Fuzz Face Bias Calculator

This calculator is designed to estimate the bias voltage you will find in the classic Fuzz Face circuit. The bias voltage depends on the transistor specs and resistance values. This calculator can be helpful for finding transistor specs that will bias well in stock fuzz circuits, or for modifying the stock circuits to bias well with a given set of transistors.

This circuit topology is at the heart of a variety of classic fuzz circuits, from the Fuzz Face to the Vox Tonebender to the germanium Schaller Fuzz.

The earliest examples of this circuit used germanium transistors, but later implementations used silicon transistors. Some guitar pedals, like the Arbiter Fuzz Face, were built either way depending on era. This calculator will work for silicon transistors, germanium transistors, or a combination of the two. Leakage for silicon transistors should typically be 0.

Transistor $Q_1$ Values
Transistor $Q_2$ Values
Overall Circuit Values
Note that some minimum values are enforced for resistors to help prevent damage to the circuit

You can enter your own circuit values below, or you can use these buttons to quickly populate overall circuit values with those found in these pedals:

Arbiter Fuzz Face (Germanium)Arbiter Fuzz Face (Silicon)Black Arts Toneworks Ritual FuzzColorsound One Knob FuzzCornell First FuzzD*A*M MeatheadDr. Tony Balls 1966Moreschi Fuzz WahSchaller Fuzz (Germanium)Tonebender Mk1.5Top Gear FuzzVox Tonebender

Tips for Optimal Results

It’s important to keep in mind that transistor beta $(\beta)$, like $V_{BE}$, is not a static value. It varies depending on the circuit conditions (ambient temperature, current, and collector-emitter voltage, among other things). Yet when discussing transistor specs, particularly in the context of guitar pedals, beta is often listed as a single value. When a “typical” beta is listed in BJT datasheets, it is typically listed with the voltage, current, and temperature conditions. For example, the Valvo AC125 datasheet gives a typical beta value of 125 at $V_{CB} = 5\text{ V}$, $I_{E} = 2\text{ mA}$ at 25°C (77°F). That means that if we were designing an AC125-based transistor amplifier which we expected to be operated at room temperature and biased at $I_{E} = 2\text{ mA}$, $V_{CB} = 5\text{ V}$, then $\beta = 125$ would be a good beta value to design for.

However, if the bias conditions are notably different, it may be necessary to determine a more accurate beta approximation. This is particularly true if the bias conditions are an order of magnitude or more off from the datasheet conditions. It is not uncommon to see beta values listed with collector currents that are over 10 times larger than those which we are likely to see in a fuzz pedal. In these cases, the “typical” beta at our bias conditions may be quite far off from the typical beta at the datasheet conditions.

Figure 1 is a chart showing beta plotted against emitter current, taken from the Valvo AC125 datasheet. There are plots for a $V_{CB}$ of -1V, -5V, and -10V.

Figure 1: Beta plotted against emitter current for a Valvo AC125

As you can see, the beta is fairly consistent with $I_E \approx 5 - 10\text{ mA}$, with a notable decrease at lower currents. The emitter currents of both transistors in a Fuzz Face can vary according to transistor specs, but a well-biased germanium Fuzz Face should have currents in the ballpark of 250μA for $Q_1$ and 450μA for $Q_2$. Our $V_{CB}$ for $Q_2$ is typically around 4-5V. Using the Valvo chart, we can find an approximate point that lines up with $I_{E} = 450\text{ μA}$ and $V_{CB} = 5\text{ V}$.