Originally Posted by
steveinfrance
Certainly not genius but anyone who has a Yaqin PA can completely eliminate speaker hum. Set the bias to the low end of whatever it should be (500mV for a 100B) when the things been on for an hour then connect headphones to one output or bring the speaker to the amp so you can stick your head in it.
Leave the DVM connected to one of the test points and very carefully turn that bias set trimmer until the hum disappears. All you're doing is making both KT88 current draws identical so any ripple in the HT is cancelled out in the OP transfo.