Problem with my Ampeg V2

[Foxfire]

to bring everybody up to speed, i bought this as a project a few years ago. some of the tubes were in the wrong place when i got it, but the amp "worked" once i sorted the tubes out. i used quotes because i could hear an unpleasant distortion on lower note/chords. i gave it to a buddy and he replaced the OT, but that turned out not to be the problem. i then gave it to my regular amp guy at the time and he wasn't able to get anywhere with it either. i then personally replaced every single cap in the amp (i just realized that this isn't totally true because i haven't replaced the caps on the vol/tone control board), rebuilt the whole power section, and replaced any resistor that looked compromised. at this point the amp was sounding good. there is a slight possibility that i was lying to myself but i'm as certain as i can be that the amp was 100%.

fast forward, a year or 2, a cap around V1 went so i replaced it. it was around then that i noticed the unpleasant distortion was back. i use so much distortion that i may have been back for a long time before the cap went and i just didn't here it, and i only heard it because i was listening to the amp to make sure i had fixed the most recent problem. i was at my wits end with the amp so i took it to a 3rd amp guy (a very well recommended one) and he miss understood what i was looking to have done and fixed/replaced a few things (jacks, sockets,etc.) thinking i wanted to lower the noise floor.

so the amp has had, all but the 4 caps in the tone control section replaced, the power section rebuilt, flyback diodes replaced, any "off" looking or out of spec resistors replaced, and the OT is new. oh and all the voltages are well within range. this is my white whale. i've got $700-$800 in to it and it has tried to kill me a few times.

here is a recording of what i'm trying to fix, the distortion can be heard at the beginning of the note/chord. i have tried different cords, guitars and cabs over the years so i feel that can all be ruled out.
Recording 26 by Foxfire3 on SoundCloud - Create, record and share your sounds for free

and here is a schematic.


Thanks in advance...

[Chuck H]

To your knowledge, has the amp ever been modified in any way? Do you NOT know? Now would be a good time to post voltage readings of ALL the tube pins.

[Foxfire]

the only mod to the amp is a Ken Fischer type 2/lar-mar PPI-MV. which i added around the point that i got the amp working distortion free. it'll be a few days before i can post voltages my only pair of 6L6's are at my practice spot. i can tell you that they were all really really close to spec the last time i wasted a day in the basement on this thing, but since i've had plenty of chances to fix this thing and have not been able to i'll post them as soon as i can.

[AL30]

I assume by "all the caps" you also mean the filters correct? I agree with Chuck - posting voltages would be helpful.

Have you checked the preamp tubes to make sure they're OK? How about swithes?

AL

[Foxfire]

Yes, every cap including filter caps minus the tone caps around the bass/treble controls have been replaced. When I "fixed" the amp I was pretty sure it was the bias feed cap C16, 0.047uF (if I remember correctly) that was the culprit. I have since changed it a few times to no avail.

I've swapped tubes in and out many times. The noise is there on both channels and with or without the reverb tubes.

I've got practice tomorrow so I'll be able to grab those tubes and post voltages soon...I probably should have waited till I had the tubes to post, but I wanted to get the sound clip up to A) make sure I wasn't crazy and B) to see if the noise was recognizable to anyone?

[Chuck H]

The distortion sounds like it might be a bad tube or a tube that has incorrect operating conditions. You do mention "swapping" tubes, but not replacing tubes. Microphonic power tubes could also be the problem. As could a mechanically rattling preamp tube that is so grossley bad that it jangles rather than rings as is typical. Voltage readings may not tell us about that. Basically, no one you have brought the amp to has found the problem and the that's rediculous. Because it's easy enough, if your a professional, to isolate the problem with the amp on a bench. Especially with a scope. Changing all those film caps is a cumbersome, expensive and time consuming shot in the dark that never had much chance of fixing the problem. It seems like whenever a novice has a problem, the first thing they do is start replacing components and then they wonder why, with all their efforts, the amp is still broken. In fact, because of other problems that can arise because of major surgury, like replacing all the film caps, the issue can become confused with new gremlins. It is ALWAYS bad to do such things when you don't KNOW that it will fix the problem. It's also bad to do mods to an amp that has problems prior to fixing those problems for the same reasons. It just complicates and confuses the troubleshooting process.

We need voltage readings. This may show any incorrect operating conditions. If not...

You need to get new tubes for the amp. Expensive, I know. But how old are the tubes that are in there??? At the very least you will have spares for some other time in the amps life. At best you may find that some of the old tubes were bad and the problem is gone. Put in the new tubes and check/set the bias. Then swap the old ones back in one by one. Discard bad tubes and keep any that still seem to perform without causing problems.

If you still have the problem you should scope the signal chain to isolate the problem and then reflow all the solder joints for the associated circuitry. If you don't have a scope you need to take the amp to a tech, other than one you've previously used, that has one. Print this post and present it to them if you must so there won't be any confusion.

Other than some rouge mod causing the problem, I'd bet six beer German that your amp will be fixed.

[g1]

For the amount of work that has so far gone into this amp, I would expect ALL joints on the circuit boards have been resoldered. If not, that would be a good place to start. These amps are prone to solder issues due to the board mounted tube sockets and associated heat.
Otherwise, I'm suspicious of a possible bias problem. These amps don't have adjustable bias. Previously you changed the bias feed cap and it affected the problem. Are you using the same power tubes all the time? You need to at least be able to measure the bias, making it adjustable would be even more helpful.

