Foxx Tone Machine tip

[Mark Hammer]

I recently repackaged an FTM I made, and decided to mod it to my tastes while I had the board out.

First, the octave switch normally connects/lifts the second complementary output from the phase-splitter.  The emitter and collector outputs of that phase splitter each go through a diode that blocks one half cycle of the signal, so that when combined the two signals produce a "folded-over" version of the original signal.  While disabling the second copy of the signal cancels the octaving effect, the remaining copy still runs through a diode, which "corrupts" its sound.  So, what I did was re-jig the octave switch so that when the octave effect is cancelled, that remaining diode in the "normal" signal path is jumpered/shunted.  I find this gives a more normal-sounding fuzz when octave is cancelled.  If you've made yourself one with a second stompswitch for octave on/off, jumpering the diode in octave-off mode will increase the contrast in tone between octave on/off.

The other thing I did was simply install a diode-lift switch for the back-to-back pair further downstream.  Normally, that clips the signal.  The clipping can be used on its own or in tandem with the octaving, but there is no requirement that the diode pair be there.  What you get when the diodes are lifted is a pleasing warmer and louder overdrive.  The octave effect is less nasal too.

So, all in all, the mod to the octave switch, and the addition of a diode-lift switch, allows the pedal to have 4 fairly distinct personalities. recommended.  Just exactly how you would implement them on a Danelectro French Toast, I don't know, but it IS the same circuit.

[aron]

Thanks Mark!

I just put the French Toast back on the pedal board. But I was thinking of replacing it with my DIY one. I will check out those mods.

Aron

[liquids]

Thanks Mark!  I've got the Foxx Tone Machine on a breadboard and am messing with it, so finding this was great info.   

My interested was particularly peaked by your idea to jumper around the diode in standard fuzz mode.  I expected to prefer it, but I didn't--it's interesting how much the feel and tone changes with that one diode! It's like two completely different fuzzes.  It became a bit less bombastic, but synthy sounding for single notes. 

That being said, I've been wrestling with completely understanding this circuit.    It was interesting to audio probe around, particularly around the base of Q3 with the octave mode 'on.'  The octave sounds likewise good all over the neck without regard for which pickup, but here it is much cleaner.   I would think that 'block' of the circuit as something those who like octave-up circuits try out in and of itself...

Anyhow, I still have some questions about this circuit that maybe you can answer, Mark--or any others who care to...

1) The RC network from the base of Q1 to emitter of Q2: Originally I thought it was a filter as mentioned in this post here: http://www.diystompboxes.com/smfforum/index.php?topic=75716.0 and It would seem this would be 'preserving' higher frequencies and sending a lot of signal to ground...... on the other hand, another look, and I could see that it's a lot like the standard fuzz face with that 100k resisor, which I've seen referred to as a kind of 'negative feedback.'

So I would think it would increase bandwidth and affect the gain with the caps affecting frequencies...but that's more of what what I know about NFB in an amp which may not apply here   R.G. said in another post "It sets the mid band gain and the high frequency rolloff of the combined feedback pair. "  All that being said, I have no concept of how the caps affect frequency in this kind of configuration, nor why the values are as chosen, or what the 'combined feedback pair' is.  Can anyone go into more depth and simplify what that all means and does? 

2) I've read posts where people say the above RC network was designed specifically with guitar in mind--how so?

3) On most schematics, the 10uF cap out of the wiper of the drive knob is flowing negative to positive---why? What purpose does that serve?  The Fuzz central schem has is as pos to neg, of note.  It may have been a coincidence, but when I first breadboarded it, I assumed the backward cap was a schematic error and placed it pos to neg, as usual. I vaguely recall the circuit it not working, so I tinkered and for one, I reversed the cap, and then it worked...that may not have been related, though maybe. 

4) Diodes: so without the octave mode 'on,' one half of any signal going in is being almost completely clipped, correct?

5) R.G. said of the octave "It trickles a little current through the diodes to make them already almost on....Look at where the DC current in the diode goes and where the signal is fed to them."   I'm looking at this and now thinking that the 100K resistor from +9v which meets at the junction of the diode anodes is what he is talking about--that it supplies enough current to get the .2v or so required to 'turn the diodes on' constant, and 'out of the way' no matter the dynamics of the input signal level...is that correct?

[Gila_Crisis]

really interesting mark! thank you for the tips.
i built one once and i must say the FTM is a really good fuzz/octave, the fuzz is really nice and thick ala big muff.
it's a shame it's so underrated in the "DIY-scene", people always has to focus only on Fuzz Face or BM 

[Mark Hammer]

My interested was particularly peaked by your idea to jumper around the diode in standard fuzz mode.  I expected to prefer it, but I didn't--it's interesting how much the feel and tone changes with that one diode! It's like two completely different fuzzes.  It became a bit less bombastic, but synthy sounding for single notes. 

I fidn the diode-less arrangement sounds "bigger", but nothing anywhere says you have to like that.   It is simply s imple change to do that yields an easily audible difference.

1) The RC network from the base of Q1 to emitter of Q2: Originally I thought it was a filter as mentioned in this post here: http://www.diystompboxes.com/smfforum/index.php?topic=75716.0 and It would seem this would be 'preserving' higher frequencies and sending a lot of signal to ground...... on the other hand, another look, and I could see that it's a lot like the standard fuzz face with that 100k resisor, which I've seen referred to as a kind of 'negative feedback.'

