parts of the amp. See if the noise changes.
Irf not that, since you have already tried everything on the grid of V1A - all thats left is noise getting into the plate of V1A...
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Does V1 have an aluminum shield over the tube like a lot of old Fenders do on the preamp tubes? If not it might be worth adding one to see if that helps.
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So when you pull the V1 tube, the hum goes away?
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Yes, when V1 is pulled there is NO noise.
Shield over the tube makes no difference.
I tried the .1 cap ...... no difference.
I replaced input jack ....... no difference.
I replaced the wire from volume wiper to V1-7 ....... no difference.
I removed the chassis from the cabinet ........ no difference.
I measured the heater wire voltages up and down the amp. They are spot on 6.3 vac.
I am truly baffled.
IF this is accurately between V1a and V1b ............... there just isn't anything left to replace or change.
Don't have the lantern battery but I am willing to try that even though I dubious it will make a difference.
Could the problem be someplace else but is more pronounced in the V1a to V1b area ??????????
with respect, Tubenit
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Jeff,
My theory is that the hum is getting into V1A - you have already proven that it is not through the grid. So I am thinking it is getting through the plate or the cathode.
I suggested earlier to detach the cathode ground and put it on an alligator clip to other places. If not that, then the noise is getting in through the plate somehow.
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I suggested earlier to detach the cathode ground and put it on an alligator clip to other places.
That was done yesterday. And I used a cap substitution box for the V1-1 coupling cap going thru a handful of values and then soldered in a brand new Orange Drop. And both the plate and cathode resistor were replaced also along with the volume pot etc...
with respect, Tubenit
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Jeff,
Are all your pots nice and tight, bare metal to bare metal onto the chassis? Input ground to your pot ground? Sorry, I could not see detail on you Photobucket page.
Jim
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Yes all pots and input jack are tight. And I quadruple checked the ground with a voltmeter.
Tubenit
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I may not have a chance til wkend to try this, but I will try these bypass ideas next to try and isolate the area definitively. AFter doing so, I'll report back again.
Can a bad on/off switch cause a severe hum? Keep in mind the hum only becomes pronounced when the volume and master volume are at above 5 or 6. The amp has no noticeable hum with the vol and mstr vol. at zero. It's completely quiet.
Tubenit
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How far are you willing to go with this? I'd like to see you abandon that ground buss on the back of the pots. Think about it. Every pot provides a potential ground loop thru the body/nut. This may or may not solve your problem but it sure won't hurt. And as a bonus, if you ever need to change a pot it will be much easier.
This Ceriatone buss would be very easy to do on that amp and would look much cleaner. And the buss gets connected to chassis at only one point--- right near the input jack. Just connect the #14 buss to your board grounds and run short wires from the pot grounds to the buss. (Notice that the power tubes cathode resistor/cap are not connected to this buss in this Ceriatone scheme.)
(http://www.ceriatone.com/images/boardPic/marshallTMBBoard/DSCN6798B.jpg)
I prefer the buss scheme on my November or Lightning amps, but that would be much harder to do at this point than the Ceriatone buss.
(http://home.comcast.net/~seluckey/amps/november/nov_02.jpg)
I know this is a lot of work and I understand if you don't want to try it. We're kinda reaching for solutions at this point though. I'd try the lantern battery first, though I'm not very optimistic. Could be a reason to get a good lantern style flashlight though.
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I have to wonder if the PT is some how damaged and the heater winding may be picking up some osscillation from the PT. This is a very cool thread mystery. But i understand very stressful. The battery test will answer the heater question
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For whatever it's worth, I've used a buss similar to the Ceriatone setup on 3 Blackface Fender clones. It has worked very well for me.
I'm grabbing at straws, but these two comments caught my eye:
I've noticed turning the treble down on the clean tone knob lessens the hum. Increasing the treble on the OD tone lessens the hum.
The reverb pot turned all the way up is quieter. Turned down is noisier.
I know that you tried a 47uf cap across the 10uf filter cap at node "E". Did you try an outright replacement of that filter cap with a 47uf? How about a 1/100th poly cap in parallel across that last filter cap? How about a 15K isolation resistor instead of that little 1K? 1K with a 10uf cap is not much filtering in my limited experience.
Node "E" feeds V1a & V1b, but it also feeds the reverb recovery triode (V3b). Would it be difficult to try feeding V3b from node "C"?
Like I said, this is just grasping at straws but it seems like you've tried everything imaginable except re-working the power supply.
Keep at it Jeff!
Chip
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Have you tried dumping the V1 plate(s) to ground with something like a .1uF high-voltage (film) cap (to see if that kills the hum)?
