“Correct Leakage Numbers”

Big Monk

Well-known member
There you go, you answered your own question. Sometimes a leaky transistor needs to be used in order to bias the transistor correctly in the circuit so the transistor "turns on" and conducts in order to pass signal. MK2 is an example of this. Cathode-base resistors can be used but I feel it's a "cheat" and a deviation of the original circuit.

It doesn’t take much though. Like I said, 15 microamps is all it takes to get a no-bias Q1 like the MK II to work.
 

jeffwhitfield

Well-known member
Back to my OP.

Let’s take a circuit I know pretty well: The Classic Tonebender MK II.

If you take 3 middle of the road OC75s with medium to high leakage, that stock circuit will bias up to vintage spec almost as an afterthought. The OC75 does have a desirable and definite upper frequency thing going on. Let’s bookmark that.

Now you can pretty much take any low leakage Germanium transistor in a reasonable gain range (60-100 hFE) and tweak the circuit for bias and get most of the way there. What you can them do is tweak the input, treble bleed and emitter caps to get back some of that OC75 “zing”.

What you get with the tweaked circuit that you don’t with the vintage spec is temperature stability. You can make the tweaked circuit sound like the Vintage spec but the Vintage spec will never have the bias stability that the tweaked circuit will.

That’s my whole beef with leakage in a nutshell.
As with anything, the answer is always: it depends.

High leakage, low leakage, high hFE, low hFE....really depends on the circuit itself and what you're going for. Personally, I haven't paid too much attention to leakage. I tend to focus more on hFE which gives me an indication of how much gain I can expect. Now, some of the circuits I've tried run the gamut of either being the typical sputtery fuzz or something that's more tame and yields more of an overdrive quality. One thing I've noticed is that there can be quite a difference in circuits that default to PNP transistors as opposed to NPN ones.
 

HamishR

Well-known member
It's funny - I've made a heap of great fuzz pedals. Some of them I really like, some are just meh, some I have taken apart and used the bits for other things or given to guys who like them.

But I never use one on my board. I have zero use for them. Maybe one day!
 

Chuck D. Bones

Well-known member
HFE and AC signal gain are not the same thing. I discussed this at length in The Boneyard.
Some circuits are sensitive to HFE and most are not.
Some circuits need high leakage, some need low leakage and some don't care. Big Monk is right about leakage biased circuits being inherently unstable thermally. The TB Mk 1 is the only leakage-biased Fuzz that I've bonded with, it just sounds right to me. But I know it will drift all over the place if it gets hot. There are ways to make a circuit insensitive to leakage variation. I did that with my FET-Ge Boost. The Ge transistor has a large emitter resistor and smallish base bias resistors. That's all it takes. The emitter resistor is bypassed with a cap so I don't lose gain with the emitter resistor. I'm more interested in building a circuit that sounds good than building a circuit that is an exact duplicate of something else. If I want to get fancy and stabilize the bias without emitter resistors, then a servo is in order.

Vintage PNP Ge transistors are lower leakage and lower noise than vintage Ge NPN. Because they performed better, they were more desirable and more plentiful. That is why so many vintage Ge circuits, not just guitar pedals, used PNP. The difference in PNP & NPN performance was a consequence of the manufacturing process at the time. The Ge transistors manufactured in the 70s & later had much better performance and there was less difference between the NPN & PNP transistors. My experience with the Russian transistors I've used (mainly MP38A & P28) is that the PNPs have significantly lower leakage.
 

thewintersoldier

Well-known member
It's funny - I've made a heap of great fuzz pedals. Some of them I really like, some are just meh, some I have taken apart and used the bits for other things or given to guys who like them.

But I never use one on my board. I have zero use for them. Maybe one day!
I say the same thing all the time. I can love riffing on a fuzz alone but in my personal playing I find it hard to use. I have built so many that I realized it's for certain occasions. I generally find my RAT to be easier to use and gives more favorable results tonally.
 

Big Monk

Well-known member
I say the same thing all the time. I can love riffing on a fuzz alone but in my personal playing I find it hard to use. I have built so many that I realized it's for certain occasions. I generally find my RAT to be easier to use and gives more favorable results tonally.

This is a pretty key point I think.

For starters, I don’t play in bands so my Fuzz knowledge is all personal (plus some feedback from a handful of commission builds I’ve done) and also confined to me riffing in my bonus room.

I always seem to think of Fuzz as this wild sound but all my favorite recorded fuzz sounds (barring a few wild ones like on The Stooges records, etc.) are really more refined and/or modified to provide a sort of fuzzy overdrive sound.
 

thewintersoldier

Well-known member
This is a pretty key point I think.

For starters, I don’t play in bands so my Fuzz knowledge is all personal (plus some feedback from a handful of commission builds I’ve done) and also confined to me riffing in my bonus room.

I always seem to think of Fuzz as this wild sound but all my favorite recorded fuzz sounds (barring a few wild ones like on The Stooges records, etc.) are really more refined and/or modified to provide a sort of fuzzy overdrive sound.
Most fuzz that has the fuzz sound is scooped in the mids. Unless your playing in a dou or using the right gear in a trio, I find fuzz doesn't work well if you want to be heard in the mix. I find the adding a mids knob to a fuzz makes it easier to cut thru but changes the tone of the fuzz in an unpleasant way. It takes away all the fuzz character and turns it into an overdrive on steroids.
 

Big Monk

Well-known member
Most fuzz that has the fuzz sound is scooped in the mids. Unless your playing in a dou or using the right gear in a trio, I find fuzz doesn't work well if you want to be heard in the mix. I find the adding a mids knob to a fuzz makes it easier to cut thru but changes the tone of the fuzz in an unpleasant way. It takes away all the fuzz character and turns it into an overdrive on steroids.

It always seems to a study in compromises. I always have a Pre-Gain pot (input resistance) on my MK II/Supa/MK 1.5/Fuzz Face builds.

When you get them so your power chords crunch instead of mush out, the lead sound is gone and vice versa.

It’s still such a compelling sound though. It’s so much fun to play and maybe that’s because I’m simply a hobbyist and I don’t have to sit in a mix anywhere.
 

thewintersoldier

Well-known member
It always seems to a study in compromises. I always have a Pre-Gain pot (input resistance) on my MK II/Supa/MK 1.5/Fuzz Face builds.

When you get them so your power chords crunch instead of mush out, the lead sound is gone and vice versa.

It’s still such a compelling sound though. It’s so much fun to play and maybe that’s because I’m simply a hobbyist and I don’t have to sit in a mix anywhere.
I agree it's a cool sound but everything is in the right context. I find for the most usable fuzzy sounds are amp pushed past the edge like a Marshall or tweed fender.
 
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