Build - Pedal Project: Carlsbro Fuzz-Tone
The Carlsbro Fuzz-tone is a unique fuzz circuit that was housed in a dark hammered-blue FZ-1-like wedge enclosure. It is a 3-transistor germanium fuzz circuit powered by a 9V battery, but as can be seen in the schematic, its circuit topology is quite unusual compared to more well-known 3-transistor fuzz pedals. Instead of being powered by -9V, Q1 and Q3 get their collector voltage from the emitter of Q2. The emitter voltage of Q2 is quite low (a little over 1V in at least one original unit), reducing the available headroom.
Even with the volume knob set to max, the limited headroom results in a fairly low output. The build pictured here can be just a bit louder than unity gain at max volume. There is a 220k resistor at the output which can limit the resulting output level, depending on what follows the Fuzz-Tone in the signal chain. The original Fuzz-Tone appears to have included this resistor to limit the amount of fuzz that will bleed into the clean signal when the effect is bypassed, as the Fuzz-Tone uses a less-than-ideal switching scheme.
If this build is wired for true bypass, the 220k resistor can simply be bypassed, and the output can be taken directly from lug 2 of the volume pot. The pictured build is less compressed and doesn’t have the screaming sustain you’d find on something like a Tonebender Mk2, but it has a smooth decay (no gating) and a classic fuzz sound. The “Attack” knob functions more like a timbre/texture control. The fuzz amount can be controlled to a certain extent using your guitar’s volume knob.
The original Fuzz-Tone used 3x OC71 transistors. OC71s tend to have a moderate amount of leakage current, which plays a role in the bias of each transistor stage here. Using transistors with little to no leakage is likely to result in the build not working at all. The transistors used in the pictured build are 3x OC83Ns. Their measurements can be seen in the below table, taken with a Peak Atlas DCA75 at room temperature.
Most germanium fuzz circuits can vary a lot depending on transistor specs. Three transistors are included in the BoM. We have chosen a transistor type which tends to fall into a similar range and tends to have similar leakage to OC71s for proper bias, but as always with germanium transistors, the specs can be inconsistent. You may need a larger batch of transistors you can try out in the circuit.
This is a PNP germanium build. It should either be run on battery power only or considerations need to be made for how to provide -9V. For more information, 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. The pictured build here includes a battery disconnect switch for disabling battery power. This switch is optional and not included in the BoM. If you would like to build this with a battery disconnect switch, the pictured toggle switch is P/N P-H495. If using a battery disconnect switch, see Figure 5 in our Guitar Pedal Footswitch and Jack Wiring article for the recommended footswitch and I/O wiring.
|1k||1||R-I1K||Carbon composition resistor|
|4k7||2||R-I4D7K||Carbon composition resistor|
|10k||1||R-I10K||Carbon composition resistor|
|22k||1||R-I22K||Carbon composition resistor|
|100k||2||R-I100K||Carbon composition resistor|
|220k||1||R-I220K||Carbon composition resistor|
|1M||1||R-J1M||Carbon composition resistor|
|OC83N||3||P-QSFT353||Germanium PNP Transistors|
|25k linear||1||R-VA25KL||Volume pot|
|50k audio||1||R-VA50KA||Attack pot|
|Switchcraft 112BX||1||W-SC-112BX||Input jack|
|Switchcraft 111X||1||W-SC-111X||Output jack|
|Davies 2150||2||P-K226||Control Knobs|
|Adhesive battery cushion||1||S-H165||Battery mount|
|Adhesive metal clips||1||P-HCABLECLIP-9D5||Cable clips|
|Fiber battery clip||1||P-BATC-SN-1F||Battery connector|