I also have a G50. I have done quite a bit of work on the modulator. The receiver has issues as you mention. The thing is dual conversion with a 2.3 Mhz 1st IF and 455 Khz 2nd IF. I am not sure whether the HF oscillator is high side or low side, but either way, this is an oscillator that free runs at around 6 m wavelength. This thing is going to drift, but what can you do? Well first would be to back out 50 plus years of aging, dirt, spider webs, bad caps, bad tubes, etc. Then I would look at V8 the 6U8, and try substituting a few other 6U8's. I also have a Gonset G76 which uses the 6U8 in the same place. Every 6U8 I've tried had a different sensitivity to line voltage change. After selecting the best V8, then install a small muffin fan inside the rig to limit the build up in temperature. Start with the easy stuff! If you expect to receive SSB with your G50, then I think you will be disappointed.
It is pretty sad to use a G50 with transmitter VFO, and try to break in to a SSB QSO
Jim
Wd5JKO