[Foxfire]

I should've been more clear, I did the cap job. In fact I did everything but replace the OT, a few jacks and 5 tube sockets. I replaced all the film caps because the caps used were notorious for leaking and I only replaced them in response to having one go on me not to fix the distortion problem. I initially replaced the bias feed cap C16 because they take a fair amount of stress and are a common problem with V2/V4 amps. I was given good advise to replace the original films and because I don't have a scope I just decided to replace everything I could within reason.

I did the ppimv at point when I knew the amp was working well. So I'm pretty sure it is not the cause. I only noticed that the distortion was back after fixing the leaky cap well after I put in the ppimv. But like I said I use a lot of distortion so I can't swear as to when it came back.

As for the tubes, I've had maybe 4 sets of 6l6's in it over the years for testing. I have a pair of winged C's for it because of the voltages and because I haven't been using the V2 I put them in another amp that I keep at my practice spot...Same with the other tubes, I have swapped them in and out with known good tubes many times.

I'm using my phone to reply so I'll have to go back and reread your post to make sure I'm not missing anything...

[Foxfire]

For the amount of work that has so far gone into this amp, I would expect ALL joints on the circuit boards have been resoldered. If not, that would be a good place to start. These amps are prone to solder issues due to the board mounted tube sockets and associated heat.
Otherwise, I'm suspicious of a possible bias problem. These amps don't have adjustable bias. Previously you changed the bias feed cap and it affected the problem. Are you using the same power tubes all the time? You need to at least be able to measure the bias, making it adjustable would be even more helpful.

Yup, it seemed as though replacing the bias feed cap solved the problem for me the first time around. I had it setup for adjustable bias but put it back stock before I took it to the last guy as a way to rule it out as the culprit. Ampeg liked to run them cool but the last time I checked them they were in the 60-70% range.

I have reflowed maybe 75% of the solder joints so I guess it wouldn't hurt to do the rest...

[Foxfire]

i found a spare set of tubes so i spent the last 2 nights messing with it. i ran the preamp out in to another amp and i am 99% sure can't hear the distortion. i then started probing with an audio probe from the preamp out back towards the power tubes. the first place i could hear the distortion is on pin 6 of the 12wd7 but found that it went away if i pulled V4. that is about the only concrete discovery i made. i didn't take voltages for anything in front of V3 since i'm getting a clean output from the preamp out. i included the front half of V3 just cause...and again thanks for everything so far.

With 120VAC at the wall,

Pin----V3-----V4----V5-6
1___342v__218v
2___101v__24mv
3___214v__9.9v__559v
4______________561v
5______________-69v
6___210v__220v
7___4mV__45mv
8___2.04V_9.9v

i apologize for not knowing how to use the list function...

[Chuck H]

Well, as per the schem nothing is glaring wrong. But a bit high here and a nit low there. Everything should be a little high due to modern wall voltages. The low readings are suspect then. Replace the 12DW7 if you haven't already. If you have then check all the V3 circuits for drifted resistor values. I'll look a bit more and repost if I think of anything else.

EDIT: Did think of something else. There was a tube maker making a 12dw7 with the triodes reversed. Make sure you have the right pinout on the tube you plug in there (or have plugged in there). Still looking...

Check to see that the NFB circuit is correct and on the 8 ohm tap.

[Foxfire]

Yeah, JJ had a habit of reversing their 12dw7 tubes apparently, I tried both and a few different 12ax7's and au7's in there without any change to speak of.

The NFB is coming from the 8 ohm tap.

I'che ked just about everything around V3 and V4 but I'll keep looking cause I clearly have found it yet...

[Enzo]

JJ purposely makes both versions of the tube, to meet market requests, they give it a different number. As far as I know, they don;t make any "wrong."

[Foxfire]

ok so i started poking around V3 and i've found that the distortion seems to go away when i remove the feedback from the OT. unfortunately i don't know what this means...i have checked the feedback resistor and cap and they are good.

[Chuck H]

It seems like a problem with the feedback loop. The components in the loop include the cathode circuit for V3 pin 8. These could be checked for value. Also, did you check the cap in the feedback loop for leakage? This cap is a tiny value for this circuit and only passes frequencies above 10kHz. It's clearly there for parasitic supression. You could remove it. If the amp remains stable and the ugly is gone you win. Another also... What kind of load are you running the amp into? Is it a mismatch? A mismatch on the high side may cause voltage spikes that the feedback loop is passing on to V3 even though the spikes aren't part of the looped signal. This could cause odd distortions. If your clipping the PI or power tubes then a similar thing applies. The signal being fed back is different from the signal out of V3 and this could also cause odd distortions. Many amps do it, yes. But all amps are different. And many overdrive much better with adjustment or removal of the feedback loop. If your power tubes are old and behaving badly this might also contribute to a similar problem. This may just be a common problem with this circuit. Not many people have the nerve to crank these amps up. So if this is part of the equation it would seem like a rare issue. I say check all the component values, including V3 pin 8 cathode. If all checks out then lift only the cap in the feedback loop. If the problem goes away try a different cap. If the problem returns just leave the cap out.