Third knob on a diy Fuzz Face, what is it for?

[Psychophonic]

I always have to preface my posts with "I'm a diy noob". So there's that out of the way.

I recently picked up 3 diy Fuzz Face pedals from a guy, thinking it would be a good way to begin learning about the FF circuit. He described them as Arbiter germanium FF copies. Housings are plastic, no markings at the knobs/jacks, so I'm on my own to figure out what does what.

So what is the third pot used for? It is wired to R4 on the pcb. The pot meters 8k. In action, this seems to control what I can describe as the overall shape or color of the fuzz.

[r080]

That looks like it adjusts the bias on Q2. It looks like it was originally intended (on that circuit board) to be an internal trimmer pot.

http://geofex.com/Article_Folders/fuzzface/fffram.htm

[Psychophonic]

That looks like it adjusts the bias on Q2. It looks like it was originally intended (on that circuit board) to be an internal trimmer pot.

http://geofex.com/Article_Folders/fuzzface/fffram.htm

Excellent, thank you. Forgive my ignorance, but this pot would be used to dial in the 4.5 volts when I connect my meter? If so, would I want to set it 4.5v and then leave it alone?

[Marcos - Munky]

If you compare the resistors of the board against the schematic, you'll notice a 100K, a 33K and a 330r resistors. The board is missing the 8K2 one, which is exactly the resistor to set Q2 bias, and it's usually swapped by a trimpot because the resistor value depends on Q2 specs for properly bias.

While some may think it's a nice idea to set and forget the bias trimpot, some think it's a better idea to have it as a external pot, because changing bias also changes how the circuit sounds, so you have more tonal options.

[Psychophonic]

If you compare the resistors of the board against the schematic, you'll notice a 100K, a 33K and a 330r resistors. The board is missing the 8K2 one, which is exactly the resistor to set Q2 bias, and it's usually swapped by a trimpot because the resistor value depends on Q2 specs for properly bias.

While some may think it's a nice idea to set and forget the bias trimpot, some think it's a better idea to have it as a external pot, because changing bias also changes how the circuit sounds, so you have more tonal options.

So with my external bias pot, how does this idea sound? Meter for 4.5 volts, then put the knob on with the pointer at the 12:00 position. Increase or decrease as desired, using the 12:00 4.5v position as the reference point. 

[antonis]

4.5 Volts should be considered optimum bias point for an ordinary CE amp..
(actually, it should be at (Vcc+V_E)/2

Here, you have to face up 2 different situations:
1. It's a FuzzFace (way far from conventional CE amp behavior)..
2. Bias point isn't at Collector but at the junction point of 330R & trimpot..
(speaking from output signal point of view..)

[Psychophonic]

I'm using this illustration for reference.

[nocentelli]

So with my external bias pot, how does this idea sound? Meter for 4.5 volts, then put the knob on with the pointer at the 12:00 position. Increase or decrease as desired, using the 12:00 4.5v position as the reference point.

You could do that - What is the range of voltage available throughout the pot's rotation, and how do you like the sound throughout this travel?

You would probably be better just adding the pot to the front panel as it is, setting it to where it sounds good, and not worrying about how close that is to 4.5v - It's not a magic number.

[tonyharker]

Haven't you got the battery leads the wrong way round red should go to +ve?

[antonis]

You can use Jack's calculator http://www.muzique.com/lab/fuzz_face.php as a starting point..

[Marcos - Munky]

So with my external bias pot, how does this idea sound? Meter for 4.5 volts, then put the knob on with the pointer at the 12:00 position. Increase or decrease as desired, using the 12:00 4.5v position as the reference point.

Joe Gagan did this for one of his fuzzes (Nitro Burner by Nine Volt Nirvana), which is indeed a modded Fuzz Face. You can set by ear, as said 4.5 isn't a magic number, but it may looks cool.

Haven't you got the battery leads the wrong way round red should go to +ve?

It depends. If the board was designed with NPN transistors in mind but PNP transistors were used, the battery leds (and electrolytic caps) should be reversed. I think that's the case.

[Psychophonic]

Haven't you got the battery leads the wrong way round red should go to +ve?

I believe the reversed wiring is correct in this case, due to the PNP transistors. With my inexperience, it's confusing me a bit as I try to meter things and whatnot. Still reading/learning.

[Psychophonic]

So with my external bias pot, how does this idea sound? Meter for 4.5 volts, then put the knob on with the pointer at the 12:00 position. Increase or decrease as desired, using the 12:00 4.5v position as the reference point.

You could do that - What is the range of voltage available throughout the pot's rotation, and how do you like the sound throughout this travel?

4.6v to 6.6v is the range, if I am metering correctly. The PNP wiring with everything backwards is confusing me a bit. I am following the illustration above, I think.

Cable plugged into input jack. Effect is on.
Positive lead on the red wire of the output jack coming off the pcb.
Negative lead on Q2 transistor. Not sure which pin is the collector, but only one of the 3 pins changes states as I work the bias pot. That's where I'm metering.

[Marcos - Munky]

Is the jack metallic? If yes, put one probe on the metallic part that's outside of the enclosure (which is connected to ground) or on the enclosure itself (if it is metallic). The other probe goes to the wires of your bias pot. I bet it's the yellow wire. Measure the voltages, turn the pot, measure again. If it doesn't change the readings, then put the probe on the green wire.

It doesn't really matter which probe goes to where. If you swap the probes, the only thing that changes is you'll get either a positive or negative reading (in this case, the negative is the correct one), but we are just going for the numbers and not the signals.

Also, be sure to measure the dc voltage.

[Psychophonic]

Is the jack metallic? If yes, put one probe on the metallic part that's outside of the enclosure (which is connected to ground) or on the enclosure itself (if it is metallic). The other probe goes to the wires of your bias pot. I bet it's the yellow wire. Measure the voltages, turn the pot, measure again. If it doesn't change the readings, then put the probe on the green wire.

It doesn't really matter which probe goes to where. If you swap the probes, the only thing that changes is you'll get either a positive or negative reading (in this case, the negative is the correct one), but we are just going for the numbers and not the signals.

Also, be sure to measure the dc voltage.

Ok, with the probes on the yellow wire on the bias pot and the outer part of the metal input jack, I am getting a range of 5.69 to 9.14 volts.
DC voltage measures 9.22v when metering at the VCC and GND terminals on the pcb.

[Marcos - Munky]

Yep, now I see I was wrong. The correct wire is the yellow one, since the green one is connected to the output cap. So the green wire is the one "before" the pot and the yellow one is the one "after" the pot.

If you add a meter to your pedal, that's the range of readings you'll get. So maybe a nice option is to get one of those digital small meters, like this one: https://aliexpress.com/item/4000608743214.html

[Psychophonic]

Based on my voltage range on the bias pot, should I add a resistor to the green wire in order to at least reach the optimum 4.5 volts? I realize it isn't a magic number, but shouldn't I at least be able to achieve 4.5v?

[Marcos - Munky]

You can add a resistor or use a bigger value pot, which may be easier to do. Then you can go even lower than 4.5V. What's the value of the pot?

[Psychophonic]

You can add a resistor or use a bigger value pot, which may be easier to do. Then you can go even lower than 4.5V. What's the value of the pot?

Pot is 8K. Also, I find that at the higher end of the range, the fuzz isn't very useful. Sounds awful.

[antonis]

I realize it isn't a magic number, but shouldn't I at least be able to achieve 4.5v?

In case you insinst on 4.5V, this should be obtained with pot travel at middle setting..
(to be able to swing up & down for reasons beyond my understanding..)