Well, the GA20 was serviced and restored today with excellent results. I had an excellent day because I got to watch every move and do some small chores to keep the process going.
Some basic stuff was done first. The awkward thin power cord was replaced with a modern one and the chassis is now connected to earth. I got a new external 220-110V transformer that could power three of these amps on full tilt so it's running nice and cool. The inside of the back panel was covered with metal foil for screening, which helped to reduce noise. The valve sockets were cleaned. Some were filled with an earwax-like substance that had leak out of the busted main filter caps.
The tech then measured almost every seperate part to assess if it still performed somewhere near the specs as in the circuit drawing. Apart from replacing the main filter caps all other electrolytic caps were replaced too, as well as some carbon resistors that had drifted too much. Funnily enough it turned out that there was a dead resistor just in front of the driver valve so the amp was running only at half power all the time 8) For nostalgic reasons we cleaned up and refitted a strange sand resistor in the power stage that is common to Gibson amps. (I first thought it was rotting or something but I found out they were supposed to look like that). We left the tremolo alone for now. It doesn't work properly yet but repacing the 6SQ7 might fix it so I'll try that sometime soon.
I'm not gonna ramble alot about the sound but boy.... The nasty noise and hum is gone, the amp is literally twice as loud as before and it sounds much livelier. It will easily handle bar and medium sized venue gigs, it's incredibly loud for it's demure size. The single tone control works nicely. From very dark to normal on the first half of it's travel, from there the sound loosens up and the volume increases. With my BKP Casino and Banzai booster it's instant Neil Young now. Will try and post pics and a soundclip soon.
