This circuit is the same as the Marshall 2203 from the "low gain" input onward. Thus, when the input section "voice" control is set appropriately, the overall signal chain is similar to the 2203 from the "high gain" input.
I use the available 4th triode section for a cathode-follower at the output, to buffer the passive tone-stack and level pot.
One of the circuit elements most responsible for the "Marshall sound", is the first triode section: this triode is set up as a "cold clipper", through having a relatively high cathode resistance (10k), and no bypass capacitor. This section therefore clips the waveform asymmetrically, which accentuates the even harmonics, and this leads to a tone with a pleasingly bright and intense distortion edge, while still preserving readable dynamics and some of the original resonance of the guitar, similar to a "clean" tone (because half of the peaks are clipped, but the other half preserve their original shape).
In Soldano amps, I've read that a similar cold clipper circuit is employed, but the cathode resistor is even higher, 39k. In this circuit, I provide a "pull colder" switch which brings the cathode resistor fairly close to the Soldano value: 37k.
Notice, in this schematic and in the one for the "F+" section, I mark a dividing border between circuitry closely based on the named original (there may still be slight differences), and elements I have added which are not part of the original ("BWK-M").
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