Knight of Cups wrote:
Make r10 a 25k pot.
I was thinking about this while doing some things this morning and remembered something I should have said earlier. By turning R10 into a 25k pot, effectively you'll be taking the champ from whatever NFB ratio it has now, to "max" negative feedback. From an operational standpoint this may cause the amp to oscillate (reference: http://www.aikenamps.com/GlobalNegativeFeedback.htm
, this is in reference to a push pull output stage, but the concept is the same); you would want to test to see if the amp oscillates with full NFB, the extra phase shifts in a PP stage will increase the likelihood of oscillation over a single ended stage like what the Champ has. From a tonal standpoint, this will reduce the output level and make the amp more "controlled" because of the increased speaker dampening.
IMO, it would be more interesting to look at the other end of the spectrum, using less NFB. To see the max of where this could go simply remove any of the wires in the NFB path. If that sounds good, then find a resistor that's large enough to give you something similar to that sound. maybe 100k, maybe 250K (keep them similar to stock pot sizes, and my guess is that you'll want these to be log (audio) pots). Cool, that's the minimum NFB you'll want. Now remove those and find the max NFB by subbing in smaller resistors, like the stock value of 22k, or see how much more NFB you can get away with/what sounds good, maybe reduce R10 to 10k or 4.7k (these are just suggested values I'm pulling out of thin air, experimentation will yield useful values). Cool, now you've book-ended the "no" NFB condition, and "max" NFB condition, so put the pot (wired as a variable resistor, "NFB Pot" on the schematic) and the resistor in series, now you have variable NFB that swings either side of the stock value for a more raw or more compressed sound. A pot with a switch in it would be cool to use if you want a true "no" NFB condition, or wiring a stand alone switch ("NFB Switch" on the schematic) in series.