The Mostly Harmless pedal is conceptually quite straightforward – it offers octave up and octave down, each of which can be variably mixed together with as much clean as you want. The clean element is transparent but the two octave elements have a (common) variable gain drive stage in front so you end up with four controls: Drive, Clean, Lean (octave up) and Mean (octave down).
For the octave down there are a few contenders, the two I considered being the Valve Wizard’s U Boat and the Shocktave by (c) Joe Davisson – the Shocktave circuit is elegantly simple so I went with that. For the octave up I was planning to go for the Green Ringer but instead opted for a different (and simpler) multiplier circuit from guitarhq called a “Pushme Pullyou“.
The Pushme Pullyou is the top section and the Shocktave is below, both share a simple common drive stage. The buffered outputs are mixed using a virtual earth mixer and the individual mixer input resistors can be adjusted to give balanced levels. With one op amp left over from a quad package, the Vref supply is op amp buffered.
The Shoctave octave down is very capable and generally tracks lead playing well without sounding too awful if some chords are hit. The octave up is less pronounced in its effect but is still a good sound to mix in, particularly with some distortion from the drive circuit. There is a second version (as yet untested) that implements a Green Ringer in place of the Pushme Pullyou – both designs are on Github
A PDF of the circuit (which is easier to read) can be downloaded here: