Marshall amps have several places where clipping can happen:
1. Preamp triodes (V1, V2): each can clip when overdriven. Voiced by cathode bypass values and plate loads. 2. Cathode follower (V2b): can clip asymmetrically. 3. Phase inverter (V3, the long-tailed pair): clips when the master volume is set high and the previous stage is hitting it hard. 4. Diode clipper (Jose mod): hard clipping at the MV wiper, voicing set by zener voltage. 5. Power tubes (EL34s): clip when the speaker can't sink any more current.
These all sound different because of: - Where in the signal chain they sit (so what comes after them, especially the tone stack and PI, shapes the clipped harmonic content). - How they clip (soft via tube curve, hard via diode V-I). - What feeds back to them (NFB loop affects the PI and power tubes; preamp clipping is largely "open loop" and feels different).
The Jose recipe puts most of the clipping onto the diode clipper (and earlier preamp stages from the cascading) and takes some clipping off the power tubes (via NFB reduction). The result is preamp-dominant distortion at any volume.
Common confusion: PPIMV (post-phase-inverter master volume) ≠ Jose MV. PPIMV puts the MV after the phase inverter, which means the PI is always seeing a high signal and clipping at any MV setting. Jose MV puts the MV BEFORE the tone stack and BEFORE the PI, so the PI only clips when the MV is up high — closer to the Plexi behavior.