This weekend I had a chance to spend considerable time perfboard experimenting, in detail, with this piggy-back idea on a "jig" I made up some time ago. The "jig" has trim-pots for every resistor in the FF circuit and multiple caps for C1 and C2 (2.2, .1, .068, .022, .0033) and sockets for adding bypass caps across the collectors and from collector to gnd.
Using this "jig" makes it painless to dial-in transistor bias and tune the FF circuit, and eliminates any need for trim-pots on the circuit board. It allows tuning of the circuit "by ear", in real time while playing thru it.
Anyway here's the values I've settled on, using four 2N5087 transistors (hfe around 350) in the piggy-back mode:
R1= 33k
R2= 47k
R3= 150k
R5= 2k
R4= 10k
C1 and C2= .068
Q2 bias set at= 4.8v
I've added a 100pF cap across the collectors, and a 56pF from Q1's collector to the juction of R3/R5 (which is ground on my negative-ground version of the FF circuit).
I've chosen a 16k resistor where you have you base resistor "X1" and no base resistor on Q2, and I've added a textbook 680 ohm resistor on the emitter of Q1 bypassed with a 33uF cap.
Now it sounds like a very well behaved Fuzz Face.
In the past when I built Si versions of the FF circuit they never sounded quite right to me ...until now... this piggy-back idea seems just perfect in this application.
Great work Brett, and thanks for sharing the idea.
