In article , Mike Coatham
wrote:

Check the faulty amp to see if it has a ground lift resistor or
capacitor that has gone short circuit. This would explain the sudden
onset of hum from that amp. It shouldn't be that hard to find as you
have another amp to compare with.

That was also my reaction. However you may be unlucky. It may be that your
amp's transformer has an earthed 'electostatic shield' and it has become
shorted to the windings in a way that gives 0 Ohms. That can mean
significant earth currents that should be flowing in the Neutral. Not good.

Slainte,

Jim

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