The Prunes and Custard is a two stage circuit. There is a preamp stage, the gain of which is adjusted with the Drive control. This stage followed by a special waveshaping distortion circuit. There is some treble roll-off between the preamp and the special distortion circuit.
The frequency at which this treble roll-off happens can be changed with the Guitar/Bass switch. The special distortion stage produces strong harmonic overtones, mostly odd harmonic. Higher harmonics are progressively introduced with increased Drive settings and signal level, so a decaying bass guitar note can sound like a mean swept filter.
Playing double notes, like 4ths or 5ths can produce some strong intermodulation effects, rather like a ring modulator, but more musical, albeit more distorted! The Mix control is a balance between the preamp stage output, (CCW) and the special distortion stage output (CW).
