I have a very low volume 60 cycle hum. This is with No tubes at all in the amp. It has to be AC getting into the output some how. I have tracked down all sources of AC that I can think of: Transformer mains are putting out 320vac each, bias is 50vac, heaters are 5.4 volts & 6.5 volts. I have done the following: Rechecked grounds, HV center tap, Heater center tap, Bias ground. HV center tap is grounded at first filter cap. Tried different house circuits, no change. ( other amps don't hum at any house location) My PT & OT are rather close so I shielded with copper foil & metal plate both transformers, no change. Paralleled another 50/100 bias cap with the current two caps, no change. Power supply caps all read good, and grounds seem good. Ran heaters off of a 6 volt battery, no change at all. Re-soldered the entire rectifier leads. I can probably live with this hum as you need to be within 2 or 3 feet of the amp to hear it and it does not change with volume, when tubes and guitar are connected.. I would love to know how this is occuring, very strange.