Fender Blues Deville - Intermittent Red Plating

[Grantorino]

Hello again EL34 world.

A Fender Blues Deville had a feedback issue that was caused by a microphonic preamp tube. Replaced the tube and the issue is gone. However, when the amplifier was powered up yesterday on initial inspection, the power tubes were red-plating and the negative bias on pin 5 of the sockets was at 0 V. Today, having changed nothing in the bias supply circuit, the bias is now stably back at -52 V. This is the original Blues Deville, with a fixed bias.

I'm looking for advice on the most prudent way forwards - disassembly and inspection of solder joints? Replacement of diodes in the bias circuit? I tried switching the power tube positions and could not recreate the red-plating issue. I'd be concerned about installing new power tubes until the underlying issue is remedied. Unfortunately I do not have a tube tester to test the tubes, which are 6L6GC.

Thank you.

[tubeswell]

Seems like you have a dodgy connection somewhere in your bias supply. Take the 6L6s out and don’t put new ones back in until you have found and fixed the problem.

[shooter]

disassembly and inspection of solder joints?

yep, dig into ALLLLLLLLL the "issues" Fender has with those amazingly cappy PCB amps
if you want to hone your troubleshooting skills, ya got the right amp
NOTE: some Psychological damage may result 

[Grantorino]

Thanks chaps - I will disassemble and get stuck in.