For the circuit design of the Fzzxkrrrxzt!, we had Johnny and Conny's original vintage pedals shipped to Denmark to be traced and analyzed by my electronics engineering partner.
Interestingly, the FZ-1S is not exactly identical to the schematics floating around the internet, and we choose to build it to the exact specifications of the bob hund pedal.
The FZ-1A proved to be a different kind of challenge, because the old parts, dating back to the early 1960s, have aged and drifted heavily in values. For this circuit, we are adding a switch inside the pedal to shift between original (non-drifted) parts values and the bob hund-specific values. Ther latter setting having less low-end.
The orignal FZ-1A pedals were made with germanium transistors, and because of the unreliabily of these, and the problems with RoHS compliance (EU environmental regulations), we have succesfully recreated the exact sound of the original pedal using modern silicon transistors, with the addition of other parts to precisely emulate the electronic properties of the germaniums (leakiness, internal capacitance etc. etc.) with an amazing result.