In my amp the issue was obviously the multiple chassis to ground bus connections.
Actually, isolating the speaker jacks from the chassis did not make any difference.
Also the jacks for the effects loop (which was not populated yet, except the jacks were grounded to the ground bus so making a ground bus - chassis connection) on the back of the amp did not change anything.
But isolating both inputs from the chassis changed everything!
Hum basically gone.
The remaining hum has a 100 Hz peak now, but very low level.
After the two inputs were isolated, I changed the ground bus to chassis connection (only one now) from the power plug to the first (leftmost) input (in the opposite corner of the chassis, front left instead of back right).
There was another reduction of hum (according to the spectroid app) of 3 dB. Wonderful!
My amp has one Marshall JMP / Bassman channel (cathode follower, the grounded one now) and one Fender Blackface channel without reverb.
If I turn up the JMP channel the hum will get louder the more I turn the volume up. But now by an expected level.
Different with the Blackface channel. The hum will get louder till the volume pot reaches 3 o´clock, then it get´s less again if I turn it up further.....
Any idea for this behaviour?
Isolation the inputs (and I isolated the effects loop and the speaker jacks as well) I had to change the jacks to "Marshall style".
https://www.tubesandmore.com/products/1 ... -metal-nut (I used the Neutrik brand, looks the same though)
The Switchcraft 11 and 12 have "only" about 7 mm of thread, my chassis is 3 mm aluminium, the face and back-plate 1 mm each, and the isolation washers (Switchcraft S1028 / 9) have about one mm each (front and back).
Adds up to about 6 mm, no way for the Switchcraft.
Guess I´ll get used to it
Signal return ground:
very interesting!
In a Blackface channel:
After V1 the plate is connected to the tone stack.
The next ground is the mid resistor/potentiometer of the tone stack.
So the ground sides of the V1 cathode resistor and the tonestack mid resistor / potentiometer should "meet" at the ground bus?
My mid pot is like 2 inch away from the cathode of V1. Should I use a 2 inch wire to accomplish that?
Thanks a lot for all other hints as well!
I actually did try elevating the heating in a different project, that was raising the hum (at least in this case).
Stephan