Bigger cap for Fuzz Face attack pot?

[bluesdevil]

I've been experimenting the last few days with changing out stock values of the ol' FUzz Face circuit on my breadboard and have come up with some improvements (to my ears): .1uf for input cap, 250k volume pot and 27k for Q1's collector resistor.
     I think my ears may be fried at this point and may be hearing differences that aren't really there, so could use some help in understanding what effect does the 22uf cap that goes on the attack pot have by raising or lowering the value?  I put another 22uf in parallel with it and could have sworn I heard a heavier more distorted sound, but now I'm not sure. Has anyone here experimented with that part in the Fuzz Face circuit?
     Yes, I've read the GEO "Technology of....." and did my best with the search function, but would like to perhaps hear some more recent experiences.
               Thank you!!
       

[brett]

Hi
That 22uF cap bypasses (for AC) some of the resistance of the 1k pot.  The gain of Q2 is very close to proportional to the amount of bypass (there is a small, fixed resistance in the base-emitter junction that can't be bypassed).  More bypass, more gain.  Simple.

Now, you can calculate the AC resistance of that 22uF cap, using the R = 1/(2.pi.f.C) formula.  At f= 200Hz, it is 36 ohms, at 1000Hz it is 7 ohms, and at 5 kHz it is 1 ohm.  From these figures, you can see that it is almost a resistance-free bypass for the 1k pot.  If you doubled that 22uF to 47uF, the resistance would be half, but they're already really low when compared with that 1k pot.

But it can't hurt.  I use 100uF caps for bypass and decoupling in many situations.
cheers

[tiges_ tendres]

I recently breadboarded an Orange sunshine fuzz and noticed the same thing.  I really like how it works like a clean boost with a low value cap in there.

[bluesdevil]

Thanks for the explanation, Brett. I really appreciate it!! So a larger cap would produce more gain (fuzz) and more low end. Maybe just a very small difference with the paralelled 22uf's..... maybe I'll rest my ears and experiment more later.
     Hey Tiges Tendres, I also experienced the overdrive sound with a smaller cap value, but like that big sound  of a larger cap..... funny how the absolute fuzz monster Tonebender mkII has only a 4.7uf value for that cap position!!!!! What the hell??

[R.G.]

The cap can only reduce the gain for frequencies which make its impedance less than the bypassed part of the 1K pot.

For 22uF and 1K, this is 7Hz. As you turn the pot down, the frequency goes up as the gain goes down. However, this is just the half-power point, so the cap's impedance is 1K there. It's 100 ohms at 70Hz, near the low end of the guitar. Doubling the cap makes it half the impedance at a guitar's low end, and could contribute a sense of more bass response.

Here's the kicker: the gain is determined by the collector resistor as well. So once the cap is less than the 1K resistor, the cap's value compared to the collector resistor determines the gain until the cap's impedance becomes so small you get diminishing returns against gm*Rc.

It's a small effect, but I can see where some people could hear it.

[bluesdevil]

Thanks for also replying, R.G..... I'll have to print all this stuff up and save it. I should mention I used a fixed 1k resistor on the breadboard in place of a pot and just placed the cap(s) in parallel with it.
       When I changed the collector resistor of Q1 to 27k. I couldn't notice much of a difference with the double 22uf's as before, but putting a 47uf beside the 22uf the sound was there again. Not really sure if  the low end is actually increased or just high end is decreased and volume raised. When I box it up I may put in a switch to toggle betweem 'em.... but then again I may be just splitting hairs and nobody else would hear the difference anyway. Playing around with this thing for hours on end has probably driven me completely insane.... I just found out my guitar wasn't plugged in all this time!!