Its the first trim on the bottom left of the circuit board. Clockwise is less, CCW is more. Mark it before you turn it because too much can make it not work(it won’t damage it). There is also a trim in the top middle that messes with the mod depth or speed I can’t remember. Mine will only self oscillate when it is distorted, so I turned it down and it sounds beautiful just no oscillation. That’s a fine compromise for the beauty of its sound to me!