[cathoderay57]

If the mains transformer doesn't have an earthed centre tapped winding for the 6.3v ac heater supply, a quick and easy fix is to try fitting a 'humdinger' as follows. First disconnect the chassis earth connection from one side of the 6.3v ac heater supply and fit a couple of 2W 100R resistors in series across the 6.3v winding, and earth the centre connection between the two resistors. This emulates a centre tapped winding and often works well. You could also use a high power wire wound 200 - 500R potentiometer if you can find one - a carbon track device isn't man enough for the job though. The slider of the pot goes to chassis and you can adjust it for minimum hum. Cheers, Jerry