I just bought a 250w GP12 SMX from 04/1993, which had some hum issues
It hummed even at pre and master at 0, I did more or less what lenky76 said here: Earth switch-Trace Elliot GP12 SMX
but instead of routing the new cable that way, I used a stereo shielded cable (shield connected to chassis) which I routed along the front of the amp (under the pots), the left side and finally to the input of the pwr amp.
Hum is 99% gone, not worse that a good GTR valve amp with pre and post at 0.
I discovered that the trick was to keep the preamp/pwr amp connecting wire as far away as possible from the 4700uF caps.
So my question: are those bad? I cannot think that the amp left the factory humming that way.
Second question: when I turn it off, after 15/20s it pops 4/6 times (each pop 1 s away from the previuos one). Anybody solved the problem?