"Chris Morriss" wrote


Do you think the hum is coming from the directly-heated filaments of the
300Bs, or from the HT line? (Heater hum is likely to be 50Hz, HT hum is
more likely to be 100Hz)


It's looking like the hum is 100Hz, but my scope telling me that it's
left-channel weighted. (If the scope can be trusted.....???)


If it's the filament, then you could try using a resistive centre-tap on
the winding and earthing the CT, (or taking it to the bias circuitry or
whatever).

You may have a CT filament winding already of course....


The filaments have their own transformers where the CT (if it is one - grey
and blue secondaries twisted togetther??) is taken up to a 200V rail which
it shares with the cathode resistors and bypass capacitor as in:

http://www.apah69.dsl.pipex.com/keit.../ampkit124.jpg

The red and yellow secondaries supply 2.5V @ 3A directly to the filament
(measured about 2.69V, IIRC)