I built a software simulation of the first fuzz pedal, the 1962 Maestro Fuzz Tone FZ-1, based on the documentation in Glen Snoddy’s US Patent #3,213,181. The Maestro Fuzz Tone was used by the Rolling Stones on Satisfaction in 1965.

Designed to reproduce the sound of a faulty amplifier, Snoddy’s circuit was the inspiration for many subsequent fuzz circuits, including the Tone Bender and the Fuzz Face.

I built the software simulation using SPICE, an analog circuit simulator. SPICE solves for the voltages in an electronic circuit over time, based on physical models of the components. In this case, I used it to solve for the output voltage of the fuzz circuit as the input voltage is sampled from a clean guitar signal.

Here’s the output for two different settings of the attack potentiometer (labeled 12 in the patent diagram), along with the clean guitar signal that was used as the input:

  • riff (max attack)
  • riff (min attack)
  • riff (dry)

The challenge was finding a good SPICE model for the transistors in this vintage circuit.

The patent doesn’t suggest a specific part for the pnp bipolar junction transistors (labeled 16, 25, and 32 in the diagram), but we know that the original Maestro Fuzz Tone used RCA 2N270 germanium parts, and that the AC128 is an equivalent replacement.

The problem is that these germanium devices haven’t been manufactured for a long time and there are no manufacturer-supplied SPICE models for them. I was able to find several AC128 SPICE models online, but none of them gave the expected results in the circuit.

Although germanium transistors were state-of-the-art at the time, they are terrible by modern standards — high leakage current, very temperature sensitive, etc — and circuits designed around them may not function as expected with “better” components. That seems to be the case with the Snoddy circuit, which requires very leaky transistors to work at all.

I suspected that the AC128 models I was able to find weren’t leaky enough, so I started digging into how SPICE transistor models work, with the idea of making my own leakier model.

SPICE uses a Gummel-Poon model for bipolar junction transistors, falling back to a simpler Ebbers-Moll model if certain parameters are missing. Unfortunately, the AC128 datasheets I could find didn’t have enough information to fill in all of the Gummel-Poon parameters. I was stuck.

That’s when I found “Comparison of Germanium Bipolar Junction Transistor Models for Real-Time Circuit Simulation”, Proceedings of the 20th International Conference on Digital Audio Effects, Edinburgh, Scotland, Sept. 2017 by Holmes, Holters, and van Walstijn, in which they analyzed a number of vintage germanium OC44 and AC128 transistors and extracted Gummel-Poon model parameters!

I was able to use their extracted parameters to build an AC128 SPICE model that worked as expected in the patent circuit.

Writeup and code here: https://grwhitehead.github.io/us3213181

AC128 model here: https://grwhitehead.github.io/germaniumbjts/

More info

For more infomation about the Maestro Fuzz Tone and its place in distortion pedal history, check out these videos:

Here’s the demo record used to market the Maestro Fuzz Tone as a replacement for horns: