Some more experimenting:
Swapped the boards, both wired that way:
(The configuration in which channel alone is quiet)
Output ground - output jack
amp_ps ground - ground on amp board
heater_ps ground - star ground
SG on amp board - star ground
GLB between SG and mains ground on the AC-inlet
The hum is still on the left side after swapping the channels.
So the "left" board is not the problem.
Well, this conlusion seems wrong or at least not complete...
Having everything wired like the "alternate wiring scheme",
now the right channel exhibits the hum. (The board that was left before...)
This is really crazy.
Having all wired after the normal wiring scheme the hum stays right.
Edited by Sathimas - 1/12/14 at 6:13am