Build - Pedal Project: One Knob Tonebender MK1
The Tonebender Mk1 is one of the earliest fuzz pedals, and its sound, circuit topology, and construction reflect that. It tends to have a raw, untamed fuzz character, and the circuit is notoriously picky about which transistors will work and sound good. Originals were built on a hand-drilled phenolic board without eyelets or solder pads. This layout is adapted for more standard turret/eyelet boards, and this variant removes the “Attack” control entirely. In the Mk1, the “Attack” control is not a standard Fuzz/Gain control, but rather a method of changing the bias of transistor Q2. Often, Mk1s will have a sweet spot on the Attack control where the pedal sounds best. This circuit is as if the Attack control is set fully clockwise at all times.
Removing the Attack control, which allows you to adjust the Q2 bias, makes proper transistor selection even more important. Three OC41s are included in the BoM below, but you may need a larger batch of transistors you can try out in the circuit. Alternatively, you can use lugs 1 and 2 of a 50K trimmer in place of the 51K resistor to use it like an internal “Attack” control. That will help dial in a sweet spot for Q2.
Q1 needs some leakage current for that stage to function. If the signal coming out of the emitter of Q1 is weak or silent, it is probably not a leaky enough transistor. A leakier transistor would fix the issue, or you can connect a resistor from the collector of Q1 to its base. This allows for additional current to flow from the collector into the base of the transistor, simulating transistor leakage. It is best to start with a large value resistor (e.g. 1M5) and decrease it from there until the output from Q1 is strong. A smaller resistor results in more simulated leakage. The less leakage the transistor has, the smaller the resistor needs to be.
This is a PNP build and uses battery power only. With no LED indicator, this pedal will last a very long time on battery power. If you would like to build this pedal with a DC jack, please see our PNP "Positive Ground" Pedal Considerations tech article.
See Figure 4 in our Guitar Pedal Footswitch and Jack Wiring article for the recommended footswitch and I/O wiring.
|100 nF||2||C-MKT-D1-400||Film cap|
|10 nF||1||C-MKT-D01-400||Film cap|
|500k Audio||1||R-VAM500KA-SS||Volume Control|
|15 kΩ||1||R-I15K||Carbon Comp Resistor|
|180 kΩ||1||R-I180K||Carbon Comp Resistor|
|1 MΩ||1||R-J1M||Carbon Comp Resistor|
|2.2 kΩ||1||R-I2D2K||Carbon Comp Resistor|
|2.2 MΩ||1||R-J2D2M||Carbon Comp Resistor|
|47 kΩ||1||R-I47K||Carbon Comp Resistor|
|51 kΩ||1||R-I51K||Carbon Comp Resistor|
|8.2 kΩ||2||R-I8D2K||Carbon Comp Resistor|
|Fairchild arrow knob||1||P-K347-52||Knob|
|Vinyl battery snap||1||K-P155||Battery snap|
|Low Profile jack||2||S-H534||In/Out jacks|
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