Getting rid of octave in foxx tone machine when octave is "off".

[ianmgull]

Just put together a FTM. It sounds great. Problem is there is a pretty noticeable octave/ring mod thing going on even when the octave should be off. Anyone know a way around this? or is that just the nature of this circuit?  I thought I remember reading somewhere about grounding D1 and D2 somehow but I wouldn't know where to start. Anyone?

[Mark Hammer]

I think there is a thread on this very topic that you can search for (though I'm too lazy to).  The issue was that when the octave switch is off, there is still a diode at play (D2 in this drawing: http://www.generalguitargadgets.com/diagrams/ftmsc.gif ).  One suggested solution was to use a DPDT switch for octave-switching, such that when the D1 path was disconnected/lifted, the other half of that toggle would be used to shunt D2.

Not having done the experiment, I can't vouch for it, but certainly it brings the signal back to a relatively "normal" one so that it can be clipped in classic fashion by D3/D4.  Do note, however, that the robust octaving produced by the FTM most likely results from the network provided by  C2/C3 and R4-6.  Since the suggested mod does nothing in regards to those components,, there may well still be a slight tendency to produce octaves.  Still, taking D2 out of the signal path should fix things up in non-octave mode.

[ianmgull]

Hey Mark, thanks for the info. Would that involve sending one end of D2 to ground? If so which end?

[Mark Hammer]

Nothing gets sent to ground.  When the switch is in the octave position, D1 is connected as indicated.  When the switch is in the other position, D1 is disconnected and the other half of the switch simply places a link of wire across D2, mimicking what would happen if there were no D2.

[ianmgull]

Got it, thanks Mark!

[Processaurus]

Hey Ian, please let us know how it goes, I'm interested, always thought it was strange that the no octave signal goes through a diode.  Also something I don't understand is why there is a 100K (R13) pullup resistor to +9v at that area.

Mark, you could do it with the spdt switch in there already, if you hardwire the D1 to SW2 connection on, and use that switch to instead switch C6 (+ side) between the R10/D2 (no octave) junction and the R13/D2/D1 (octave!) junction.

[ianmgull]

Hey Ben, turns out the guy I made the box for kind of liked the residual octave so I never got a chance to test it. I'm thinking of putting together one for myself in the near future which I will definitely post results here (if somebody doesn't beat me to it).