This pedal employs the EIGHTEEN circuit which is officially licensed from www.runoffgroove.com, This is what runoffgrooove site says:
Around 1965, Marshall introduced a few amps built around an 18 Watt EL84 powered circuit. While these models were intended for practice, the sound was much better than other small amps. Unfortunately, only a few hundred of these combos were manufactured over the next three years. These rare amps were given the model numbers 1958 for the 2X10" combo, 1973 for the 2X12" combo, and 1974 for the 1X12" version.
The features were two channels with Volume and a single Tone on each, along with Tremolo on one channel and an optional Reverb. The 18W amps consisted of 3 12AX7 type tubes and a pair of EL84 power tubes in push-pull configuration. Legend says Jim Marshall bought the circuit from Watkins Electric Music. Watkins sold a very similar amp they called the "Watkins Dominator."
An internet group for cloning these amps has been established at www.18watt.com. On this site, you will find a verified schematic for the 18 Watt combo. After hearing the sound samples there, we decided to give this circuit a try as a distortion stompbox with FETs in place of the tubes.