Blowing Filaments?

[Mark Black]

That "-38V" figure seems mighty low for 6L6's to me, especially with plate voltage that high. More like EL34 territory...

I would check values of the resistors, function of the diode, and probably just replace the filter caps in the bias supply to start with (make sure the caps go in with the correct polarity!).

Does it look like anyone has fiddled with the resistor that is soldered to the body of the bias balance pot? Changing that will change the bias voltage (bigger value resistor = more bias voltage).

I'm also suspicious of the 270V screen voltage. Should be close to the same as the plate voltage if the circuit hasn't been hacked and the tubes are working.

[Tom Phillips]

...I just thought that since the bias circuit in this amp has been untouched, that -38V would be acceptable. The tubes that cherry plated were installed in the Peavey Butcher before, and that thing totaled about 20.5 plate watts before I installed an adjustable bias, so I figured they could handle it.

Nope. That’s not a valid assumption. Based on the voltages you posted I’m not surprised that the tubes are red plating. I would expect the bias voltage to be much more negative than –38V. IMHO something is not right with the existing bias circuit. If the amp were mine, I would cut to the chase and put in an adjustable bias circuit. Then add 1 Ohm cathode resistors to allow easy measurement of the current as you adjust the bias. Are you familiar with that? If not we can point you to existing further information. You need to be able to measure the plate current (or cathode current) and calculate the actual dissipation with the tubes that are installed.

Regards,
Tom

[apehead]

Yes, I am familiar with both of those modifications, and I planned on performing them as soon as I had received some corrospondence from the jerk who sent me this broken POS in regard to partial refunds, retubing costs, etc. I just got a little impatient, and wanted to hear what it sounded like .

Also, the whole amp circuitry looks completely untouched, so I think this high voltage is the result of component failure.