Modding RollFuzz with a "fuzz" level pot

[joakim]

Hi!

I want to build this Roll Fuzz effect, but I thought I'd try to add a "fuzz" dial (pot) so the level of effect can be set.

http://www.doitfuzz.com/forum/diy_pedal_project.php?effect=rollfuzz_simple_fuzz

My thinking was to replace R4 with a pot, but I don't really have any understanding of whether this is correct.
Does it make sense to vary the level by changing the resistance at R4?

[antonis]

In theory, it should work..
(but it should also upset bias)

For particular CE amp, gain is set both by R4/R1 and R3*Ic/0.025, so better replace R1 with a pot, wired as pot..
(lug 3  to C1, wiper to Q1 Base and lug 1 to GND)
You'll have a "dual" effect - input signal attenuation and Q1 stage gain variable..

[joakim]

I'll try that! I've experimented quite a bit with R4 replaced by a 1MOhm pot (yeah, largest I had) and the results were really not very audible.
A scope did show some response (on a 440Hz sine wave) but of course a visual change might not give any "interesting" effect on the sound (right?)

Thanks for the input!

Btw, speaking of input, why is the pulldown (R2) so large?

[antonis]

Btw, speaking of input, why is the pulldown (R2) so large?

For particular circuit, it's value doesn't make any difference..

In general, pulldown (antipop) resistor is considered in parallel with next stage input impedance so, the bigger the resistor the less the equivalent impedance domination effect..

[pacealot]

What I'd like to know is where people are getting 5088s with hFEs of 815!    I can't even get a 5089 that's that high these days!

[antonis]

Maybe it's derived from 10μA working current..

[johngreene]

In theory, it should work..
(but it should also upset bias)

For particular CE amp, gain is set both by R4/R1 and R3*Ic/0.025, so better replace R1 with a pot, wired as pot..
(lug 3  to C1, wiper to Q1 Base and lug 1 to GND)
You'll have a "dual" effect - input signal attenuation and Q1 stage gain variable..

I would change the order of R1 and C1 though. Otherwise you will get bias current flowing through the pot and that will not only create noise on its own but will scratch loudly as you adjust it.

[pacealot]

And that would also render the pulldown resistor R2 unnecessary, which is also handy... 

[antonis]

IMHO, there shouldn't be any audible difference for gain variations..
(other than those harsy top waveforms for signal negative input..)

100mV_RMS, 800Hz   

[joakim]

So, are you saying there isn't any point in a "fuzz" (level) pot for this effect? Not on R1 or R4?
I'm essentially looking for some simple 2n5088 based fuzz effect, with two pots (volume and level) for my little project.
Maybe I should be looking for a different schematic alltogether? Any recommendations?

[joakim]

What I'd like to know is where people are getting 5088s with hFEs of 815!    I can't even get a 5089 that's that high these days!

I know... I bought a bag of 20 2n5088 transistors, and all of them measure very close to 400.