Vox AC100/2 build

[SoundmasterG]

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

[pompeiisneaks]

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

[pompeiisneaks]

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

[Willabe]

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.

[pompeiisneaks]

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

[shooter]

get this popping visible in the scope with a dummy load

so where does it start, before V1, after PI?

[pompeiisneaks]

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

[shooter]

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

[pompeiisneaks]

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

[sluckey]

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.

[SoundmasterG]

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

[pompeiisneaks]

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

[pompeiisneaks]

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

[SoundmasterG]

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

[pompeiisneaks]

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...   

~Phil

[SoundmasterG]

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

[pompeiisneaks]

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.)

[pompeiisneaks]

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

~Phil

[pompeiisneaks]

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

[shooter]

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.

[SoundmasterG]

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

[pompeiisneaks]

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. 

[pompeiisneaks]

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

[pompeiisneaks]

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

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

[pompeiisneaks]

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

[sluckey]

A 3 watt resistor will be fine.

[pompeiisneaks]

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

[SoundmasterG]

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

[sluckey]

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.

[pompeiisneaks]

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

[SoundmasterG]

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

[pompeiisneaks]

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 :)

 
 
 
 

This group is so full of smarty amp gurus I'd never have done it without you

Did I say thanks?

~Phil

[pompeiisneaks]

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. 

[sluckey]

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.

[pompeiisneaks]

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

[Willabe]

Well son of a gun, plate stoppers fixed it.         

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?   

[SoundmasterG]

Well son of a gun, plate stoppers fixed it.         

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

[pompeiisneaks]

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

[SoundmasterG]

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

[pompeiisneaks]

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

[shooter]

Glad you're a happy camper!

put some earplugs in and annoy the neighbors

[pompeiisneaks]

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

[pompeiisneaks]

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

[SoundmasterG]

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

[pompeiisneaks]

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

[SoundmasterG]

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

[pompeiisneaks]

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

[pompeiisneaks]

And the reason for the last update, is that the video is here with the audio of the finished amp.

~Phil

[SoundmasterG]

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

[pompeiisneaks]

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