Gibson GA-9 NFB

[Colossal]

Hey guys,

I am finally getting around to cooking up a Gibson GA-9 derivative. The stock GA-9 preamp topology is similar to a Champ with V1a at 100k/1k5/22uF, V1b with 100k/1k5 (unbypassed), and 0.022uF coupling caps. Basic, low-loss Volume and Tone with A1M pots.

I am modifying those values to keep the low end and overdrive tighter, but essentially, quite similar. One difference with the GA-9 is that it injects 22k NFB back into the output stage at the 220k grid leak, where a Champ does so at V1b's unbypassed 1k5 cathode resistor.

I have a little space on the board where I will add in a trimmer in series with the stock 22k NFB resistor to vary the amount of NFB. I'm sure this will be set-it-and-forget-it, but just wondering if any of you guys have dabbled with this? Usually a variable NFB mod in a Champ amounts to a "presence" control.

I've attached the schematic for reference.

GA-9.gif

[sluckey]

That's quite different from the GA-9 schematics I've seen...

https://el34world.com/charts/Schematics ... on_ga9.pdf

[Colossal]

Steve, it's probably closer to the GA-8, but the schematic above is close to what I'm doing. Parallel single-ended power amp, with NFB at the grid leak. I just wanted to illustrate the NFB difference vs. a Champ.