Gibson GA79RVT - No Screen Resistors

[RightLurker]

I am tinkering with a ~1967 Gibson GA79RVT stereo amp. The schematic shows the screen grids tied directly to the OPT center taps (there are two OPTs), with no screen resistors. The circuit in the amp matches the schematic. The schematic also says the screen grids should measure 322v and the plates 317v at idle, which sounds backward to me. I have no doubt the schematic is correct, but can someone enlighten me as to what is going on in this circuit? Thanks in advance.

[sluckey]

That was very common practice back then, especially when B+ was not really high. Look at some more Gibson amps from that era to see even more examples.

[RightLurker]

Thank you, Sluckey. Good tip, and good to know.

[Stevem]

This arrangement was also done during a time / era where tube quality was far better, especially in terms of EL84 / 6BQ5 tubes.

If your working on this for a customer who is going to actually play it and needs reliability I would add 150 ohm screen resistors.

Such a mod should not change the tone of the amp and barely effect its power output.

[martin manning]

The schematic shows the screen grids tied directly to the OPT center taps (there are two OPTs), with no screen resistors. ... The schematic also says the screen grids should measure 322v and the plates 317v at idle, which sounds backward to me.

Note that the B+ is connected directly to the screens, but plate current passes through the OT primary, resulting in a 5V drop.

[RightLurker]

Thanks to all. I'm strictly a hobbyist. Years ago good friend picked the amp out of someone's road-side trash and gave it to me, sans speakers, knobs and back panels, but otherwise in remarkable cosmetic condition. I have a '62 ES-345, and I want a stereo amp to go with it. They should make a nice set.