Hi,
I have made several alterations in an attempt to make the octave up circuit a bit more to my likeing. The first is the addition of PSU filtering to eliminate any hum from unregulated 9 volt supplies sneaking through. The circuit runs quite happily from between 9v and 12 volts with a more cleaner tighter sound at 12 volts. Additionally I felt at 9 volts the bass was still too dominant so to reduce this I changed TR3s emitter capacitor (10uF) down to 2u2F. This is cap C110 in the FT. This value could be less (1uF?) when using humbuckers.
The third and most effective mod is the addition of a 150K ohm resistor connected from the 9 volt rail to the anode of the switched doubler diode which is driven from the emitter of TR2. The 10uF cap polarity will need to be changed, (C107 in FT) this is an attempt to negate the momentry drop in signal level when the octave switch is operated. This switch function could now be assigned to a second foot switch to bring in the doubler effect on the fly.
If anyone is interested in the circuit changes I can mail them.
John G