This is a tube emulator. It utilizes FET transistors, which are voltage controlled, to emulate the sound of vacuum tubes. This design uses J201 FETs. The design has full tone stack so you can tweak it to your liking. FET emulators use Field Effect Transistors to replace vacuum tubes. FETs are way cheaper than vacuum tubes and only run on 9v. Vacuum tubes generally run around 300v or so. They clip very similarly so it sounds great for a guitar pedal.