Fuzz Face - External Bias Pot vs Fixed Resistor AND Tone Controls

[nation]

Hi All

Apologies if these are basic questions. I may be coming up with a few more like this as I'm painting by numbers here.

First Question - I often read that there's a preference for installing a fixed resistor to bias rather than leaving in a trim (or an external) pot. Is there some inherent weakness (noise, hiss etc) with a pot that makes a fixed resistor preferable?

Second Question - Similarly for a tone control. Is there something about an external tone pot that makes them less desirable than having no tone adjustment option for fuzzes? And if yes, would wiring up a tone switch as an alternative to a pot be as good as having similar value fixed capacitors in the circuit i.e. no adverse effects on the fuzz's performance whether caps are wired via switch or fixed to the board?

Thanks all



I often read that the preferred approach to biasing is using a trim pot to determine the correct value then installing that value fixed resistor. 

[MaxPower]

1) A resistor costs 1-3 cents where a pot costs significantly more. Also, one usually sets the Q point (bias) and then leaves it there so a pot is unnecessary.

2) No. The BMP and plenty of other fuzzes have tone controls which use pots.

Pots can be noisy in some situations but usually they work fine in our guitar circuits.

[antonis]

I often read that the preferred approach to biasing is using a trim pot to determine the correct value then installing that value fixed resistor.

That's true for FET biasing, due to wide spead of parameters even among identical items, but rare for BJT biasing (unless you use a h_fe/V_BE/Temp variations greatly affected bias method, like single resistor bias..)

[nation]

Thank you for the information. Looks like pots/switches are ok. My main issue was with noise introduced by the extra components/wires or issues like tone loss akin to a guitar's tone control (i.e. a guitar with a tone crl vs one with no tone crl pot).

[POTL]

I did not find much sense in the displacement of the first transistor, but the displacement of the second transistor strongly affects the sound of the circuit (I use Russian transistors)
The bias circuit can be changed, you can use 1 or 2 fixed resistors together with a potentiometer, depending on the position of the resistors and the potentiometer, the bias will work a little differently, experiment 
Adjusting the tone may not be very useful in this scheme, it sounds great without a tone, as well as MXR Distortion + or BOSS OD-1
Some tone controllers are not needed