Seven Fuzz Topologies of the 1960s, Pt. 3: The Tone Bender MkII
Part 3 of Seven Fuzz Topologies of the 1960s, a bench-reference series. New here? Pt. 1 covers the drawing conventions and how to read the clipping diagrams.
3
3-Transistor Modified Darlington
Sola Sound Tone Bender MkII topology · 1966
A Fuzz Face core (Q2/Q3 with the 100 k feedback loop) with an extra common-emitter gain stage in front of it. The pre-stage overdrives the core so hard that the output is a near-symmetrical square with long sustain, and a hard gate when the note dies.
- Devices
- PNP germanium ×3 (OC75 / OC81D), 9 V, positive ground
- Supply as drawn
- +9 V
- Controls
- ATTACK · VOLUME
- Clipping
- transistor saturation, two clipping stages in series
Stage by stage
- Q1 is a plain common-emitter stage with collector-to-base bias (R1 100 k / R2 10 k). Its gain (×40–60 with a leaky germanium part) is enough to start flattening the signal on its own.
- Q2/Q3 are the Fuzz Face pair, but now they are fed a signal that is already many times larger than a guitar pickup delivers. Both halves of the wave hit the rails, so the result is squarer and more symmetric than a Fuzz Face and it sustains far longer.
- The ATTACK pot is the Fuzz Face’s FUZZ pot: it bypasses Q3’s emitter with C3 (47 µF) to raise the gain of the core.
- Because the core is biased so hot and starved of headroom, it drops out abruptly when the input falls below a threshold. That "gated" decay is a feature of the MkII, and the reason the Mk III and Mk IV added an emitter resistor and tone control to tame it.
Bench notes & values
- Values follow the Sola Sound Professional MkII. The 8.2 k at Q3 is usually built as a 470 Ω resistor in series with a 10 k trimmer so Q3’s collector can be set to 8.0–8.5 V below the rail; Q2’s collector wants 1.0–1.75 V.
- Typical hFE targets: Q1 55–60, Q2 70–80, Q3 100–140. Leakage matters as much as gain; a modern low-leak silicon part in Q1 needs a smaller feedback resistor (try 47 k).
- The video’s sketch shows an emitter resistor on Q1; the MkII grounds Q1’s emitter, the Mk III/IV do not. Both count as the same family.
- Bench cost: about $3 silicon, $25–50 for three matched germanium PNPs.
Same topologySola Sound Tone Bender MK IISola Sound Tone Bender Professional MKIISola Sound Tone Bender MK IIISola Sound Tone Bender MK IVTone Bender MKIII / MKIVVox Tone Bender Mk IIIColorsound Tone Bender Mk2Colorsound OverdriverBurns Buzzaround