Some Big Muff resistor questions..

[manson]

I've been looking at schematics of the various big muff's. I have a few questions about some of the resistors:

What does changing the value of the collector resistors do to the sound? Some have 10k, 12k or 15k in that position, or combinations. Q4 sometimes has a 390k or 470k.
What is the function of the very first resistor after the input? Most muff's have a 39k there. Is that some kind of signal limiter thing going on?
The 470k negative feedback resistors, how would higher or lower values impact the sound?

Thanks for looking any answers?

[GibsonGM]

I think playing with the collector and feedback resistors will affect the biasing of the BJT's. Could have interesting implications for the "quality" of the fuzz you get, or changing them could just misbias transistors and make it splatty...
The input resistor looks like it functions with the .1uF cap as a filter to set a cutoff frequency of about 40Hz, essentially lowering the high pass frequency to a more guitar-oriented zone.  And probably increases the input impedance of the transistor as well (BJT's tend to have low input impedance compared to opamps)...

[brett]

Hi
the BMP gain stages can take a while to understand. 

R1 is part of a voltage divider to ground with R2 (often 100k).  The gain is mostly set by the ratio of R3, the feedback resistor, to R1.  I say "mostly" because there's also a feedback filter cap (470/500 pF), which lets extra highs through, and filters out high frequencies.  There are also losses between stages, due to the "sustain" pot and impedance mis-matching.  R1 also has a role in setting the input impedance (R1 + R2 ?).

The collector resistors determine the bias and set the output impedance of the stage. 
cheers

[manson]

Thanks guys, I'll mess with the values and see what happens