Peavey C30 oscillation

[rrrip]

Hello again guys, nice to see this helpful place is up, running and full of nice people I already posted here a problem with this damn C30 a few months ago, though you probably don't remember me. But anyway.
The problem can be described quite shortly - when turned on, the thing sqeals instead of properly amplifying my guitar signal No guitar sound, just high pitch oscillation plus the usual little hum. The frequency of oscillation is decreacing during time, though I never ran it longer than 3-4 seconds. I've figured out the problem is after the preamp - it's not affected by any pots, it's still there if i insert a cord into the return jack, and the sound picked up from the send jack is fine. But the amp is silent if I pull the phase inverter out. Swapping 12AX7's around doesn't solve the problem, so it's not the V3 tube.
That particular amp had some soldering flaws around the power tube sockets, so I had to disassemble it a few times to resolder. One time after resoldering it started to work like this: 15mins of normal working, and then it cutted out and developed a faint oscillation, with frequency going down slowly. First minute after that it looked completedly like a bad leads connection/soldering joint - it cutted in/out if i moved the amp a little. Next it died out completedly. If turned off and cooled down, the amp began it's thing from the start - 15 mins running normal, then dying out etc.
After two times of disassembling/assembling in effort to sove the problem, the amp started to act as said previosly - just loud oscillation right from the start.
There's nothing bad looking on the board, no bad smell and all the jumpers are allright, so there's totally nothing I could suspect. I don't have the oscilloscope. The circut boards are facing each other when the amp is assembled so I can't even use a chopstick. Damn thing drives me insane, it probably spent more time disassembled and being repaired than actually working. I don't have much free time so now it's not working for, like, month and a half.
Please help me locate the source of a problem.
Cheers.

[Enzo]

I find it pays to be systematic disassembling those. Not bragging, but it takes me less than 10 minutes to get the boards out.

Are you sure all the jumpers are OK? Did you TUG on each and every one?

[rrrip]

Actually, I just used the multimeter Okay, I'll try some harder tests in the evening. Thank you.

[rrrip]

Done it. All the jumpers seem alright, the oscillation is still there

[Enzo]

OK, so much for the simple stuff.

All the tube heaters are wired in series, so if you pull one, they all go dark. So pulling the phase inverter tube kills the entire preamp. Actually the tubes are in two groups - the power tubes and the preamp tubes. Pull one of either group make the whole group go dark.

Look on the schematic bottom center, just to the right of hte power supplies. R61,R62 connect various grounds together. MAke sure neither of those is open. You'll have to hunt for them on the board. They are near the + end of C46, straight back from the VR3 POST control.

Does plugging something into the effects return jack make a difference? That separates the preamp from power amp.

Oh, wait. Unplug the output transformer primary leads and reverse the plug. Does that fix it?

[rrrip]

Oh, wait. Unplug the output transformer primary leads and reverse the plug. Does that fix it?

It does!! But... HOW? I thought the poweramp should be balanced - the tubes are amplifying neg/pos half of the signal, each pair; so it shouldn't be a matter which way the OT is connected? Or is something still wrong with the poweramp, even if it's working with this plug position?
Also, I thought I've always assembled the amp with the plug in the same position - the one that doesn't work now, and it did work. I'm not so sure about this now, though.

[Enzo]

The output stage is balanced, yes, but you are forgetting the negative feedback from the speaker winding. When you reverse the OT primary, it then becomes positive feedback, so it oscillates.

[rrrip]

Ah, of course. Thanks for the explanation!
Well, bad news here - previous issue with quiet oscillation came back after I put everything together. Probably there's still a bad connection somewhere there, since the amp worked with chassis sitting upside down on the main box, and stopped working after I installed the chassis back where it should be. The question is again what might be wrong with it The symptoms are now different - the oscillation is very quiet and is affected by the main guitar signal in some bizarre way. I recorded it, it's in the attachment. I turned the amp on with the guitar connected to usual input jack. After it warmed up a few seconds it started to make sounds - that's right where the mp3 starts. Next I play a few notes, then pull the guitar cord out and put it into return. As you probably will hear, it didn't stop oscillating, just changed the pitch. And you still can hear through the oscillation the guitar notes i'm playing.
If I pull V1 or V3 out, the amp becomes silent. Turning the volume pot doesn't affect oscillation volume, it affects it's pitch a little bit instead
Update: I've turned on the amp one more time, and it did work for a few minutes. Next it started crackling on some "resonant" notes, and ten seconds later started oscillating just as in the mp3.
What could that be I'm just as puzzled as when I've started the topic - the soldering seems allright, no signs of burnt components, nothing wrong! Sorry, but I need help again

[rrrip]

Ummm.. Totally no suggestions?

[rrrip]

Had some free time today, decided to wander arond the amp a bit. What I did:
- Resoldered again the socket pins - those that I resoldered just before the amp started to glitch.
- Checked the soldering at the signal path - from the C31 to the R55,52 etc, including cathode bypasses etc, not including the power supply - since all tubes glow allright.
- Checked the R61&62 ground connection resistors.
- Tried swapping the tubes
- Found (accidentaly) that if the amp is powered without a speaker the "oscillation sound" is still there - looks like it comes from the OT at least partly. When the speaker is connected the sound differs a bit, though.
- One time after i assembled the amp (partly), it ran and played fine for a 45 minutes or so, so the problem isn't completedly thermal. After that, I decided that if the amp will stay fine after one more reassembling, I'll leave it and will think it's fine. It won't.

Actually, I've got only one idea left - to swap all the parts after the return jack with the new ones. But it's a bit hard to get the needed parts, so i'd really want to try everything else before that...
Any ideas and help will be greatly appreciated!

[Amp Kat]

You need to isolate the problem from preamp to power amp and find out what is causing it. Sounds like a preamp problem to me and maybe an open wire or even a short. Hook up to the input and take the send out and go to the input or return of another amp and see if it still does it. If so the problem is in the preamp and would be around the 12AX7's. If it doesn't do it the problem is from the return jack to the power tubes and could be verified by taking the send from another amp to the return of the C-30 and it should squeal. Remember the power tube heaters are in series so if you remove one it kills all of them and you'll get no sound. The noise I'm hearing on the MP3 does not sound like an oscillation from the OT but a bad connection or something going somewhere that's not supposed to be. What is your province ?

[rrrip]

I've done that already - it seems like the problem is after the return, somewhere around the phase inverter and power section - the signal from send jack is allright, squealing volume isn't controlled by preamp volume pot.
Sorry but what do you mean by province? Under my nickname it's stated that I live in the cold icy Russia

[Enzo]

KB is from Louisiana in the far US south. Folks down there sometimes get Canada and Russia confused.

I keep them straight this way. The Canadians drink the beer, eh? and the Russkies drink the Wodka.