Some mechanical humming from power transformers is not unusual even under normal circumstances and results from flowing current. So, electricity and mechanical humming are definitely associated. Sounds like he described the mechanical hum as varying with the setting of his bias pots. The varying mechanical hum sounds like a symptom of another problem, probably whatever is causing his biasing difficulty.
If the amplifier was working fine for some time and then this problem suddenly came up, it means some part or parts, including possibly tubes, in the amp are failing. Questions:
- were there any changes to the amp shortly preceding the onset of the current symptoms?
- is a 3A fuse in the fuse holder, and not more than 3A?
If the problem is in one channel only, I'd try swapping the tubes from one channel to the other and see if the problem follows the tubes.
I'd also still refer back to Post n°13.