Crate Vintage Club 50 Redplating

[judgeamps]

Hey all, long time lurker.

A customer brought me a Crate Vintage Club 50 that wasn't putting out a signal. I found the 1/2 amp fuse coming from the standby switch was blown, the standby switch itself wasn't operating properly, and a few filter caps were visibly bulging. Replaced all of those things and everything looked good. After testing for about 5 minutes, all 4 power tubes red plate.

Schematic attached.

Here's what I've found:

Tried new tubes, same issue.
All Test Points are pretty much spot on.
I suspected a bad bias, but that section seems pretty good. I'm getting -15.6 V on pin 2 of the EL84's.
I replaced R52 because it looked questionable. (crumbling)
I tested the coupling caps (c27 and C28) coming off of pin 2 and they also test out fine.
R58 going into the standby switch differs from the schematic. It's 220k (schematic calls for 150k and I don't have that value on hand) Looks like it could have been replaced at some point in it's life. Would that cause red plating?
I'm not seeing any cold solder joints

[sluckey]

R58 going into the standby switch differs from the schematic. It's 220k (schematic calls for 150k and I don't have that value on hand) Looks like it could have been replaced at some point in it's life. Would that cause red plating?

no

[judgeamps]

Thanks, sluckey. I was 99.9% certain, but wanted to make sure I wasn't having a "brain fart."

I also realized I swapped the standby connections J31 and J37 when re-attaching.

[judgeamps]

Update:

After about 5 minutes, voltage at TP 17 starts to increase (moves from -15.5 to -15) and starts red-plating.

Out of curiosity what's the point of C54? 470pf 500v ceramic cap.

Thanks in advance.

[sluckey]

It does just the opposite of C40.

[sluckey]

After about 5 minutes, voltage at TP 17 starts to increase (moves from -15.5 to -15) and starts red-plating.

I would pull the output tubes and monitor TP17. If the voltage still changes it means that the coupling caps (C27 and/or C28) may be leaking. Or there is an issue with the bias supply, most likely C49.

[judgeamps]

I thought the same thing and replaced all 3 of those components this morning. Same problem. Voltage at TP17 doesn't seem to budge when the power tubes are pulled.

Thanks again.

[shooter]

are the tube sockets mounted on the PCB?

have you looked at the underside of the PCB, I've found interesting things there before

[judgeamps]

Oh yea, I've been spending a lot of time under this darn PCB. Haha. Re-soldered pretty much everything.

[judgeamps]

Seems like one of those replacements did the trick and I just didn't realize it because this amp runs so hot. I let it idle for about an hour and TP17 is always around -15.54 which is .06 off of the chart on the schematic, but no red-plating.