Help with 120Hz hum issue on Bogner Clone

[versital]

I Built 2 Bogner uberschall clone amps and they both suffer from a 120hz hum. Amp sounds great minus a loud 120HZ Hum I can’t get rid of. The Hum can be heard with all volume pots off and gets louder as you raise the volume. I’ve been trouble shooting with one and I have done the following things trying to fix it.
Upgraded and replace Filter caps
Upgraded and replace Choke
Replaced Preamp Tubes & Power tubes
Repositioned OT orientation
Upgraded and replaced Bias rectifier, filter caps
Upgraded and replaced DC heater circuit for preamp tubes
Removed ground loops with FX loop and Footswitch jack
Revised Grounding paths
Upgraded Switching relays
Retraced all solder and wire connections points

I assume its related to the B+ power supply after rectification and can be heard with volume off but not in standby. It appears once Amp is warmed up.
I as a last effort I started replacing all the 180k and 100k ½ watt metal film resistors feeding from B+ and replaced with 2w Metal Oxide Resistors. The result was it lowered the Hum by 25% in general and 80% on the clean channel.

Does Anyone have any other ideas what will cause a 120hz hum??????.

[Enzo]

Yes, stop throwing parts at it. Especially in home built amps, it is way more likely a problem in ground wiring.

[R.G.]

There is one common problem that is not obvious at all, and requires a specific fix. The first filter cap and the wiring to it is critical. The negative side of the rectifiers (in a full wave bridge circuit) or the CT of the power transformer (in a full wave center tap circuit) must go to the negative side of the first filter cap - nowhere else. The DC and signal grounds leave that same point to the rest of the amp and for a ground to the chassis. If the transformer ct or rectifier (-) goes anywhere else, you will get 120Hz buzzy hum that cannot be cured.

There are many other hum sources, but this one is subtle, and maddening. It cannot be cured any other way. The higher the amp's gain, the worse this is.

[Mick Bailey]

Have you tried removing the PI tube to determine if the power amp hums on its own?