Fuzz Face biasing - collector vs emitter

[disorder]

I recently read a comment here explaining how biasing the fuzz face circuit should be done by changing Q2 emitter resistance.

I believe you tack a resistor/trimmer from Q2 emitter to ground. But I am unsure of what to do with the fuzz pot and bypass cap.
Do you connect the bypass cap also to the emitter in parallel with this new resistor and then connect fuzz pot as variable resistor in series with bypass cap with one end to ground?

[GibsonGM]

Know what, Disorder? I STILL haven't tried that description I posted back then!   I recently built a very nice silicon Axis Face...I put the trimmer at the collector and just dialed in the 4.5v there, and everything worked out awesome    2k fuzz pot (so HALVED the Q1 collector R to 18k)  I did it because that circuit is 'optimized' for output volume (no loss doing it that way).   The thing is a monster and is biased properly...but I digress lol

Not sure if it's 'worth' going to all the trouble with the emitter...there are a few fuzz guru's in the forum sure to come along tho and let you know their experiences with it!

[fowl]

I use a trimmer on Q1 collector (replace the 33K), and leave everything else stock values.

[antonis]

You can see what "disorder" comes out of messing with Q2 Emitter resistance..

The lower its value the lower the amount of feedback (via 100k resistor) resulting into zero feedback for Fuzz pot set at CW extreme end..

[Elektrojänis]

How about this: http://www.muzique.com/news/fuzz-face-mod/

[disorder]

Know what, Disorder? I STILL haven't tried that description I posted back then!   I recently built a very nice silicon Axis Face...I put the trimmer at the collector and just dialed in the 4.5v there, and everything worked out awesome    2k fuzz pot (so HALVED the Q1 collector R to 18k)  I did it because that circuit is 'optimized' for output volume (no loss doing it that way).   The thing is a monster and is biased properly...but I digress lol

Not sure if it's 'worth' going to all the trouble with the emitter...there are a few fuzz guru's in the forum sure to come along tho and let you know their experiences with it!

I appreciate the honesty and reply. I get caught up in all the possibilities all the time and need to explore everything. All this time I should have just done what nearly all of us do, and have done for awhile. Dial it in at the collector. Glad you ended up with a killer fuzz!

[GibsonGM]

I posted that back during MY big 'what is the right way, oh wise one??' period Sounds like you're in yours now, ha ha.   Long story short - if it biases up well using the collector, and all the voltages look good,  and it sounds great...why overcomplicate it?  *shrug*

[Phend]

Ok I am desufnoc.
Isn't the collector of Q2 usually (normally ) where biasing takes place in these circuits.
See fuzzcentral axis face for instance.

[Rob Strand]

I believe you tack a resistor/trimmer from Q2 emitter to ground. But I am unsure of what to do with the fuzz pot and bypass cap.
Do you connect the bypass cap also to the emitter in parallel with this new resistor and then connect fuzz pot as variable resistor in series with bypass cap with one end to ground?

As a first-order approximation they are sort of achieving the same goal.   Adjusting the emitter resistor is much less convenient on a fuzz-face.   An intermediate solution is to parallel resistors across the 1k pot but that only works if you want to decrease the Q2 collector voltage.

[fowl]

Ok I am desufnoc.
Isn't the collector of Q2 usually (normally ) where biasing takes place in these circuits.
See fuzzcentral axis face for instance.

Either way will allow for some bias adjustment, but changing Q2 collector resistor will also change the 470/8200 voltage divider ratio, and thus output volume.  That may or may not be a good thing, depending on your goals, but if you want to stick to the original circuit as closely as practical, I recommend a trimmer on Q1 collector.

[cab42]

Ok I am desufnoc

So am I!

Either way will allow for some bias adjustment, but changing Q2 collector resistor will also change the 470/8200 voltage divider ratio, and thus output volume.  That may or may not be a good thing, depending on your goals, but if you want to stick to the original circuit as closely as practical, I recommend a trimmer on Q1 collector.

I recently built a Fuzz Face with a pot on the q2 collector. I got 4.5 volt at q2 collector with a resistance  around 4K ( though I prefered the sound around 7k)

If I wanted to bias at q1 collector instead, should I then measure at q1 collector? And should I stil aim for app. 4.5v as a starting point?

[antonis]

If I wanted to bias at q1 collector instead, should I then measure at q1 collector? And should I stil aim for app. 4.5v as a starting point?

It depends on what you're looking for..

For n-p-n Fuzz, Q1 Collector biased at 4.5V means Q2 Emitter biased at 3.9 - 3.8 V, setting Emitter, hence Collector, current (I_C = I_E= V_E/R_E)..
That current sets Collector voltage (Vcc - I_C X R_C)..

[Rob Strand]

You can adjust Q1's collector resistor in order to set Q2's collector voltage.      It will work when Q1 is germanium.   Perhaps ok for low gain silicons on the Q1 position.    I'd recommend adjusting Q2's collector resistor on silicons.

[fowl]

You can adjust Q1's collector resistor in order to set Q2's collector voltage.      It will work when Q1 is germanium.   Perhaps ok for low gain silicons on the Q1 position.    I'd recommend adjusting Q2's collector resistor on silicons.

I thought it worked because they are directly connected, didn't know it had anything to do with germanium or silicon.

[PRR]

> If I wanted to bias at q1 collector instead, should I then measure at q1 collector? And should I stil aim for app. 4.5v as a starting point?

You can not set Q1 C arbitrarily. It has to be what Q2 B needs to support Vbe2, V(Re2), and bias drop in Rb1. The feedback loop tries to force this. The parts around this loop are picked to force useful currents at this condition.

Q2 C voltage *is* user selectable because it affects how it "breaks up". Centered, high, and low all sound different.

If you don't have a preference, start "center" because it will work. Then try low and high (by changing both of Q2's collector resistors).

[Rob Strand]

I thought it worked because they are directly connected, didn't know it had anything to do with germanium or silicon.

The actual reason is the Q1 gain.  It just turns out that germaniums tend to be low gain.

Lowering RC1 means more Q1 collector current.
more Q1 collector current
  ---> more Q1 base current
  ---> more voltage drop across the 100k
  ---> more voltage drop across the emitter resistor
  ---> more Q2 collector current
  ---> reduced Q2 collector voltage VC2.

Raising RC1 does the opposite.

When Q1 is high gain (and usually that implies silicon) the base current remains small and the voltage drop across the 100k is still low.  So while the general effect outlined above is still present, changing RC1 doesn't have a strong effect on VC2.   Changing RC2 on the other hand has a direct effect on VC2 and makes more sense.   That's why  I'm not so fond of using RC1 to tweak VC2 in general, and especially for silicons.   It can work for germaniums.

[Rob Strand]

Actually, I did these tests just a month ago,

https://www.diystompboxes.com/smfforum/index.php?topic=127926.msg1229614#msg1229614


You can clearly see how the gain of Q1 affects the collector voltage of Q2, and it's a bigger effect when Q1's gain is low.

[fowl]

I'm a simpleton.  Q1c is directly connected to Q2b.

[Rob Strand]

Q1c is directly connected to Q2b.

Yes.

[fowl]

Q1c is directly connected to Q2b.

Yes.

So even without the feedback, changing Q1c voltage would re-bias Q2, no?

Damn Fuzz Face, just a few parts and yet still more stuff going on than I can really comprehend.