Hi! I think it good be great to share our experience regarding Fuzz Face circuit.
You start reading about this tiny circuit and you end up like crazy looking for some old ac128/nkt275 at ebay for some crazy amount of money. They were in the "mandatory" range of hfe so they worth it. They arrive and: fuck, high leakage ones! Sound like shit.
Well, I'm not the master at all but I'd like to share some hints that maybe some others could corroborate.
-There are plenty of different transistors that will work as well as the M A G I C ones. And lots of them are cheap and you can and SHOULD get a bunch of them.
-Leakage. IMO, low leakage is great for Q1 in order to bias easily and to have it sounding good very soon. When I find a low leakage that works well with a couple different Q2, it is likely this Q1 transistor is going to behave pretty well with a damn good amount of Q2 transistors. So I'd say firstly find a stable Q1 and then try different partners.
Something to keep in mind is that you can bias almost every set of transistors you want. You might end up with some extreme resistor values and likely a different sound...Like I said, I find pretty useful having a stable Q1 transistor that make the resistor network close to the original design.
Another point, you will find that for some reason, transistors that on spec (hfe and leakage) looks like good candidates for Q1 they just don't do it. Yes, it happens a lot. What you won't come across so easily is a transistor above 200uA leakage that behaves well for Q1.
So low leakage Q1 makes life easier.
-HFE. It doesn't mean more gain everytime and you're not going to ear changes between 20hfe. I think couples between 50 and 120 will get the sound everyone is expecting. I like to use a high hfe for Q2 around 160hfe. And that leads me to the next:
-Q2 leakage. As the transistor hfe raises up, it is very likely the leakage will do as well. I don't know if it is the hfe, the lekeage or both, but a Q2 with 160hfe and around 350uA lekeage sounds great to me. It starts to collapse at low strings with bass sounds and I really like it for leading. Then I clean with the vol pot por rythm.
I have to say that the sound I would get with a lower hfe and lower leakage Q2 it is not far far away. Of course there are differences, but they are up to taste. So, doubling a transistors specs (hfe and leakage) for Q2 will make it sound different, but maybe not as different as one might expect from reading info on the internet.
-Transistor sound: really? Again, I think it is very subtle and it is more important to find the right ones.
To sum up: low leakage is great for Q1 but not mandatory for Q2.
small jumps in hfe are subtle for the ear
buy 30 cheap transistors, measure them and try them all
make Q2 collector resistor variable
you can almost bias every set of transistors but it doesn't mean they will sound OK
God, I just wanted to share some stuff but gave me headache. I'd rather prefer an audio clip or something. Please don't take it as hard statements.
My personal preference is usually not to go too high on Q2 hfe as I feel higher gain and leakage transistors reduce temp stability considerably while (as you noted) maybe not really producing a higher gain sound.
As you mentioned the NKT275 that may pop up on the market are probably not in the ideal range of specs. The NKT275 I have seen for sale are almost never the Newmarket brand ones which are extremely rare. Zach (Rocket88) and I are both fuzz super geeks and individually tracked down small quantities of real Newmarket NKT275. We both agree that there is something special about the sound with those transistors. There is a clarity and balance that is hard to achieve with other transistors. With that said it is completely impractical to chase the Newmarket NKT275 at this stage in the game.
I dunno if I’m too anal or what but I don’t like to use a bias pot. For me it is quite easy to get it sounding good with the stock circuit. I don’t like the idea of a knob that offers a range of “bad” sounds and personally I have not had a bias pot save me when temp drift sets in. For production it is certainly easier to slap a 70/120 pair blindly in there and adjust with a trimpot but that’s not quite what this DIY thing is about for me personally
There are many valid ways to approach this. If you’re going for pure authenticity then certainly RG Keen hfe advice and trimpots go out the window but it’s easy to make a great FF either way
I really like the sounds with voltages above 4.5v on Q2 collector. I don't like the ones below but I find the sounds at 5.5v or higher pretty good. I could be my gear or just my way using it, the sounds I get are quite similar but a bit better I think.
Yeah, I get a bit more of gain. But it is not drastic. And like vomiting/collapsing sounds on low string-low notes. Very low sounds. I've done some measurements and looks like temperature it's not affecting that much so far.
Ah, I used reversed pots for volume and fuzz and that make a huge difference! The fuzz pot came alive, it was useless to me before that.
I didn't know the NKT275 magic was real. Coming that from you, now I believe it. I'm not crazy about getting ones but I would like to get them at some point in the future just to hear the differences.
It’s very true. As Travis said, I’ve got a few, 4 or 5 I think, matched sets of real deal NOS red dot NKT275. I’ve been saving them to do something special, and had them for probably 8 years or so now. Definitely not worth search for, cause if you find ones they’re going to want some ridiculous price and most likely won’t be what you need.