The early Rat™ circuit with an LM308 opamp and all the best mods I know.
*Three way clipping switch for stock "White Face," "Turbo" and a third asymmetrical "tubey" position,
*"Bite" a variable control similar to the popular "Ruetz mod." This varies the voice of the dirt between the stock aggressive, bright distortion (fully CW) and a warmer, more overdrive-like voicing (CCW).
*Clean blend - a very useful addition to retain clarity and articulation- (popular with certain Jazz players) but also useful for bassists as it helps to retain clarity and subs.