This Week on the Breadboard: A 4-stage FET Fuzz

Chuck D. Bones

Circuit Wizard
I know I said next up was the Baby Blue OD, and I will get to that one soon, but I ran across this high gain JFET Fuzz on Luciferstrip's FTP site and had to give it a whirl. I have a pile of Ge diodes so I thought this would be a good circuit to try out some of them.

This is the schematic I found:

fetfuzz.jpg


I started with this as the boilerplate design and then made mods from there. Since I used PN4393s, I had to adjust the source resistors to get the biasing right. I don't like having the GAIN control before everything because it can be too noisy. I put the GAIN control between the 1st & 2nd stages. I deleted the other gain control between the 2nd & 3rd stages. The signal level across that last pair of Ge diodes is pretty small, so I added a 4th stage to bring the signal level up and then stuck a TONE control after that. Most of the caps are film. The pF caps are silver-mica. The 4.7uF & 22uF caps are tantalum. The 100uF caps are aluminum. Here's the more-or-less finished product.

Boba FET Fuzz v1.0.png

This design has some interesting qualities. It has a shit-ton of gain and compression. The three JFET stages have 24dB gain each. The Ge diode hard clippers can be fairly leaky without degrading the circuit performance. The hard clippers clamp the drain voltage, so none of the JFET stages can saturate. Because of that, the drain voltages can be set lower without running into headroom problems. The diodes make most of the distortion and compression. At high GAIN settings, the 2nd & 3rd stages are driven into cutoff (drain current goes to zero) for asymmetric clipping. The MOSFET 4th stage does not add any distortion, it's just there as a boost.

I had a bit of trouble with this at first; it wanted to oscillate when GAIN was above noon. I cleaned up the layout, power distribution, grounding and wire routing; now it's stable. The 1N128s are lower leakage than the D2Es. With all of the gain on tap, it's easy to get feedback going, even at low volume.
 

Chuck D. Bones

Circuit Wizard
I bought those PN4393s from Mouser in January. I wanted to use something anyone could get in a leaded package. I'll probably try running PF5102s too, just 'cause I have a bunch.
 

jubal81

Well-known member
Fets, germanium diodes and crushing distortion?

images
 

cooder

Well-known member
great stuff again! With the suggested cap, do you mean C7 the 100pF on Q3 to be larger or is that a typo? Maybe C5 on Q2? Just wondering...
 

Chuck D. Bones

Circuit Wizard
I wanted to reduce the high-freq harshness a bit. C7 forms a low-pass filter at Q3. Increasing C7 lowers the low-pass freq. It may or may not be effective because it interacts with the clipping diode impedance. A cap across D3/D4 or D5/D6 might work, I'd start with 10nF. I have taken this circuit off of the breadboard, so I can't be much help at this point.
 
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