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One of the things I've mentioned that may not stand out in what I have said is how quiet the amp is at the normal level that I play it. The normal level is how loud it is when I play it with the band. Vol, and OD controls are on around "5". No complaints by anyone. About the same noise as the PR I had owned but a different noise. When I dial those same controls ....... or the clean master volume to "7". The noise is probably 7 or 8 times louder (or more) then the volume I usually use and the amp chassis actually vibrates physically.
This amp (despite the hum) is one of the most transparent, touch sensitive and expressive amps I've built at "normal" volumes.
I've got a reasonable hunch of what it might be! I think? :undecided:
If you count original builds and incarnations, I've built maybe 20+ different amps. ALL of them have been either super quiet or very reasonably quiet.
I typically use a paralleled terminal or tagboard approach with B+ running along the chassis UNDER the plate, grid and cathode wires.
However, the most recent of one (of maybe 4 or 5) incarnations was Geezer's HoSo56 with 6V6's. When I changed the position of the 5879 tube to the Tweed OD Lite, I did not have enough wire length to run the B+ wire under the plate,grid, & cathode wires so I ran it above them and somewhat close to them.
I'm betting a nickel that's the issue! It was staring at my all along. The B+ is under the LTPI wires and above the preamp wires.
I think I need to change that wire out and get it under the preamp wires and further away.
IF that doesn't work, I'll try the bypass stuff & report back. I can certainly change out the filter cap on node E.
Even though my schematic shows that to be a 10uf, it is actually a 16uf filter cap.
If that doesn't work, I am probably going to gut the amp and start over at which point I'll use the Ceriatone type buss wire.
FWIW, I have used Hoffman's buss wire on the back of the pots on every amp I have built with no issues including both of the Tweed Overdrive Special amps that I also have which have even higher gain. However, if I am not opposed to changing that buss wire to a Ceriatone type. It wouldn't be that much effort.
Currently, the reverb is completely disconnected, but I can change the B+ to reverb to node C no problem.
Guys, THANKS to all of you for your help and support and encouragement! You guys are superb and I am truly appreciateive!
With respect, Tubenit
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Did anybody take my nickel bet? ............... I owe you 5 cents. :grin: :angry:
Changing the B+ wire didn't work. Back to square one.
Between now and Sunday, I have a very busy schedule so I may not get a chance to work on this for a few days?
Picture this, volume and master volume on "5" OR volume and OD controls on
"5", there is not much noise.
However, with those same pots turned up to 7. It makes the same loud obnoxious noise level as one would get if their guitar cable became unplugged from the guitar. It is a HUGE jump in volume/hum. And the chassis will physically vibrate when in the cabinet. It is not the same type of noise as a cable unplugged but it has that type of obnoxious loudness.
For my own curiousity (if not mental health), I am next going to try my bypass plan and see what happens? Then I'll go thru the other suggestions.
Anybody have another thought of how something in another part of the amp appears to be problematic between V1a and V1b besides the node D to node E dropping resistor? (which I will attend to)
Is it possible that I have a bad PT but the problem appears to be located between V1a & V1b?
The chassis is out of the amp and the problem still exists. Because it is an open ended chassis, I can easily see under the tagboard and there appears to be nothing problematic there.
With respect, Tubenit
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Jeff - I know you've used your 1-tube reverb successfully in many builds. You haven't done a high-gain build before though, have you? I know the guys who build high-gain amps obsess over the power supply. They also obsess over grounding.
If you have TUT 3, figuring out how to get closer to O'Connor's "galactic ground" topology may help. I can't help wondering where the cap for node "E" is grounded in relation to V1 and to the reverb recovery circuit - including the reverb pot itself. Can you try just disconnecting node "E" from the reverb recovery stage?
This isn't from personal experience, so please don't take it as gospel. I just spend some time lurking over on AX84.com and they seem to have a lot more problems with oscillations with those high-gain amps with multiple gain stages than we do on our typical builds.
Good luck!
Chip
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I made a mistake in one of my high gain build a year ago. I had mistakenly wired the ground buss of my preamp section and all pots to a ground lug. That ground lug was on the PT. The amp was noisy and the hum increased along with an increase in volume. I finally saw what i had done and isolated the preamp and pots to the other end of the board at the input jacks. Hum was gone and volume worked as intended. I havent forgotten that one. That was proof for me that grounding is a very sensative thing. That a PT signal or osscillation could actually get injected into my preamp through my bad grounding error
Speaking of Mental Health where is my VOLUME? :huh:
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You know, sometimes, reverb pan orientation can have a dramatic effect on humm issues.
It probably a long shot, but if we're stumped, let's look at all possibilities.
Dave
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FreshStart,
Yes, I have done high gain amps with one tube reverb and NO problems. They were quiet.
