For those that looked at my other thread, this is the same amp that blew 2 rectifier tubes about 80 hours apart. The offending amp works normally otherwise but emits a very noticeable sound from the speaker audible from feet away. It sounds more 60hz to me than 120 hz but I cannot really be sure. I tried swapping rectifier tubes. No effect. Both amps are plugged into the same variac, swapped plugs. No effect. I moved other equipment away from the amp. No effect. I removed the small tube that's in the board. No effect. I tried removing the input RCA, No effect. Bias adjustment doesn't has no effect. When powered on the amp is silent until it warms up for a moment then the humming starts. In practice we'd normally might not notice this because we usually start streaming before the amps are on anyway. Could this be a bad can capacitor which might also explain the blowing rectifier tubes? Is this dangerous to run? Could it harm other components? Any help appreciated.