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Ok, here's what I've figured out so far... the tremolo circuit on this thing works something like this: off the plate of the first 1/2 preamp tube, the guitar signal passes through a capictor to a 5.6meg resistor in parallel with the photo-resistor side of an optocoupler. the neon lamp side of the optocoupler is part of an oscillator circuit driven by the 2nd 1/2 of the tube. the optocoupler works so that it is ON when the trem is off, which means that the resistor side is low-resistance and the signal passes around the 5.6m resistor to the tone-stack fairly easily. when the tremolo is ON, the lamp flashes on/off changing the resistance value low/high, causing the effect.
Note that the above description is correct to the best of my knowledge, but I'm no expert in interpreting schematics...
When I got my amp, the optocoupler was not really working, so the resistance of the photo-resistor was always stupid high. For kicks, I bypassed that part - and the volume of the thing was huge! After that, I looked all over for specs of the Ampeg part, to see what the right value would be... which turns out to be in the ballpark of 200k ohms (maybe you could measure yours? it's a little tricky to do, though). Effective resistance of the 5.6m and 200k resistors in parallel gets you something like 190k, which really tamps down on the gain from the first preamp stage. Taking a cue from the Fender replacement part (with an ON resistance of about 40-50k), I detached the photoresistor side of the part and soldered in a 56k resistor. Pretty good volume jump. I've been too lazy to buy the fender replacement part... but now I know that there's a lot of gain potential in there that just wants to be free.
Too much info? anyway, easy mod for me to do since I already need to replace that part, and the only real replacement option will have this effect whether I want it or not... probably not worth it if you already like what you're hearing.
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