For example, I have two Tweed Overdrive amps with me & no issues with them. Yes, they are heads instead of combos. However, this Tweed OD Lite still does this removed and isolated from a cab (as far as 4 ft away with the speaker hooked up). One of the Tweed Overdrive amps did have the one tube reverb & it worked fine with no issues. It is higher gain than the Tweed OD Lite. I did convert that though to an active effects loop per request of a buyer who wants it.
Please remember (as I have mentioned) the reverb is completely disconnected in the amp for the last several days & the problem continues. If the amp essentially doesn't have reverb, then I would anticipate that eliminating the reverb as a problem.
The wires are not hooked up, there is no reverb tube and the pan is disconnected. Node E has been disconnected from the reverb for several days.
And all my builds except this one have been successful using the same type layout format with paralleled terminals. Including a cathode biased D_mble HRM and both Tweed Overdrives, Plexi 25W with revergb & an 5881 HoSo56. The EL84 56T and 56T with 6BM8's were quiet also.
And I've always used Hoffman's grounding scheme with success in over 20 different builds. So why wouldn't it work this time? I am not aware of what would make it different. The preamp filter caps are connected to the buss wire like Hoffman shows. The power filter caps are connected to a PT bolt.
Again, the problem "appears" to be related between V1a & V1b. Which is confounding me because there are so few parts there. Using Sluckey's suggestion, grounding V1-2 does NOT eliminate the noise but grounding V1-7 makes the amp super quiet.
And while this seems to indicate that between V1a and V1b is the problem, I have literally replaced everything from the input jack up to the V1-7.
With respect, Tubenit
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This probably won't help either, but it looks like the heater wiring is very close to the grid resistor of V1b. I would bend those away from the other wires and that resistor.
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Ok,what about NOT concentrating on V1 and looking to the output section.The output jacks in particular.
1)does the amp have negative feedback and do you have a ground reference for the output jacks?
2) did you use isolated jacks or switchcraft style?
3) have you tried loosening off the bolts on the output transformer and moving it around a bit?
4) then back to grounds.I had a nasty hum on a 5E3 until I completely separated preamp filter cap grounds from PS cap grounds.
Just thoughts added to an already frustrating scenario for you.
If you take a look at the ground scheme on a D'Lite layout,it shows a great separation of those grounds,even using really long ground wires.
I even had a nasty hum on a V-Verb build that turned out to be the unused bias tap wire wrapped around the filament wires!
Go over the amp and don't concentrate so much on the preamp.It may not be there,but it gets amplified by the preamp!
I do not star ground anything and use Doug's ground method sometimes,but not always.Do not assume it works with every amp.
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I wonder if it could be a bad V1 tube socket?
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I wonder if it could be a bad V1 tube socket?
He already tried that. Multiple tubes too.
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Phsyconoodler,
Your input has given me another couple of ideas to try.
The amp has a passive effects loop. I'll try using that and see if I can hook up another amp preamp ........... or feed into another amp poweramp to more conclusively pinpoint the area.
I may have time to do that tonight. And if so, I'll post the results.
And I can disconnect the effects loop and remove it to eliminate that as an option also.
Since this is a 4th or 5th incarnation in the same chassis, I should point out that the PT, OT, grounding scheme, layout, etc............ has worked in all the previous incarnations of the amp.
I pretty much changed the preamp and left the majority of it alone on this last conversion.
As TON mentioned, it has a brand new socket. No change.
With respect, Tubenit
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Maybe a dodgy switch contact on one of the FX loop jacks? Sometimes when you tighten up the jacks, the switch and tip get squeezed. I suggest this because on a recent BFPR build I did, the same thing was causing similar symptoms to what you describe is going on with your vol control. Just a wild hunch (but given the other hunches around here to-date - I figured what the heck?) Or it could be the contacts in your O/D switch perhaps.
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I thought about the clean/OD switch also? It's a mini-dpdt. And I don't remember if it was one I rewired or new? I will work to eliminate that in the list of things to try. I have bypassed it and the noise is still there. But perhaps bypassing wouldn't eliminate it?
I can easily remove the effects loop.
I am still wondering about the on/off switch? It seems to be making a louder pop turning it off and on. OR it may be that I am hypersensitve listening for noise and it was already that way?
I already tried taking a chopstick and moving the heater wires around. No difference at all.
Guys, I sure appreciate the ideas and support alot! Thank you!
With respect, Tubenit
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UPDATE:
Using the passive effects loop, I ran the Tweed OD Lite preamp into the Tweed Overdrive and it still makes the hum.
I ran the Tweed Overdrive into the Tweed Lite OD power amp and that sounds fine.
