Foxx Tone Machine vs. PE Experience - strategic tweaks?

[MordechaiBenZev]

I've been combing through the archives here and on other sites for some of the principle distinctions between these two related circuits. I cannot seem to find the schematic anywhere for the PE Experience circuit; I know it's very closely related to the Foxx Tone Machine, and I want to see where the changes from the older circuit were implemented.  From what I can gather, the diodes are different and the pot values are as well (1n4001 and 100Ks on the Experience).  But I'm having a hard time tracking down info that would let me do a closer side by side analysis.  Any help would be appreciated.

[Locrian99]

I've been looking for the pe experience schemas well with little luck.   I found a vero layout I was going to compare it that way.   

[MikeA]

Here's a mostly complete schematic and the PE instruction sheet.  I built one of these for someone who had an original and loaned it to me, so it's based on that one.  I added C1 and a pulldown resistor, changed C3 and substituted diodes and transistors, and it still worked like the original.  The front end is from the P.E. Clean/Octave Blend, which is a modified Tone Machine.  The swell is kind of hit or miss, it's been analyzed in other threads, works with some amps but not with others.  The vero layout on TagboardEffects is good, except Q5 and Q6 were swapped.
<edit: the switches are not shown here, but are correct on the vero layout>

[MordechaiBenZev]

This is tremendously helpful.  Thank you so much for posting it.

[Locrian99]

Awesome!

[Mark Hammer]

The diodes labelled D1 and D2 are there to "separate" the complementary half cycles by blocking the one you don't want.    Keep in mind that not only do the diodes restrict which half-cycle is allowed to pass, but also block any part of that signal not exceeding their forward voltage.  By having a much lower forward voltage, Schottky diodes (the BAT variety) chop off less of the nose and tail of the signal than regular silicon ones do.

The diodes labelled D4 and D5 actually have a different function than many think.  Folks see a back-to-back pair going to ground and figure that's the fuzz, but if you lift one end of that diode pair, you'll still hear plenty of fuzz.  What those diodes do is provide a sort of quick-and-dirty limiter.  Octaves tend to get lost in the initial "harmonic haze" just after you pick, and really only come out of hiding once the harmonic content has died down a bit and allowed the doubled fundamental to be more clearly heard.  But by that point the volume has declined sharply.  Those diodes make the volume seem somewhat constant, which makes the octave more audible.  I installed a similar pair on Green Ringer builds, and the octave became much more perceptible.  The diodes DO add some clipping, though, so here's what you can do to lessen it.  Stick a small-value resistor in series with that diode pair (e.g., 100-470R), and a small-value cap to ground in parallel with them (e.g., 1-10nf), to "soften" what the diodes do.

The drawing shown does not provide for octave engage/defeat.  On the Tone Machine, that is achieved by simply disconnecting either D1 OR D2 from where they meet with R20/C9.  By omitting one of the rectified half-cycles, you eliminate the doubling.  HOWEVER, you are still left with the impact of the remaining diode, which introduces crossover distortion.  Normally it would be a simple SPST switch that breaks/makes the diode connection.  I wire up the octave switch differently.  The common of the switch goes to the junction of R20/C9.  One outside lug connects to the diode whose contact you're breaking.  The other outside lug bridges the remaining diode.  That is, it goes to the C7/R19 or C8/R18 junction, depending on which diode you eliminate.  That results in a louder fuzz, but one which includes both half-cycles, for more of a traditional fuzz tone.  Alternatively, use an on-off-on toggle such that you can select between octave (both diodes connect), octave-off (only one diode connected), or "normal" (one diode disconnected and one bridged).

Because of the impact of different kinds of diodes on both voltage drop and wave-truncation, bridging the remaining diode tends to have more impact on tone for silicon diodes than for Schottky types.

Looking at the drawing, it would seem that the Swell function can work for non-octave mode as well as octave, since it is basically an extension of the output that appears to provide a little cancellation in response to pick attack, resulting in the illusion of swell.

[MordechaiBenZev]

I remember an earlier thread where you discussed putting a resistor in series with those back to back diodes.  I took away that discussion that a 1K series resistor without using a parallel capacitor would be a good compromise for slightly moderating the tonal features of the original circuit.  I do really like it as is but the idea of softening things **just** a touch seems like where I'd want my clone to sit.

[Mark Hammer]

Well, you try it and see if you like what it does.

[MordechaiBenZev]

I'll try it with the tiny cap in parallel...I can always remove it!  Do you think something like 470p-820p would be too subtle to notice?

[Mark Hammer]

470, probably.  820 might be more audible.