This is quite a brutal high gain overdrive by Joe Davisson based on another pedal called the massacre.
it's a very mod friendly circuit - some good places to start are changing some of the capacitors (C1, C6 and C8) for example.
I actually knocked the layout up in 15 minutes but because I have been in so much pain due to a dislocated knee - it took me 2 sittings to actually build it (the valium kicked in 30 minutes into the first half)
anyway it sounds immense
if you have any issues with switching refer to FX Switching
REMEMBER THIS IS A POSITIVE GROUND CIRCUIT - it needs it's own isolated supply eg a battery
otherwise you're going to have a bad time