This circuit starts with the Fender AB763 circuit, as used in many of the Fender amps from the "Silverface" era. In front of the first triode section, which would be the guitar input on an actual Fender amp, I place the "gain" pot; this allows the gain to be trimmed down, because this section will normally be fed from the input section, which also contains a similar gain stage (i.e., more gain stages than the actual AB763). Alternatively, this arrangement allows more gain/overdrive to be obtained than an unassisted Fender amp could normally produce.
But for even more gain, I provide two more triode stages with a second tube, accessible with the "pull 2-stage" switch. The first of these triodes is set up as a cold-clipper, similar to Marshall. There is a "pull alt voice" switch, which increases the cold-clipper cathode resistor, and also changes the R-C circuitry between the two triode sections. The gain into these sections is controlled by the second section of the dual-gang "gain" pot; thus, this pot has a multiplying effect on gain when in 2-stage mode, which should give a wide range of control over the distortion tone.
The "pull raw" switch, borrowed from other Fender modification websites, effectively eliminates the tone-stack, producing something closer to the so-called "Tweed" tone. Gain is higher without the insertion loss of the passive tone-stack; and since the tone-stack fundamentally causes a "mid scoop" no matter what its settings are, the "pull raw" switch leads to more prominent midrange.
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