Build - Pedal Project: One Knob Tonebender MK2
The Tonebender Mk2 Professional is a classic effect most well-known for its use by Jimmy Page. It tends to provide more extreme saturation and sustain than a germanium Fuzz Face (or the similar Tonebender Mk1.5). Most one knob fuzz pedals use the Arbiter Fuzz Face circuit topology. The Fuzz Face is known to “clean up” well when the guitar volume is rolled back. In other words, the level of saturation in a Fuzz Face can be controlled well without tweaking the “Fuzz” control. It’s fairly common for users to leave the “Fuzz” control maxed and only use their guitar’s volume knob for control. Because the guitar volume can control the amount of saturation, a one-knob version of the Fuzz Face can still provide a wide range of tones.
This is less true of the Mk2 circuit. The Mk2 has an additional gain stage in front of the Mk1.5 and Fuzz Face circuits, and it tends to provide a more saturated sound which is less affected by changes to the guitar volume.
The Tonebender Mk2 can be sensitive to which transistors are used. Three OC41s are included in the BoM below, but you may need a larger batch of transistors you can try out in the circuit. Original units used different resistor values in two locations, which can have an effect on the transistor bias. One option for tuning the circuit is to try out the different original values that Mk2s used to find the best bias while staying true to the original units. In the below layout drawing, the optional value changes are shown in parentheses, and the resistors to tweak are highlighted in violet.
This is a dense layout, and in order to fit it all on a 2x6 board in a 1590B, there are two unusual connections. The first one to note is that one of the resistors jumps a column. In order to do so, its leads need to either cross over leads that they should not touch, or one of the leads needs to be bent to go around those components (as seen in the diagram). In either case, we recommend that you insulate the indicated green lead to prevent any risk of shorting.
The other unusual connection to note is that the emitter of Q2 is wired directly to lug 1 (GND) of the Level pot. This works well in a 1590B, as the compact layout puts the level pot right near the base and collector contacts on the board, and the lug can be easily used as the third connection. If the lugs on the level pot are not close enough to the board to make that connection, there is an alternate board layout (Figure 2) which has the emitter of Q2 wired to the board and the 10nF output capacitor wired between the board and the Level pot.
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.
|10n||2||C-MKT-D01-400||Film cap||The 10nF cap at the input is not present in all originals.|
|100k||1||R-I100K||Carbon Comp Resistor|
|10k||2||R-I10K||Carbon Comp Resistor||The 10K resistor from Q1 base to ground was sometimes 100K|
|470R||1||R-I470||Carbon Comp Resistor|
|47k||1||R-I47K||Carbon Comp Resistor||Sometimes 100K|
|8k2||1||R-I8D2K||Carbon Comp Resistor|
|1k||1||R-I1K||Carbon Comp Resistor|
|Vinyl battery snap||1||K-P155||Battery snap|
|Fairchild-style pointer knob||1||P-K347-52||Knob|
|Low profile stereo jack||2||S-H534||1/4" Jacks|
|Adhesive cable tie mount||1||P-HTIEMOUNT-BLK||Battery Strain Relief|