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
~\beta_1~:
~I_{LK1}~:μA
Note that this value is entered in microAmperes
~V_{BE1}~:V
If ~V_{BE}~ is unknown, 0.1V for germanium and 0.65V for silicon are reasonable approximations.
Transistor ~Q_2~ Values
~\beta_2~:
~I_{LK2}~:μA
Note that this value is entered in microAmperes
~V_{BE2}~:V
If ~V_{BE}~ is unknown, 0.1V for germanium and 0.65V for silicon are reasonable approximations.
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
~V_{CC}~:V
Enter the absolute value of the supply voltage here. For PNP circuits, the negative supply voltage should be entered as the numeric value (i.e. -9V should be entered as "9").
~R_{C1}~:Ω
~R_{C2A}~:Ω
~R_{C2B}~:Ω
~R_{SH}~:Ω

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

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}~.

Figure 2: Expected “typical” beta value for ~I_{E} = 450\text{ μA}~ and ~V_{CB} = 5\text{ V}~

Figure 2: Expected “typical” beta value for <span class="tex2jax">~I_{E} = 450\text{ μA}~</span> and <span class="tex2jax">~V_{CB} = 5\text{ V}~</span>

The beta value appears to be ~\beta \approx 105~, as you can see in Figure 2. This would be a more accurate “typical” beta to use for ~Q_2~ when designing around an AC125. You’ll notice that the emitter current never gets down to 250uA in the plot, which makes it difficult to determine a typical beta value for ~I_{E} = 250\text{ μA}~. Ideally, the datasheet would show us the expected beta at 250μA. It’s not too far off from 450μA, so we will use the beta value at 450μA again for a reasonable approximation. However, ~Q_1~ ~V_{CB}~ is usually ~1V or less in a germanium Fuzz Face, so we will use the ~V_{CB} = 1\text{ V}~ curve here, which gives us a beta of ~\beta \approx 90~.

This is an approximation using a “typical” beta from the datasheet, but the same logic applies to measured beta values as well. Germanium transistor specifications can vary quite a bit, particularly given that their leakage plays a role in the bias. It’s fairly common to take measurements before using them in a circuit, but it’s important to consider what conditions they are measured at. The Peak Atlas DCA75 and DCA55 are common off-the-shelf tools for measuring germanium transistors, and they both measure beta at a collector current of 5mA. The R.G. Keen method, a common minimal-parts DIY option, uses a set amount of base current so ~I_C~ varies depending on transistor beta and leakage (but it is typically much closer to Fuzz Face range for beta values you are likely to find in germanium types).

Though you cannot change the collector current which the DCA75 measures beta at, it does allow you to plot beta against collector current for a fuller picture. In Figure 3, you can see the measurements of three different Valvo AC125s with the “typical” plots from the Valvo AC125 datasheet overlaid in blue. Note that the Valvo chart shows beta plotted against emitter current rather than collector current, but they should be very similar.

Figure 3: Beta plotted against collector current using three Valvo AC125s

Figure 3: Beta plotted against collector current using three Valvo AC125s

The actual AC125 plots are not as ideally flat at higher currents compared to the Valvo chart, but they line up fairly well. Like the Valvo chart, the DCA75 does not measure these at as low of a current as we would like, but it gives a much more accurate picture than simply using the beta measurement at 5mA. As you can see in the chart, the beta at the low (<1mA) collector currents we are likely to see will be notably lower than the measured beta.

If we simply used the beta measurement that we got from a DCA55 or DCA75, there is likely to be more error in the bias estimates because the transistors are operating under different conditions from where the values were measured.

The same is true for the base-emitter voltage (~V_{BE}~).

Figure 4: Base current plotted against ~V_{BE}~ using three Valvo AC125s

Figure 4: Base current plotted against <span class="tex2jax">~V_{BE}~</span> using three Valvo AC125s

The forward voltage of the base-emitter junction varies depending on how much current is flowing into it. Figure 4 shows the plot of 3 different AC125s again, where you can see that ~V_{BE}~ varies from less than 0.1V at very low currents to ˜0.25V at ˜5mA. You may occasionally hear that 0.3V is the “nominal” forward voltage of a germanium diode or PN junction. Most small signal germanium types max out around 0.3V at a large enough current. However, for lower currents, the forward voltage can be a lot smaller. With 1mA flowing into the base of the AC125s, the voltage drop is just over 0.2V. In the Fuzz Face, the base currents are typically around 10μA or less. At these currents, typical small signal germanium BJTs will have a base-emitter voltage drop that is closer to 0.1V or even less. On the AC125s, the voltage drop with 10μA flowing into the base is around 0.07V. ~V_{BE}~ can be important for bias, but if it is unknown, 0.1V is typically a closer approximation than 0.3V at Fuzz Face currents.

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