Peavey Classic 100 - Hum For First 15 Min Then Plays Fine

[Jecarroll7]

Eight EL84 tubes in very close quarters, who needs a space heater.

Loud hum for about 15 minutes, then it's fine. Killer tone BTW.

Power supply caps and voltages look clean, hum doesn't respond to component cooler or tapping with a stick. I know unbalanced tubes could cause this, but haven't seen it stop after a 15 min warmup before.

Has anyone played around with these amps?

[Jazz P Bass]

Do any of the controls affect the hum?
How about turning reverb on & off?
We need a hint.
Is it consistantly a 15 minute time?
When it's cold.
When it's hot.
Do you gave any spare EL84 tubes that you could swap around?
Even one would do.

[Mick Bailey]

Take a really good look at the valve base solder joints and inspect for ring cracks.

[Jecarroll7]

Mick,

Great suggestion (checking valve solder joints), although someone beat me to it - it did point me in the right direction. Turns out R53 (V7) was burnt beyond band recognition (although still checked out with DVM cold). I replaced it and pulled/tested all the power tubes with a Hickok tester. V7 had significant leakage so I stopped testing on that tube. Of the remaining 7 only one hit the minimum mutual conductance of 475, the rest were between 100 and 450. In the preamp V1 tested weak as well. I suspect a new set of tubes will address the hum issue and I ordered those this evening.

I haven't checked into biasing a Classic 100, any thoughts there?