ez80

[Tony Duell]

> Make sure neither valve has a heater to cathode leakage.  Insure the DC 

It used to be common to run the rectifier heater off its own winding, and 
tie one end of that (or the centre tap if there is one) to the rectifier 
cathode. Then a bit of h-k leakage does little if any harm.

> bus is clean
> as a common old valve gear problem is dried up electrolytics. Try 
> grounding the other
> side of the filament  string or float the heater string (insure neither 
> side of the heater

The better power transformers have centre-tapped heater windings 
(3.15-0-3.15V). Try earthing the tap (rather than one side).

> winding on the power transformer is grounded.  I've also put a 100ohm 2W pot
> across the heater winding and grounded the slider to be able to balance 
> the heater

Commonly called a 'humdinger' in the UK :-)

> voltage against ground.  If all else fails run the heaters on DC.

Certainly for testing. RRun the heaters off a battery. If the hum goes 
away, at least you know where it's coming from.

-tony