We’ll start off this entry with a little history.
Going back to the “classic” germanium fuzz, everyone wants to know which transistors to get for the best tone. It’s interesting to note that the original fuzz faces used AC128’s, which were affordable and common at the time, as well as not being very consistent. After a while, they switched to NKT275’s (NKT is the abbreviated form of Newmarket), which are very similar in spec, but had a more narrow gain bucket. Remember, we are aiming for gains of 70-130 HFE, traditionally. The AC128’s, of which I have personally tested over 1,000, actually range from 40-180 HFE.
So, do we use matched or staggered gains for the transistor pair? It depends on what you want - symmetrical or asymmetrical clipping. Either way, you’ll want the gains to add up to about 200 HFE. Matched transistors will give a more symmetrical type of clipping, whereas the staggered gains (ideally 70 and 130 HFE) give asymmetrical clipping. But what part numbers sound best? It’s totally subjective! Everyone has to decide for themselves what they want their tone to be. You aren’t going to sound like Jimi, even if you used his personal pedal, because you aren’t Jimi.
But I can give you some other popular, tried-and-true part numbers to try out. Mullard GET872, OC44, OC75, or OC81d are solid options if you can test them. AC125 is a hotter option from Philips. GE made the 2N527 and 2N508, the 508 being on the higher end of the gain range, but both have great frequency response. The best PNP I’ve ever found is the Soviet ASX12D, as they were very consistent gains, low leakage, and sounded amazing. Other Soviet options are MP20A, MP25B, and GT2308 If you want to be able to use your pedal with the standard 9v power adaptor (without the need for power isolation), you would need to use NPN type transistors. Good options are ASY29, OC139, OC140, and OC141 and the Tesla 104NU71.
The suggestions and numbers above were referencing what works best for the fuzz face. When we get into tone benders and their cousins, other parts of the story need to be discussed, including leakage being required. However, that’s a story for another day.
Switching back to silicon fuzzes, it’s important to note that almost all of them are NPN type and will work with your standard power supply. Historically they used the BC108(C) or BC109(C), both of which have gains consistently 500-600 HFE but can be higher. Other popular options are: BC107(C), BC169C, BC183(C), BC237, 8, and 9, 2N2484 (my personal favorite), BC547, 8, and 9 as well as BC550. Note that all the BC types can have a suffix of A, B, or C or none. No suffix means they are unsorted for gain, the A indicates a low range, B a midrange, and C high gain. For example, a BC107 gain range is 125-900, BC107A is 125-260, BC107B is 240-500, etc.
The global supply of germanium is definitely getting lower and more expensive. If you're looking to build something with that vintage, wooly tone, do it while you still have options. We carry a good supply of rare Soviet and European germanium transistors alike that you can check out here.