I posted this on gearpage also, just to clarify:
One thing that came up in discussion is the idea that there was clipping in the FET boost circuit. Like a typical pedal circuit. Actually, there is no clipping at all in the boost circuit. It just increases the guitar signal hitting the first tube gain stage. Any extra "gain" AKA clipping is happening in the tube stages.
This boost circuit uses the same high voltage FET as our Zero Loss FX loop. In the loop circuit this unique FET allows full signal to pass from tone stack to phase inverter. Up to 90V P-P signal. Hense, the "zero loss" moniker.
By comparison, a typical chip based loop passes up to about 15V P-P max.
The catalyst for the boost circuit was a sim Steve Miller did in LTSpice emulating a typical Marshall input tube stage. His analysis led to a FET based circuit that behaves like a tube input stage.
Steve offered this to me and I tried it. Very, very cool. But, not exactly what was needed for the boost in this amp. So I tweaked it quite a bit and added a high cut and low cut EQ.
Steve's original circuit was capable of about 40db of gain, far more than necessary or useable. I lowered that to 22db and this helped optimize signal to noise ratio.
Hope that helps clear up and misconceptions about what this boost circuit does. And also gives all due props to Steve for his great design work and generous sharing off all things technical. I've learned a lot from him over the years.
george
