4 tranny fuzz analysis required.

[digi2t]

I was breadboarding a four Ge transistor fuzz yesterday, which doesn't sound half bad. I found the schematic on an Australian amp site (Ozvalveamps.org), it's called the "Hi-Fi John" Fuzz. This is the schematic;



On the whole, it sounds pretty good. Wide open, it sounds a bit like a boosted FF, and with the decay of a Harmonic Percolator (+1 octave harmonic feedback of whatever chord your playing). I'm currently using four OC72's with the following gains and positions ("Q" from left to right on the schematic, followed by gain and leakage);

Q1 - 82  0.12mA
Q2 - 95  0.06mA
Q3 - 100  0.15mA
Q4 - 94  0.04mA

I'm using 1N60's for the diodes. I did find that a 47K in place of the 33K below them works better. The clipping doesn't attenuate the volume as much, yet allows the tone pot beside it to be respectfully useful. At first look, I thought that this arrangement was a clipping control, but it reacts more like a tone control, which threw me since I'd never seen a tone control below the clipping diodes before.

I know some of you folks here are really good at simming circuits, and I was wondering if you could tell me (show me) what's going on here. Also, are there any recommendations as to transistor positions, or even transistor substitutions insofar as gain is concerned.

So far it sounds like a pretty decent circuit, I'm just wondering if it could be made better. Any ideas?

[R.G.]

I don't have it set up in a sim; this is what I can tell by inspection.

First and last transistors are emitter followers, gain of about one, and buffering. Input impedance of about 235K, input rolloff down at about 47Hz.

Second transistor, AC gain stage. DC bias with collector near 3.6V, probably a bit less because of leakage. Gain in the 15-30 range, variable with the 1K pot.

Third transistor, much the same, but fixed gain, probably about 10.

Rolloff/clipping/tone-ish thing from the diode clot.

[digi2t]

DC bias with collector near 3.6V, probably a bit less because of leakage.

Correct. Actually, I have a 1M trimmer here, and for exactly 3.6v, I've got it at 932K.

Rolloff/clipping/tone-ish thing from the diode clot.

Yeah, weird, but here's the neato thing. Remember that upper octave harmonic decay a la Percolator I mentioned? The kicker is when I strum (let's say) a barred A chord, as it decays, I get an octave A ringing in the decay, yet twisting the pot only affects the tone of the played chord. The upper octave decay note doesn't change. First time I've ever heard such a thing. What's up with THAT?

[Digital Larry]

Given that there is quite a bit of negative feedback in all stages (i.e. emitter resistance), I wonder how different it would sound (if at all) with silicon transistors?  One of my theories which I have not personally put to the test is that by the time you add emitter feedback (extreme case - emitter follower) then you might as well use silicon, because the resistors are determining the gain more than the transistors themselves.  Unless the beta is REALLY low.

[R.G.]

One of my theories which I have not personally put to the test is that by the time you add emitter feedback (extreme case - emitter follower) then you might as well use silicon, because the resistors are determining the gain more than the transistors themselves.  Unless the beta is REALLY low.

Yep. The point of feedback is to hide the individual quirks of whatever is inside the loop, so the more feedback you have, the less the insides of the loop matter as long as there's enough gain.

This gets strained with low open loop gains, which is why opamps have high gains. The second transistor here has a means to bypass the emitter resistance with that emitter cap/pot setup, so the device will "show through" at higher gains, but most of the distortion in this thing will be in the diode clot.

[Gus]

R.G. post covers most of what is going on.
now for some details
as always the circuit is the sum of the stages
I don't think the EF matter that much even if they were changed to a 2n3906 etc. you might change a resistor in the bias string to set the emitter voltage and keep the same input resistance
The two gain stages operating points and input and output resistance matter along with the cap value

The collector voltages set how symmetrical the clipping is for each stage and note the stages are inverting
The tone/ clipping is interesting and is effected by the output resistance of the third stage(a little) and by the input resistance of the output EF(470k in parallel in parallel the hfe times what is connected to the emitter this changes with setting of the volume and what the circuit is connected to)

If you try Si try medium power transistors like a tip29 NPN +9VDC or a tip30 PNP -9VDC

[digi2t]

Here's a video of the breadboard test.

[pinkjimiphoton]

wow, nice sounding fuzz... very amp like... good find dino!

[digi2t]

wow, nice sounding fuzz... very amp like... good find dino!

Yeah, it`s really a nice, middle of the road, circuit. My next test will be with some of the Russian trannies I got.

In the meantime, I`m still swapping the trannies around in an effort to find the best gain value versus position.

[digi2t]

Tried it with the SRC 54 transistors that I got from Surplus Sales tonight. Gains between 75 and 90, all with leakage at 0.06mA. Sounds great.

Had to really sift through them to find good ones, but for the price, well worth it.

[Gus]

A sim of a silicon transistor circuit based on the the Ge one for DC voltages.  Tried to keep the input resistance of the stages close to the Ge circuit and use standard resistor values and to have a good bias for each stage, used different value resistors in the bias string for the EFs because of the greater base to emitter voltage



You might be able to use PNP silicon transistors for the emitter followers and save the Ges for the two middle stages without much of a tone change.