Here’s what the Phat Phuk is: A germanium/JFET booster. A unique sounding pedal which adds a beautiful sheen to your bass’ top end and a bit of grit to the overall tone. It’s a fantastic pedal for situations where you want a lift in your overall level, during a chorus or bridge for example, with a bit of bawdy bump via the germanium transistor.
The exact germanium transistor is a Chinese PNP on the originals, but others can be used. Shoot for around 90 hFE. On originals there is a trimmer that is essentially a gain control. I've laid it out so that you have the option for a trimmer or external board mounted pot on the same board. So if you want the pot externally, don't populate the trimmer and vise versa. Here's the schematic for reference.