Power supply ground buzz....

[dave251]

Power supply ground buzz; totally irrelated to volume. In fact the amp is quiet until the standby is switched on and the amp warms up for about twenty minutes.....it's quiet when the standby is first switched on and sounds great. If the standby is flipped off and the amp let to sit idle for a half hour or so, it's again quiet until the circuit "warms up" again.

I'm assuming a cold solder joint? Any other likely causes?
Thanks....

[Enzo]

More likely a tube, way more likely. Do any of the power tubes start to glow red hot inside as the hum comes along?

Try a different set of power tube that you KNOW to be OK. Doesn't matter if they are old, as long as they work. If the amp no longer does the 20 minute hum thing, then you verified the existing tubes as bad.

Monitor the bias voltage at the power tube grids as the hum comes in. is it the same as when the amp was first turned on? or has it dropped? Measure the ripple on the bias supply and on the B+ rails. Possible leaky filter. ANy of the filter caps showing white stuff oozing out the vent hole?

[R.G.]

Dang, Enzo, you've done this before haven't you?

[Enzo]

A time or two...