So I would think it would increase bandwidth and affect the gain with the caps affecting frequencies...but that's more of what what I know about NFB in an amp which may not apply here   R.G. said in another post "It sets the mid band gain and the high frequency rolloff of the combined feedback pair. "  All that being said, I have no concept of how the caps affect frequency in this kind of configuration, nor why the values are as chosen, or what the 'combined feedback pair' is.  Can anyone go into more depth and simplify what that all means and does? 

I'm certainly not the "go-to" guy about this, but it IS the same topography as a standard scoop/notch filter.  Keep in mind that negative feedback is not simply an all-or-none thing; it can be applied selectively across the spectrum.  Indeed, this is how a great many op-amp circuits sound the way they do; caps in the feedback or ground path depetermine where more and less negative feedback is applied, resulting in different emphases.  In this case the network would appear to provide less negative feedback in the mid-band, which may account for the great octaving.

2) I've read posts where people say the above RC network was designed specifically with guitar in mind--how so?

if the scoop/feedback network is what I think it is, then it is tailored to boost the zone where fundamentals on the fretboard are located.

3) On most schematics, the 10uF cap out of the wiper of the drive knob is flowing negative to positive---why? What purpose does that serve?  The Fuzz central schem has is as pos to neg, of note.  It may have been a coincidence, but when I first breadboarded it, I assumed the backward cap was a schematic error and placed it pos to neg, as usual. I vaguely recall the circuit it not working, so I tinkered and for one, I reversed the cap, and then it worked...that may not have been related, though maybe. 

4) Diodes: so without the octave mode 'on,' one half of any signal going in is being almost completely clipped, correct?

If the entire signal passes through one diode, then an entire half cycle is eliminated simply because it cannot pass through the diode like the other half-cycle can.  The result can sound like a clipped signal for two reasons.  First, because the missing half-cycle is replaced with....nothing; which would look flat on a scope....just like a heavily clipped waveform.  Second, the diode that it passes through "eats up" signal until such time as the amplitude is around the forward voltage of the diode.  This results in what is called "crossover distortion": a clipping of the sides of the wave, rather than the tops.  It has its own pleasing qualities.

Actually, now that I think of it, a third way to switch between octave and non-octave on the FTM would be to use a SPDT switch that, instead of placing a wire jumper across the remaining diode, places a reverse-biased diode (one running in the other direction) across it, such that you have a back-to-back pair in series with the signal.  This will result in a symmetrically crossover-distorted signal, that should sound somewhere between the stock nasally spitty sound, and the bigger fuller sound of the diode-less mod originally suggested.

Perhaps the "ultimate" mod would be a 3-position on-off-on SPDT toggle that connects D1 and D2 in position one, lifts D1 in the middle position, and straddles D2 with a reverse diode in position 3.

5) R.G. said of the octave "It trickles a little current through the diodes to make them already almost on....Look at where the DC current in the diode goes and where the signal is fed to them."   I'm looking at this and now thinking that the 100K resistor from +9v which meets at the junction of the diode anodes is what he is talking about--that it supplies enough current to get the .2v or so required to 'turn the diodes on' constant, and 'out of the way' no matter the dynamics of the input signal level...is that correct?

Yes.  Which should reduce the impact of the suggested back-to-back pair.  But as you can tell by comparing the diode-less and single-diode sounds, the diode DOES have an impact.  Just perhaps not as spitty as it might otherwise.

[liquids]

I agree!  It's definitely a secret weapon!  Tone machine style fuzzes have gotten me far closer to that "Siamese Dream" exploding fuzz sound with my Fender amps than the Big Muff circuits I've built.  And I find the octave on the FTM the most usable, consistent, and toneful octave up for what I do in band context.  By comparison, I was totally disappointed with all the big muffs I've tried. I do dig the whisker biscuit, though, another surprise for me, and another fuzz pedal that probably deserves more attention than it does.  To boot, it's fairly simple to build and mod.

From the posts here, the tone machine fuzz isn't for everyone, but I think a fair number of people appreciate it.  Plenty of people like the danelectro french toast, and the ultimate octave got some press when Satriani was talking it up a bit--both of which are FTM fuzz-a-likes.  It will never be as popular as the FF or BMP or Octavia, sure, but it's still cool even if it's not voted most popular fuzz by it's senior class.   

[liquids]

Actually, now that I think of it, a third way to switch between octave and non-octave on the FTM would be to use a SPDT switch that, instead of placing a wire jumper across the remaining diode, places a reverse-biased diode (one running in the other direction) across it, such that you have a back-to-back pair in series with the signal.  This will result in a symmetrically crossover-distorted signal, that should sound somewhere between the stock nasally spitty sound, and the bigger fuller sound of the diode-less mod originally suggested.

Why didn't I think of that?     

The wife will be overjoyed when she finds out she gets to hear me do another round of my 'Cherub Rock' imitations, due to more diode tweaking on the breadboard this weekend!   

[WLS]

I am still fairly new at this and understand what you are saying about the nasal sounds of the octave. I would like to in coporate these mods into my pedal, but don't quite get the just of it. Do you have a schematic or pics showing what needs to be done? And if so could you post them so that I may view it.

Thanks,


Bill