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I've been working on a 1971 Plush P1000S guitar head, and I thought I'd post some of what I've found. For those who don't know, it's pretty much a copy of the AB763 Twin Reverb with improved transformers.
Rumors of sketchy build quality are true. After doing the general repairs like recapping electrolytics and replacing drifted resistors, I had to go through the amp carefully to find the time bombs: joints that were never soldered properly at the factory and wires that were nicked when stripped and have weakened over time to the point where they're ready to break off.
The available schematic shows a tube cathode follower line out, but this one has a later transistor emitter follower (single BJT) line out that uses the bias supply as its power supply, so it runs at a negative voltage relative to the chassis. The downside of this approach is that because the bias supply isn't nearly as well-filtered as the B+ supply and because solid-state circuits don't have the input/output isolation of tube circuits, it feeds a lot of PS ripple back into the phase inverter, raising output hum dramatically. I'm going to recommend disabling it to the amp's owner, unless he really needs the line out, in which case I'd recommend retrofitting the tube cathode follower. It looks better on paper. At least it looks less likely to couple PS ripple back into the source. I guess the SS line out buffer saved a few bucks.
Another quirk of this amp is that it pulls the current for the preamp stages from the chassis through the input jacks grounds. I can't remember seeing that before.
Rumors of sketchy build quality are true. After doing the general repairs like recapping electrolytics and replacing drifted resistors, I had to go through the amp carefully to find the time bombs: joints that were never soldered properly at the factory and wires that were nicked when stripped and have weakened over time to the point where they're ready to break off.
The available schematic shows a tube cathode follower line out, but this one has a later transistor emitter follower (single BJT) line out that uses the bias supply as its power supply, so it runs at a negative voltage relative to the chassis. The downside of this approach is that because the bias supply isn't nearly as well-filtered as the B+ supply and because solid-state circuits don't have the input/output isolation of tube circuits, it feeds a lot of PS ripple back into the phase inverter, raising output hum dramatically. I'm going to recommend disabling it to the amp's owner, unless he really needs the line out, in which case I'd recommend retrofitting the tube cathode follower. It looks better on paper. At least it looks less likely to couple PS ripple back into the source. I guess the SS line out buffer saved a few bucks.
Another quirk of this amp is that it pulls the current for the preamp stages from the chassis through the input jacks grounds. I can't remember seeing that before.
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