Original WAD 300B pp Question

[david]

Ok you guys with the original 300b pps, I ahve a question for you:

Clive Meakins and I met up last night to take our scratch builds forward. I also got hold of a pcb via Clive (thanks Greg!).

If I look at the circuit diagram, then the heater circuit for the 6072 and 5687 is simply connected to the floating ac from the mains tx. However, the pcb is clearly designed so that the heater ac is earthed via Pin 9 of the 6072, thus sharing the same earth run as the input valves. This seems a sure fire recipe for hum to my mind and I gather some of you building the amps did have problems with hum.

So, my first question is why not leave the heater for these valves floating as it is in many other designs and as indicated by the circuit diagram?

I would certainly like to try this option rather than opting for a DC heater solution in the first instance.

Thanks
David