Fuzz Face Q1 bias - worth doing?

[m7b52000]

I am building a series of fuzz faces using largely identical components apart the transistors for direct comparison. I note that the "Bonamassa" style Ge version is the only version described that recommends using a Q1 bias trimmer and aiming for approx -0.9 V at Q1C.
Is this because the benefits are dwarfed by the effort with silicon transistors or is it a function of the low gain Ge transistors used in the Bonamassa style circuit? Previous fuzzes I have built, both Si and Ge, using "typical" resistor values have tended toward slightly higher Q1C bias values.

thanks

[antonis]

Just to have a handy comparison reference..
(sorry Jack for deleting bypass switch..)



By trimming Q1 Collector at 900mV you actually set Q2 Emitter at about 1.2V (considering 300mV V_EB drop for Ge junction..) resulting into 1.2mA Q2 Collector current so you're able to set Q2 Collector voltage in the range of 9V -  [ R4/(R3+R4) <--> (VR2+R4)/(VR2+R4+R3) ] X 1.2mA..
(didn't use minus PS signs for clarity..)
Considering low h_FE toghether with high leakage currents for Ge FF versions, ability of trimming Q1 Collector voltage it's a wise idea in the mean of customizing particular pedal to your particular working conditions and wishes..
(e.g. 900mV Q1 Collector setting already is marginally low for Q1 stage and, despite NFB bias from Q2 Emitter, it could easily result into almost permanently bottomed transistor in case of high leakage current..)

Jack, on the other hand, got rid of all those "nuisances" by designing a more "robost" FF..
http://www.muzique.com/lab/fuzz_face.php

[mozz]

Q1 bias adjust is the only way to fly. Leave the 8.2k alone.

[Mark Hammer]

Josh Scott has a nice video on this topic, and demonstrates what to expect.

[Electric Warrior]

I am building a series of fuzz faces using largely identical components apart the transistors for direct comparison. I note that the "Bonamassa" style Ge version is the only version described that recommends using a Q1 bias trimmer and aiming for approx -0.9 V at Q1C.
Is this because the benefits are dwarfed by the effort with silicon transistors or is it a function of the low gain Ge transistors used in the Bonamassa style circuit? Previous fuzzes I have built, both Si and Ge, using "typical" resistor values have tended toward slightly higher Q1C bias values.

thanks

-0.9V is rather high for Germanium. Vintage Fuzz Faces usually measures between -0.3 and -0.7 here.
BC108C Fuzz Faces are usually between 1.2 and 1.3V.

[m7b52000]

so far I have been told that:

1) -900mV Q1 Collector setting already is marginally low

2) -0.9V is rather high for Germanium

and 3) according to Josh Scott's FF mod vid I could bias anywhere between 0 and 9 volts (for NPN)

My brain is melting...

[nocentelli]

My brain is melting...

You're not the only one with brain-melt - Josh has drawn the fuzz pot part of the schematic wrong in his educational video:

[Gus]

For Si transistors it should help with how resistor values change the operating points of the transistors in a FF like circuit.
In a FF the circuit is adjusted by more than the 2nd stage collector resistor value.
https://www.diystompboxes.com/smfforum/index.php?topic=114153.msg1058953#msg1058953

[antonis]

so far I have been told that:

1) -900mV Q1 Collector setting already is marginally low

2) -0.9V is rather high for Germanium

It should not be difficult to read about some basic/elementary BJT working regions to decide by yourself..
e.g. for a Ge BJT of 300mV B-E drop, if you bias its Collector at 300mV you'll result into an almost saturated transistor - both due to very "loose" Base-Collector junction reverse bias (0V) and very low positive (or negative, according to PS ground set) headroom (0.3V - V_CEsat)

You can, of course, work on a FF with permanently saturated Q1 (as many "experts" suggest from time to time) if it sounds pleasantly to your ear..

[Electric Warrior]

It should not be difficult to read about some basic/elementary BJT working regions to decide by yourself..
e.g. for a Ge BJT of 300mV B-E drop, if you bias its Collector at 300mV you'll result into an almost saturated transistor - both due to very "loose" Base-Collector junction reverse bias (0V) and very low positive (or negative, according to PS ground set) headroom (0.3V - V_CEsat)

You can, of course, work on a FF with permanently saturated Q1 (as many "experts" suggest from time to time) if it sounds pleasantly to your ear..

That's pretty much how my MK1.5 Tone Bender is biased at room temperature. It sounds great.

Battery: -9.99V
Q1 C -0.31V B -0.08V E 0V
Q2 C -7.64V B -0.31V E -0.24V

The only problem is that it can't handle high tempertures. Currently it's 28°C here and Q1C has dropped to -0.8. It's gating – and not in a pleasant way. It's probably one of the better behaved MK1.5s though. Many are biased around 0.15V at room temperature..

[antonis]

The only problem is that it can't handle high tempertures.

With a Ge B-E junction forward bias with only 80mV drop it's a miracle that it's not gating in the North pole..
(if you look at Ge diodes Voltage/Current graphs you can see that 80mV forward voltage drop could easily be doubled of halved even with a realy negligible current variation..)

[Electric Warrior]

The only problem is that it can't handle high tempertures.

With a Ge B-E junction forward bias with only 80mV drop it's a miracle that it's not gating in the North pole..
(if you look at Ge diodes Voltage/Current graphs you can see that 80mV forward voltage drop could easily be doubled of halved even with a realy negligible current variation..)

It wasn't gating at all when I took the measurements.