The Phobos Germanium Fuzz is a clone of the Colorsound / Sola Sound Tone Bender Mk III. While the first version was essentially the same topology as a Fuzz Face, and the second version added a gain stage in front, this third version from 1967-1968 was a completely different circuit, using a Darlington-pair transistor configuration to drive a third transistor, followed by a tone control which the earlier versions were missing.
While the Mk. III Tone Bender was itself a variation of the Baldwin-Burns Buzzaround, it had a few variants of its own, both official and unofficial. The Vox Tone Bender was a licensed variant with a few parts substitutions. The Park Fuzz Sound by Park Amplification (Jim Marshall’s side-brand) was a contemporary clone. The Elka Dizzytone and the Prescription Electronics Yard Box were two others. Aside from the Buzzaround, all of the rest of these variations can be build on this PCB.
A note about the name: the original Colorsound / Sola Sound unit was not called the “Mark III”, but the Vox-licensed version was. As a result, this circuit is commonly called both the “Mark III” and the “three-knob” Tone Bender. This project will refer to it as the Mk III to differentiate it from the Mk II, which is available as another project called the Deimos.
The Phobos project has a voltage inverter which allows you to power the effect with a standard center-negative adapter while maintaining the positive-ground operation of the original. The PCB also includes biasing trim pots so you can dial in a perfect bias without having to swap out resistors.
David Main from D*A*M has written up a history of the Tone Bender up until Mk II, so you would do well to check out his page on the history for the full background story. His history stops at the release of the Mk III so this is where we’re picking up.
In 1968, the Mk II circuit was “updated”, which is another way of saying “completely revised and bearing no resemblance to the previous version”. The new version had three transistors, with the first two transistors being arranged in a Darlington configuration. This has the advantage of coaxing higher gains (HFe) than germanium transistors are normally capable of, while also being much easier to bias and being tolerant of poor-quality transistors.
The gain control on a Fuzz Face or Tone Bender I / II is a pot off the emitter of one of the transistors, directly controlling the gain of that stage. In contrast, the TB Mk III cranks up the gain to a fixed amount in the first stage and then lets you bleed off the signal before it hits the second stage. In other words, it’s a gain cut control wired in reverse, so it has the illusion of increasing it in the same way as other fuzzes or overdrives.
The Mk III also had a tone control, which is something sorely missing from any of its predecessors. It essentially has two fixed passive tone paths at the tail end of the circuit, one that cuts the treble & enhances bass and one that leaves most of the treble intact, and then a potentiometer that pans between them.
The Mk III circuit was also released in a smaller enclosure and called the “Mk IV”, although it’s the same circuit. The two were sold alongside each other for a period of time before they transitioned completely over to the Mk IV.
Later Tone Benders such as the Supa Tone Bender from 1973 and the Jumbo Tone Bender from 1976 were silicon Big Muff clones and again bore no resemblance to any prior versions. (They’re fantastic and worth a look—just not related to prior Tone Benders at all.)