Cloning and Modifying a Mosrite Fuzzrite

Part I: Cloning

Mosrite created and sold the Fuzzrite in the 1960s. It was an effect for the guitar that made the instrument sound like an angry hornet. It's not a sound that everyone likes, but it holds a special place in my heart for its use in some Spaghetti Western film scores.

If one looks up this circuit online, one will quickly find a schematic that looks like Figure 1. Note that Q1 and Q2 are given as the common 2N2222 BJT simply because I could not find conclusive information about the silicon BJTs used in the original. Basically, this circuit works by cascading two simple BJT gain stages together and crudely mixing their respective outputs with a potentiometer. Stick another potentiometer on as the output volume control and it's done.

incorrect schematic

Figure 1. An incorrect schematic for the Mosrite Fuzzrite, commonly found online. Note that R4 and R8 make up a ~350k potentiometer that controls the amount of "fuzz" in the circuit, and R9 with R11 make up a ~33k potentiometer as a master volume control. I could find no information about potentiometers' taper, but it's safe to assume logarithmic taper for both.

correct schematic

Figure 2. The correct schematic, showing the 22k resistor hanging off of Q2's collector.

Most schematics for this fuzz omit the 22k resistor shown in Figure 2. This is unfortunate, as it is a key to this effect's unique sound! In the original effect, the 22k resistor was found not on the PCB, but rather behind it and soldered directly to the one of the "fuzz" potentiometer's leads. Given that, it's easy to see why this resistor is often forgotten. To see how this resistor effects the circuit, I ran a SPICE simulation while changing the value of the 22k resistor.

One of the simulations ran includes the case where R10 = 10MOhm, which is roughly equivalent to removing it from the circuit entirely.

results of a simulation when varying R10

Figure 3. Simulation results when varying R10, showing that low frequency content is reduced with lower values of R10. As the curves move "down" the plot, R10 decreases from 10MOhm to 2.2k. The gray trace represents the case where R10 = 22k.

It can be seen that omitting R10 would give the effect a totally different sound than the original design! Without R10, the fuzz effect would have a lot more low frequency content and sound similar to other popular fuzz effects of the same era. The mechanism is simple - R10 and C3 form a simple high-pass filter together that is "before" the "fuzz" potentiometer.

If one intends to get the famously deadly buzz-saw tone this pedal is known for, it's best not to forget the 22k resistor!

Part II: Modifying

I play a baritone guitar tuned a fourth lower than a standard guitar, so the frequency response of this effect is not quite right for my own use in its stock form. Simply, the lower register notes on the baritone guitar are very quiet when using the effect. We can't have that!

The above simulation of the effect of R10 in the Fuzzrite circuit showed a frequency peak around 120Hz. Specifically, this peak is centered at the B that's two frets above the A string on a normal guitar. To modify this circuit to be more appropriate for a baritone guitar, the obvious choice would be to move this peak down to the F# that's two frets above the E string (~92Hz).

The location of the aforementioned peak in the low-frequency response of the Fuzzrite is largely controlled by C1 and C4. They are the input coupling capacitors for the two BJT gain stages. Below are the results of a simulation where the values of these capacitors are changed in tandem.

results of a simulation when varying C1 and C4

Figure 4. Simulation results when varying C1 and C4 together. Note the change in low-frequency response. The red trace shows the response peak near 92Hz, which corresponds to C1 = C4 = 68nF. The blue trace shows the response peak near 120Hz, corresponding to C1 = C4 = 50nF, their "stock" values.

Not only does increasing C1 and C4 in the circuit move the low-frequency peak closer to 0Hz, it also decreases the magnitude of that peak. Choosing C1 and C4 to be 68nF to place the peak at ~92Hz, the peak will be a few dB down from the original. Recalling Figure 3, one can increase R10 to get back some low-frequency amplitude. A small adjustment in the location of the peak required only a small adjustment in R10 to recover the lost amplitude. 33k was used in the modified circuit instead of the original 22k. This way, the circuit will sound as close as possible to the original, while at the same time be more suited for a baritone guitar.

In addition to adjusting the low-frequency peak of the effect, I thought it may also be prudent to add a simple tone control to tame the high frequency response. I personally like the buzzy nature of the pedal, but it's not hard to imagine the high frequency content being a bit too much in some situations. To create this tone control, I simply copied the designs people use in tone knobs on guitars and stuck a 100k pot with a capacitor to ground on the circuit's output. A simple technique like this is not without side effects, but fuzz pedals are not expected to be well-behaved. As is standard, a logarithmic potentiometer is used for tone.

a sweep of the tone control's range

Figure 5. The range of the tone control.

Finally, the circuit needed some other small modifications to accommodate modern parts. The "fuzz" potentiometer in the original circuit was valued at 350k, but one can't get such potentiometers anymore. With a resistor across lugs 1 and 3 of a more common potentiometer, one can approximate more esoteric values. In this case, I used a 1M resistor across a 500k potentiometer to approximate the original 350k. The volume potentiometer in the original circuit was valued at 35k, but for this one I grew lazy and simply used a 50k model. Coupling capacitors C2 and C3 were originally specified as 2nF, but 2.2nF versions are close enough and much easier to come by.

The modified circuit as-built is presented below.

fuzz rite schematic with mods

Figure 6. The schematic as modified for use with baritone guitar, with an additional tone control, and using more easily sourced components.

As a parting note, I'd like to add a statement about transistor selection in circuits that use the "self-biasing" technique employed in the Mosrite Fuzzrite. Good biasing technique makes a given transistor circuit's behavior depend on anything but the Beta or hfe of a transistor. This is typically done because of the wide spread of Beta / hfe values that exist among transistors of the same part number. It's not hard to imagine why designing such variance out of a circuit is desirable. The "self-biasing" or "DC-feedback" biasing technique used in this circuit is a good example of such a biasing technique and makes the circuit fairly immune to Beta / hfe variation.

What does this mean for the budding circuit guru? It means that swapping out different transistors in this circuit, hoping to achieve a different sound, is a fruitless activity. For a wide range of transistor Betas / hfe values, each stage has about 44dB of gain and each collector will sit at a voltage of about 0.6V, give or take 50mV.

Some people will of course claim that they can hear a difference between two transistors with an hfe of 150 and 200 in this circuit, but I'll leave any "golden ears" discussion to the audiophile crowd.