Gated

"Gated" as a fuzz adjective can refer to any type of fuzz that gives a splattery, stuttering effect, especially as the note decays. Here, we are only considering fuzzes that employ an actual transistor gate circuit. These appear to be relatively rare. The Fuzz Tone FZ-1B (Version 2) is a vintage example.

The above topology example isn't the only way this could be done, but it does the trick and its a good starting place. Q1 is a shunt feedback amplifier. In the FZ-1B(V2)'s case, the collector resistor is made ridiculously high that it doesn't function very well as a linear amplifier, but makes an interesting fuzz amplifier. Q2 replaces what would normally be a series feedback resistor, or a straight connection to ground. If Q2 is not conducting, Q1 will also not conduct.

What state Q2 is in is determined by the rectified signal that passes through the diode and resistor to Q2's base. The resistor divider at the base, and the reservoir capacitor can be adjusted to change the effect.

FZ-1B(V2) Simulation

Adding in the FZ-1B(V2) component values, and applying some computer simulation of a sine wave input, the mechanism reveals itself.

The input sine is the green trace, and the output signal is the yellow trace.

The red trace is the collector voltage of Q2 and the emitter voltage of Q1 (same node).

Note that at the top of our input signal's swing, the red trace has a positive bump which causes Q1 to cutoff. This cutoff is seen at the output as a negative spike.

Not shown here, but as signal level is reduced the output is cutoff. There is a certain minimum level required to activate the amp. Also, too large of a signal increases the bump seen in the red trace, and this too lowers the output again.

Reducing the ridiculously large 470k collector resistor increases the output significantly. This was probably set up like this to act as a natural limiter. If more output is desired, try reducing 470k to 47k, or even 10k. At 10k, you should be able to get spikes going up and down. A voltage divider or diode clippers will tame the output before the jack.

Messing with Q2's base resistor and the reservoir cap can change things as well. In simulation, the shape of the fuzz can be dramatically changed this way.

Fig 3

Fuzz Factory

The Zvex Fuzz Factory can be summed up as a Fuzz Face with every resistor variable. It is slightly more complicated than that, but that's the basic idea. A simplified topology is shown in Fig 3 above. One of the five controls is labelled GATE and this changes the resistance of the Fuzz Face's Q1 collector load, which determines the operating point of the circuit. By moving the operating point to or near cut-off or saturation, a "gated" effect can be created. This control is very interactive with the COMP control (Fuzz Face Q2 emitter resistor), which also affects the operating point.

Examples:

Maestro Fuzz Tone FZ-1B(V2)

Zvex Fuzz Factory