Last night, after many hours of use, the amp began to hum, not through the speakers, but the thing itself. Today I started troubleshooting by measuring Bias. It measures only 0.45 volts instead of 1.56! When I adjust on the pots, the value does not increase above 1 volt. The hum varies with the adjustment. Tried to replace the rectifier tube but no difference.
Any help on troubleshooting is very much appreciated!