Fuzz Face emitter cap question

[tonedawg]

So I've been looking up turret builds of fuzz faces, and though every schematic I've seen of the fuzz face has the fuzz control and 20uf cap coming off of Q2, every turret build I find has it off of Q1. Pics for reference below, does it just not matter what emitter is used?

[Mark Hammer]

It does matter, and it goes to the emitter of Q2, where it helps to set where the AC gain is applied.

[pacealot]

If you trace that layout carefully, you'll note that the cap is in fact connected to Q2E, through the fuzz pot. Where it is connected to Q1E is simply the common ground connection. The fact that those components terminate at the same location is probably just a matter of layout convenience, along with the fact that they are the only two components on the board that connect directly to ground.

[PRR]

[tonedawg]

DUH! thanks for helping me see that! One last question, does modifying this cap's value change the gain structure at all?

[anotherjim]

The only time I'd expect to see that cap directly on Q2 emitter is when the design has opted to do without the Fuzz pot in favor of a fixed 1k resistor.
Changing the cap value changes the frequency response. Smaller would favor treble.
Basically, the DC voltage from Q2 emitter biases Q1 base. It also provides negative signal feedback that reduces gain and cleans it up. When the Fuzz pot is adjusted, the 20uF cap on the pot wiper sends the feedback signal to 0v instead of Q1 base. When the pot is effectively connecting the cap to Q2 emitter, all of the negative feedback is taken away and maximum gain is achieved. However, the DC bias for Q1 base isn't affected since the cap doesn't pass DC.

[FiveseveN]

does modifying this cap's value change the gain structure at all?

If you'll excuse my allergy to guitarists' misuse of jargon: if using the original (proper) meaning (appropriate signal levels), then no. If using the term as players understand it (the timbre of saturation) then yes.

[anotherjim]

Ah, you might also see the order of the cap and fuzz pot reversed - then the cap could be directly on the Q2 emitter and the pot wiper.
This could be a way of using a log fuzz pot in a reverse direction to get a better control sweep.

[antonis]

What Jim said..

If you isolate Q2 stage, you have a Common Emitter amp with Emitter degeneration..
https://www.electronics-tutorials.ws/amplifier/amp_2.html

There are two negative feedback mechanisms here..
1. Fuzz pot, by its own, serves as Emitter resistor mainly for bias stability..
2. Fuzz pot/100k resistor voltage divider serves as negative feedback factor configuration so by reducing its AC effective value via 20μF cap you reduce negative feedback amount hence you raise gain..
https://www.allaboutcircuits.com/textbook/semiconductors/chpt-4/feedback/