fOXX Tone Machine question...

[jpiddy118]

Built one, added Fulltone UO mods, works fine, LOVE the fuzz, but after all i've heard about these, i'm not impressed with the strength of the octave... based on clips and descriptions from others, i'm starting to think something is up in the octave circuit. My Tycho octavia clone is much stronger. Where exactly should I be looking to remedy this? Don't want to mess with an otherwise awesome fuzz tone.

[Mark Hammer]

If you find others have a stronger octave then you most certainly have made an error somewhere, because the basic FTM gives a very strong octave, and there is nothing in the Fulltone version of it that would take away from that.

Are you sure the diodes  that rectify the signal coming off Q2 are oriented properly?  Are you sure the electrolytic caps coming off the emitter and collector of Q2 are oriented right (+ on transistor side)?  Are you sure the emitter and collector resistors on Q2 are equal value (4k7)?

[jpiddy118]

Yep, double-checked all of that after work tonight... diodes, caps, Q2 resistors both 4k7. Voltages correspond very closely to those on the GGG build instructions. The octave is there, it just doesn't sound good compared to clips of others... its got a thin raspy-ness, whereas i've heard others (both FTM & Fulltone UO) that sound thicker and smoother on top.  Also, when i kick on the octave, there's a slight drop out and then a quick fade up in sound. Bizarre.

[Mark Hammer]

So everything else about the unit, apart from the octaving aspect, works as it should?  I.E., it gives you significant volume levels above bypass?  The tone control works, and the distortion intensity varies with the drive level?

[jpiddy118]

Yep, plenty of juice, decent range on the fuzz/sustain pot, tone stack has a nice useful sweep, made mine in permanent "fat" mode rather than the Fulltone toggle switch. It might be a better stoner-rock fuzz than my BC108 FF.

[Mark Hammer]

Here's an idea to help with the detective work: flip the octave switch around.

Normally, Q2 puts out two identical, but opposite-phase copies of the input signal, one from the emitter and one from the collector.  The design sets the collector output as the default "always-on", and simply adds the emitter output to produce the doubling effect.  You could just as well use the emitter output as the "always-on", and add the collector output to produce doubling.  Essentially NO difference, other than the phase of the always-on portion of the signal.....assuming there is nothing amiss with the emitter output.

Presently, your problems seem to center around the addition of the emitter output, via the octave switch, which suggests to me that you should see/hear something very different if you flip things around as suggested.

Also, note that the stock FTM/FT-UO design leaves the diode in place for the non-octave setting.  This introduces a kind of crossover distortion by requiring the collector signal to be at or above the forward voltage of the diode before allowing it to pass.  This chops the sides of the signal.  What I've done on mine, to increase the sonic palette, is use a 3-position toggle.  In position 1, it connects the emitter output to generate octaving.  In the middle position, it breaks that connection to produce the stock FTM non-octave fuzz.  In position 3, it now bridges/shunts that diode on the collector side so that there is no chopping of the sides of the waveform and the full signal passes.  A much more conventional fuzz sound, that is also a bit heftier, without having to forfeit the stock sound (if you like it).  three-position on-off-on SPDT toggles don't cost more than regular SPDTs, although if your pleasure is to engage the octave via a stompswitch, then I guess, you're out of luck, or will need to use a 2nd toggle (SPST)  to have octave/non-octave and regular/altered fuzz.

[jpiddy118]

Wow, thanks for taking the time to help Mark! I will try your suggestions later tonight when I get home. Much appreciated!!!

[jpiddy118]

I'm looking at my FTM, and the schematic... would it be as easy as flipping the orientation of Q2? Since i'm dealing with a pcb, i can't exactly re-route the circuit, if i'm understand your suggestion of flipping things around correctly.

Also, read that trick about jumping the 2nd diode somewhere else on this forum... thought about trying it just for fun.

Thanks again!

[Mark Hammer]

Don't do anything with the transistor.  It,s a suggestion that involves the leads going to the switch.  Right now you have the diode after the emitter going to the octave switch, and from there to the mixing point.  The diode from the collector already goes to the mixing point.

Disconnect the emitter-side diode from the octave switch (leave the other wire in place), and solder it to the wire that leads to the mixing point (it will now connect the emitter-side diode to the mixing point ).  Unsolder the stripe end of the collector-side diode, lift it,  and connect a wire from there to the point on the switch where you had previously connected the emitter-side diode.  You have now reversed which diode/path stays connected all the time, and which one gets added to produce the doubling.

[jpiddy118]

ah now i get it... i'll mess with this later.

also, could changing C3 from .1 to .047uF affect the octave tone drastically? Can't recall much about the FTM tone pre-mods except that overall it was too thin & harsh for me.

[sansaat]

I had the same problem that jpiddy has. I used the ultimate octave schem which had the two caps from Q2 emitter and collector backwards, - on the transistor side. Turned them around and now the octave effect is working properly. It still has a little delay when I hit the Octave switch.

[jpiddy118]

my electrolytic caps are oriented correctly per GGG schem:
http://www.generalguitargadgets.com/pdf/ggg_ftm_sc.pdf?phpMyAdmin=78482479fd7e7fc3768044a841b3e85a
Not sure if this is diff from the Fulltone schem.

The little switch delay is pretty annoying, too. Figured it had something to do with voltage, but i'm a novice. Could a drop-down resistor help?

[jpiddy118]

Think i solved my issue... decided to change C3 back to .1uF as per original, and noticed that the .047uF i had in there was loose on the lead to ground. I swapped it anyhow and made sure it was soldered in there good. Without being able to do a direct A/B, i think the octave is a bit smoother now... (either that or i just got used to it, hah)

still would like to do something about the volume dip when engaging octave