The Colorsound Tone Bender has been invented in the legendary Denmark Street in London.
According to the legend, a customer visited Macari’s (who still produces the Tone Bender today) longing for an overdrive pedal with more gain than his Maestro Fuzz Tone provided.
The result of this customer wish was the Tone Bender MKI.
The first version used two transistors. This version has been slightly altered (MK1.5) and finally became the MKII which is based on three transistors and can be heard easily on the records of Led Zeppelin.
The experts still argue about the question if the pedal of Jimi Mullard used OC75 or OC81D.
I think, both could be possible because the transistors have been chosen because of their gain factor (hFE) and not because of their type designation.
At the end of the 60s, the MKIII, which additionally offered a tone control, has been invented.
The MKIV uses actually the circuit of the MKIII with a different enclosure.
The germanium transistors of the first MKIV pedals soon have been replaced by cheaper, and at those days pretty modern, silicon transistors.
The Supa Tone Bender and the Jumbo Tone Bender, both produced in the early 70s, have no longer any relation to the primary circuit and are, in fact almost identical to the EHX Big Muff.
The circuit of the Tone Bender has also been sold under different names (like the Marshall Supa Fuzz or the Vox Tone Bender).
There were also some attempts for reissue pedals but their circuits mostly varied al lot from the original one.
Because of its unique sound, which is full of sustain, the Tone Bender still is very famous, and many boutique manufacturers offer clones of the original.
One of the most famous boutique Tone Bender is the Fulltone Soul-Bender (which is based on the MKIII).
Another very nice Tone Bender clone is the Ramble FX Twin Bender which allows to switch between the MK1.5 and the MKII.