It is definitely in the Tweed OD Lite preamp section PRIOR to the LTPI.
Then I removed the effects loop just in case that was a factor and it still makes the noise, so the effects loop isn't the culprit.
I think Sluckey has it figured out with the V1a to V1b area.
I am going to try the bypass idea next and further pinpoint where it is. I may not be able to work on it again til Sunday?
However, after I try it, I'll give another update.
Depending on what I find, I might rebuild V1b and change B+ from node D to node E.
with respect, Tubenit
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If it was me, I would let it alone and forget it for three weeks.
Then, as you'll be fresh-minded, it will be the right time to debug it again.
Maybe you'll be tempted to dismount it entirely and re-build it from the start... I did this on a 6G13A and it helped a lot :wink:
Good luck and BR,
Danskman
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Your close Jeff, don't give up now!
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What I lack in knowledge, I try to make up for with persistence!
:grin: :laugh:
I'll get it fixed! Will post again Sun or Mon with an update.
Thanks guys!
With respect, Tubenit
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I still wonder about vibration and a dodgy switch contact(s). If you brush your finger-tip against the tip contact(s) of the input jack(s), does it/they rattle? I had this problem once with an amp, and it annoyed the feck outta me until I found it. I found that the tip and switch contacts were ever-so-slightly on the verge of being unconnected, and this induced a buzz/rattle in the chassis at certain frequencies or certain volumes. Worth a look at maybe.
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Tubeswell,
From a previous post on the thread
And while this seems to indicate that between V1a and V1b is the problem, I have literally replaced everything from the input jack up to the V1-7.
Input jack was replaced with a brand new switchcraft. It's not the issue. It's making good contact and no rattle from it.
I'll get back to trouble shooting it probably tomorrow & post what I find out.
with respect, Tubenit
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I bypassed V1-b into LTPI & noisey. No OD
I bypassed V1-a into LTPI & noisey. No OD
I concluded it was either the mini-dpdt or the dropping resistor between node D and node E.
I removed the DPDT from the wiring altogether & left OD wired in and changed the dropping resistor from 1k to 4.7k.
Amp is VERY quiet at idle now with only typical hiss and NO hum!
That resolved it!
THanks guys! with respect, Tubenit
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:smiley:
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Congratulations! You went through the trials of Job on this one.
So which was it? The switch or the under-sized power rail resistor? :wink:
BTW you really need a "listening amp".
http://www.el34world.com/Hoffman/tools.htm
I built a little box similar to what Doug shows at the bottom there and plug it into a TradeMark 10 amp I don't like much anymore. http://www.el34world.com/Forum/index.php?topic=3950.0 It's fantastic for figuring out where noise is coming from... at least sometimes.
Cheers,
Chip
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Yippee! Congratulations Jeff!
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Yay! another victory in the struggle for tube amp perfection
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Do you think the DPDT switch was overheated when soldered to causing it to go bad?
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Guys,
Unfortunately, I don't know which it actually was? Here is what happened ............
Neither bypassing V1a (and using V1b as 1st stage) or bypassing V1b eliminated noise. So I thought at that point, that I'd just try a couple of last things and then gut it and start over.
I was not anticipating the last few things to resolve the issue. I was presuming the amp would be gutted.
So, I removed the master volume, the DPDT & replaced the dropping resistor between node D and E ........ all at once. Something in that removed the hum and it is very quiet like my amps are typically.
Wished I had done those one at a time, but I can say that I am pleased with having a really quiet amp again.
:grin: :wink:
THANKS for all the guidance, support and encouragement!!!!!! You guys are fantastic!
Best regards, Tubenit
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Boy I wouldn't have suspected either of those. Congrats.
Dave
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So, I removed the master volume, the DPDT & replaced the dropping resistor between node D and E ........ all at once.
It may not have actually been any of those. You may have fixed a problem solder joint in the process. Of those 3 components, I think the dropping resistor would be the most likely suspect. Dropping resistors for the preamp are usually fairly large to drop the voltage and to provide a higher degree of decoupling from the previous B+ node.
Glad you got it. Now to start stitching all the bell and whistles back in. :grin:
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AMEN! I have a similar issue myself now but a little different. When i turn up the volume in my Bassman normal channel with no input i have a hum that increases as the volume is turned CW. The vibrato channel is dead quiet and does not do this. Is this Hum month?
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You have a Bassman with a Vibrato channel? :huh:
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You have a Bassman with a Vibrato channel? :huh:
Sorry i ment a Bandmaster Reverb. That is a big difference isnt it?
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Actually, I knew what you meant just from your recent Bandmaster thread. They both start with "B". I mix up words like that all the time. Ask my wife! Don't know why she doesn't understand what I'm talking about. :grin: