Peavey VT classis cuts out at high volume.

[flashmiles]

The amp cuts out when at the master volume is above 3 or so and is sent a loud signal. There is also some quiet oscillation in the background when using the automix channel.
I recently replaced the caps in the phase inverter, as it was red-plating.
But now this?

I would have assumed the caps to the output transformer were to big, but I've checked the value several times.

[stokes]

The oscillation is the most likely cause of the amp "cutting out".If the PI was red-plating as you describe,it would most likely be a problem with the cathode resistors probably being shorted,although I've never seen a PI tube red plate.What caps in the PI are you refering to?By the caps to the output tranny I can only imagine you are talking about the filter caps.Are they stock?Did you change them for some reason?In any event you need to find the source of the oscillation, if you can,if not get the amp to someone who can.

[Enzo]

I could be wrong, but I think it is his power tubes red plating, so he changed the couplers from the PI, in case they were leaking +DC through to the grids.

And the PV Classic doesn't have a PI tube.

If you changed the caps and used the same values, then they are not too large. The power supply is solid state anyway, so that is not an issue.

The VT series has a preamp out jack, so monitor the signal there. Does the signal remain during the cutout? Or does it disappear there too? We want to isolate the problem to the peramp or the power amp.

[flashmiles]

Sorry about the terrible spelling and grammar in the last post.
Enzo was right on all counts.

Running the preamp out into a SS practice amp(Damn good idea, wish it came to me earlier) with the power tubes removed, has the signal cutting out just as before.

[Enzo]

OK, then it is not the power amp.

With the power tube back in, you can also verify the power amp. Plug a signal into the power amp in jack and wail. The power amp should sound clean and strong.

You have the VT, not the VTX, so it uses op amps

Isolate the problem. it does it on Automix, now try input to only the Normal input, then also try it plugged only into the Bright input. if it does it in both, then it is something common to them, if it does it in one only, then obviously that channel holds the problem.

Is the power OK? Are both +15 and -15 clean and present at all the op amps? Pins 8 and 4. Are any op amp output pins offset DC from zero?

PLug a cord into the preamp out jack and use the free end to measure for DC voltage. SHould not be any. If DC is present one of the clamp diodes is leaky.

Note, lots of 8 leg ICs, but lot of them are not op amps. The TL604 switching ICs and teh 478 OTAs are not op amps.

Apply a signal and turn it up to cause the symptom, and scope the signal stage by stage to see where it disappears.

[flashmiles]

OK, with the preamp cranked a DC voltage appears. It varies with the amount of signal going in, to a maximum of 100mV for a hard strum from a CIJ Mustang, both pickups in phase.
I assume the clamping diodes are the ones, pseudo schematically, from:

-15v ->output-> +15v

Right before the 680R output resistor?

[Enzo]

No signal, and 100mv wouldn't matter anyway. I was looking for several volts. yes, those diodes. What you measured was your signal.

My idea was that several volts offset might be OK for the signal at low levels, but gets in the way anything higher. Not here apparently, but do go through the amp swith those other tests and look for DC offset on the various chip outputs in the signal path. Again, the DC measurements with no signal, and ignore those of a few millivolts.

[flashmiles]

Thanks for your help.

There is 21v coming off the positive rail, and 16v on the negative.
The caps have 26v over them on the positive rail, 27v on the negative.
The 5w 100ohm resistors have a 10v drop on the negative rail but only
5v on the positive rail.
I assume the resistor is at fault, it looks old though
not obviously damaged.

[Enzo]

Aren't those resisztors feeding zener diodes that set the voltage? I'd replace any 15v zener that had 21 volts across it. The resistor is likely just fine.