The legendary Guild Rotoverb,used by Nils Lofgren,Neil Young,Hendrix,to name a few. I did some research on this effect and I think I figured out which circuit it is. During this time in the 1960's a lot of companies were contracting other manufacturers to build their effects and license their name on it. This pedal in particular was actually made by JEN in italy. The cct is gooped,but I would bet that this cct(circuit) is a Farfisa Sferasound!