Vox MKIII Preamp Distortion

The only factory schematic I can find (courtesy of Korg UK) is pretty low res and fuzzy, but I am working on a trace to fill in the unreadable values and voltages.

This is a pretty unique fuzz/wah circuit. When playing through one of these the other day, a co-worker said it sounded "like a giant fuzz/wah," which is exactly what this is. I like it, although it's definitely not for everyone!

TR1 is the input amplifier and it takes negative feedback from the negative side of C12. TR2 (next transistor, down and to the right from TR1) is normally cutoff and makes for an open circuit there. When the distortion switch is engaged, TR2 turns on and connected the bottom of the DISTORTION control (VR1) to ground. This has the effect of reducing the negative feedback to TR1, and thus increasing the gain of the circuit.

The distortion switch also activates TR5, which is the closest transistor to the switch (on the schematic). The 3 transistors in the immediate area of TR5 are germanium transistors with their bases and emitters tied together in a sort of "reverse diode" connection (TR4, TR6, TR7). As far as I know, this is something unique to this distortion circuit. The "reverse diode" germanium transistors probably have a higher and perhaps gentler/curvier clipping action this way. The distortion switch, when it activates TR5, brings these transistor clippers into effect.

The middle transistor is TR3, and this provides an intermediate voltage amplification and phase inversion stage. Overall output of the distortion stage is phase coherent.

After the distortion circuit, the Vox MRB circuit is reached. This is a fun thing to play with, and is allegedly how the Vox wah came about. SW1 is a rotary 3 position switch that makes a wah effect when switched back and forth. A toggle switch, SW2, changes the mid boost. The overall effect is like having a wah pedal in a fixed position while you play.