If the amp can be cycled off and on with the hum going away for a while, its not a bad solderjoint.
Tubes can do this if they have a cathode/filament short. But the problem is in both channels so I don't think its that.
A filter capacitor in the power supply could be failing. I've seen them act in exactly this way, and the amp is old enough and they didn't have very good filter caps to begin with, so I think that's the problem. This should be attended to- it will only get worse and could damage the power transformer if the cap is also leaky!
Tubes can do this if they have a cathode/filament short. But the problem is in both channels so I don't think its that.
A filter capacitor in the power supply could be failing. I've seen them act in exactly this way, and the amp is old enough and they didn't have very good filter caps to begin with, so I think that's the problem. This should be attended to- it will only get worse and could damage the power transformer if the cap is also leaky!