5F1 hums, quiets down when volume at maximum

[motzu]

Motzu: After reviewing your pictures once more, I have a question: what is the surface of your bench covered with? It looks like some kind of felt?

Ah, not a workbench. I was just working "quick and dirty", right on the floor at that moment. Which I wouldn't recommend to anyone, if only because it gets very uncomfortable on your back after quite a short while. But I though that not much more work needed to be done at that point so I didn't move my work somewhere more convenient.

So, not a workbench/felt, just a somewhat nasty-looking rug.

[jmaf]

I'm sorry, I've missed this post, didn't mean not answering.

No sweat, glad you ended up solving it.

Just a note: If the amp is properly isolated from mains, reversing the leads does not change anything electrically.

But.

Reversing the leads reverses the polarity of the magnetic field and eddy current induced in the steel chassis by the power transformer. Your hum was ground current on the chassis, due to a difference between the circuit ground and the induced current.

Just recently there's been a discussion about something related, hope this helps: http://music-electronics-forum.com/t20415/ - Elevating the circuit ground from the chassis just a bit, through a resistor, and grounding AC through a cap kills this sort of hum instantly.

[motzu]

Reversing the leads reverses the polarity of the magnetic field and eddy current induced in the steel chassis by the power transformer. Your hum was ground current on the chassis, due to a difference between the circuit ground and the induced current.

Just recently there's been a discussion about something related, hope this helps: http://music-electronics-forum.com/t20415/ - Elevating the circuit ground from the chassis just a bit, through a resistor, and grounding AC through a cap kills this sort of hum instantly.

Thank you for the explanation! That thread is quite interesting.