I’ve started working on a tube distortion pedal using 6021W tubes. Since doing so I’ve been collecting and analyzing circuits for other tube distortion pedals, which I’ve started to summarize here. I’ve simplified each circuit into block diagrams to illustrate their architecture.
If you know of a tube pedal not listed here, please let me know!
The Tube Driver is a somewhat famous pedal, I’d guess it to be the most well-known tube-based distortion pedal. It runs the 12AX7 stages at very low plate voltage—about 9 volts. The 2 triode stages appear to be fixed biased—their cathodes are grounded, with the first stage getting grid bias voltage with a 3.3K resistor from grid to ground, and the second stage getting its grid bias voltage derived from the B+ voltage.
Doug Hammond’s Pentode Driver pedal is a variation of the tube driver using 6AK6 miniature power pentodes in place of the 12AX7 triode stages. Aron Nelson’s Shaka Tube pedal is another variation, with a single SS gain stage, and more traditional 12AX7 tube stages—still low voltage, but cathode biased & bypassed.
This is something from the October 1981 issue of Guitar Player. It’s another starved-plate opamp/tube hybrid pedal design, using a 12AU7 for the tube stages.
The schematic link I had for this one has disappeared.
Fred Nachbaur’s Real McTube II looks like your basic tube preamp, with common cathode triode gain stages. It has a pair of 12AX7 gain stages, with a gain & output level control. It uses a clever power supply with back to back 12VAC transformers to get 140V B+, which is low for a guitar preamp, but definitely puts this in the high voltage pedal category.
The Matchless Hotbox is a large pedal, with a traditional transformer-based power supply supplying 240V of B+. While I’ve left out bypass switching on all these diagrams, the Hotbox actually doesn’t have any—the clean triode is always in the signal path. The circuit itself has no surprises if you’re familiar with tube amps.