Fuzz Face Emitter bypass cap value

[j_flanders]

I was under the, apparently wrong, impression that the emitter bypass cap's large value was similar to an input or output cap in the sense that if a 220nF input cap includes lows to 80Hz you can also use 1µF or 10µF or whatever and that the emitter bypass cap was just 'oversized' for no other reason than to make sure that all AC would pass.

Sorry to refer to the Electrosmash article again, but there I read:
https://www.electrosmash.com/fuzz-face#lin42

In terms of design, the bypass capacitor C2 should have a reactance, at the lowest frequency you are interested to amplify, less than the value of RFUZZ. We can use the formula:

fc=12πRC=12π⋅Rpot1⋅C2=12π⋅1K⋅20uF=7.9Hz

All the frequencies over 8Hz get full amplification. The 20uF is so big that almost all the frequencies get full amplification.

I googled "emitter bypass capacitor calculator" and the first hit was:
http://www.learningaboutelectronics.com/Articles/Bypass-capacitor-calculator.php
I enter 80Hz (low E string) and 1000 Ohm (fuzz pot) and the calculator gives me: 20µF. That happens to be the standard bypass cap in a Fuzz Face. But 80Hz is not 7,9Hz. According to the calculator it'd have to 200µF to include 8Hz

But then I come across this great post from PRR:
https://www.diystompboxes.com/smfforum/index.php?topic=114587.msg1064059#msg1064059

> all the AC above 48 Hz should be heading to ground through the cap

Gain won't max-out until Z(cap) is similar to *emitter internal resistance*, not the bias resistor. There is a range where "some" of the AC is "heading to ground", a range of rising gain.

Emitter internal resistance is 26 Ohms at 1mA current. Here you are running near 1mA. On that assumption, 26r with 10uFd is 640Hz; 2KHz with 3uFd.

But-- we must also add the impedance seen at Q2 Base (around 10K) divided by hFE. This is maybe another 50 Ohms. So 75 Ohms total. 200Hz for 10uFd, 700hz for 3uFd.

These may be "appropriate" values for a fuzz. But I wonder, if you go too far with this low-cut, if it will be working on harmonics more tnan fundamentals. As harmonics phase crazily on pluck and decay, this may give unexpected sounds.

Not knowing what to think, I simply tried several caps in series or parallel to the 22µF and see how low or high I could go and still hear a difference. I find that I can keep increasing the capacitance to 220µF (!) and still hear a clear difference compared to lower values. My next value was 470µF at which the circuit choked.

With 220µF it sounded more like a Big Muff, huge!
Going back to the stock 22µF it 'suddenly' sounded trebly and lacking bass and low mids.

Could this also explain why I get a kinda sputtery, buzzy, zippery decay on the high(er) strings with the guitar volume rolled back, going for a clean sound?

With (much) higher capacitance the cleans never get really clean but become an ever present slightly fuzzy or hazy, mellow tone. This sounds more like the inbetween settings on my Double Muff (a Si FF variant) which does not have an emitter bypass cap, and I assume therefor all frequencies get amped equally.

Another thing I noticed, or at least that's the impression I got, with the stock 22µF cap and the guitar volume rolled back for inbetween sounds, the fuzz turns into a mix of dry/clean and distortion. It sounds slightly distorted but somehow I seem to hear some (low frequency) clean tone in there as well.

On my P90 hollow body, which is overly bassy, I seem to prefer a lower than the stock 20µF cap, around 10µF or so. For the first time it actually cleans up, which it didn't previously. Not even with the guitar volume at 1.

With my (bright!) Jazzmaster I really like the 220µF. It won't cleanup but it's not a ragged edge or sputtery or grainy, raw fuzz either, more like a quieter, smooth, overall fuzzy tone. Maybe compressed would be a better description.

But on both there are a lot of different sounding distortions with different caps between 20µF and 200µF. It really feels like pre-clipping voicing.

So, what's up with my transistors or circuit that I can increase the cap to 220µF and still hear a difference?
If I follow PRR's post, that would imply a very low resistance. Much lower than the 25 or 75 Ohm he mentions.

I can only assume there's something up as I rarely read about voicing the distortion/fuzz by changing that emitter bypass cap. People seem to only do that with the input and output cap.

By the way, that trick PRR suggested in the other thread (cap to ground in the feedback loop) gives a somewhat similar behavior.

Here's the circuit (Red Dunlop FF) I'm working with:

[PRR]

> the bypass capacitor C2 should have a reactance, ......less than the value of RFUZZ.

Wrong. Not completely wrong. But incomplete.

To get ALL! the gain of Q2 alone we must bypass the emitter internal impedance. Since Q2 runs near 1/2mA this is 50-60 Ohms. In Hi-Fi we want >200uFd.

But also! If the emitter is not bypassed utterly dead, signal leaks through 100k to Q1 base. And Q1 works at gain near 100. And loading at the INput jack is light and unpredictable. So even a slight leak-back can cancel a lot of gain.

OTOH, when you SLAM the FuzzFace all the caps charge up/down. Part of the over-over-drive dynamic. If the caps are *huge* then a big slam may let it "faint" for a large part of a second (not what you intended).

It's a remarkably complex circuit for so few parts.