fOXX Tone Machine; question

[Bernardduur]

Hey all!

I have a small question; I've build a fOXX Tone Machine just I did before but just now something hit me; something I haven't noticed in my former builds. In the non-octave mode I already hear a hint of octave in my sound..... is this normal?

[Gripp]

Good question!
Mine does that too and I wondered about the same thing. Guess it might be normal then.
My clipping diodes are probably not symmetrical as there were no matching for the clippers. Did match the octave diodes well though.

[Jurgen]

In the octave mode, the signal is full wave rectified, this doubles the fundamental frequency, giving an octave effect. In the normal mode, the signal is half wave rectified, which doesn't kill the fundamental but can give a lot of second harmonic, which is heard as a bit of octave.

[Bernardduur]

Thanks! I think my first build was different then as I had no slight octave!

[royzic]

The same thing happened to me,
are you using 2n3565 in it?

i swapped mine fore 3904's and it sounded a lot better.

[Bernardduur]

Nope; I use BC140's

[royzic]

I would suggest trying diffrent transistors ,
the first TM i've built had 2n3565 in it and sounded good
the second one was technically identical but had the same problem you've had with yours
replasing the transistors made it sound normal.
(i like the tone machine with 2n3904s and silicon diodes)

[caress]

i used bc547 or 557s in mine and it wounded great!  a bit buzzier...but in a good way.

[Mark Hammer]

As near as I can figure it out, the network between the base of the first transistor and the emitter of the second transistor plays a strategic role in the FTM sound.  You will note that it is VERY much like the mid-scoop filter at the other end of the circuit.  I gather that it assists in providing additional boost for the lower harmonics of the signal such that the octave-up is easier to extract in an audible manner.  Owners/builders of the FTM probably have noted that the octave is a much more obvious and "throatier" one than in some other pedals.  I sauspect that is because the "octave-advantage" provided by the aforementioned network is on the job whether one has selected the octave doubling position or not.  In other words, the octave is just champing at the bit (NOTE: for non-English speakers, this is an expresssion to describe a horse that is eager to race and must be restrained by the rider), and you'll hear some of it when the octave switch is off, and all of it when the switch is on.

[Bernardduur]

What would be the best way to get rid of the octave effect in the non-octave mode?

[Mark Hammer]

Maybe the simplest thing to do is to use a DPDT toggle for the octave switch and shunt the remaining diode when you switch to the fuzz mode.  Remember that the 2-transistor input stage shares much in common with the Fuzz Face type of circuit, and there are 2 clipping diodes just ahead of the tone circuit, so you should not want for distortion by removing diodes from that earlier part of the circuit.

Actually, now that I think of it, you only need a SPDT toggle to do this.  Make the common of the switch the junction of the 100k resistor, diode, and 10uf cap just before the Sustain pot.  When the switch is flicked in one direction the link to the other diode is cut and the remaining diode is shunted.  When the switch is flicked the other way, the second diode is re-connected and the diode shunt is lifted.

[Bernardduur]

OK; that worked!

I tried it before and got no sound but rigth now it works perfectly and gets the sound I heard in my head!

Thanks Mark!!!

[Mark Hammer]

I can't believe that something I suggested before I even finished my morning coffee worked!

Is the difference between shunting the diode and not shunting that obvious?

[Bernardduur]

Yep

But I guess that after I unshunt the diode the signal gets a bit octave bleed.......

Dunno; I really like it now; just like I heard the sound in my head!