BK Butler Tube Driver

Schematic image found in the (expired) U.S. Patent for the Tube Driver.

Good links for info on this pedal:

https://www.google.com/patents/US5022305

http://www.kitrae.net/music/Music_mp3_Tube_Driver_Sound.html

This pedal does indeed include a real 12AX7 tube inside the enclosure. However, the DRIVE control is controlling an op amp distortion stage with loads of drive. Max gain is on the order of x500, and a log pot is used (using patent schematic), instead of a reverse log, so the gain is incredibly scrunched. The gain control is pretty useless, in my experience.

One particular customer wanted more variety out of this pedal. He liked the lower gain settings, and wanted more from that range. I can't recall what I did exactly; I'm sure I tried raising the input resistor value to the drive stage, and adding a resistance in parallel to the drive pot. Either one of those mods or a combination ended up being satisfactory, and the customer was happy with the tamer version. He had tried using different 12A_7 tubes, since the gains of those tubes are different - this won't work to tame the drive. The drive is all happening in an op amp before it hits the tube.

Phase is inverted at the output. (There is 1 non-inverting stage followed by 3 inverting stages.)