NPN Tonebender Mark I -- Q2 gain/leakage

[MordechaiBenZev]

I have some good NPN transistors for a MK I build, at least for the Q1 and Q3 spots: an AC127 Hfe 65 with 450mA leakage, and an AC176 Hfe 90 with 500mA leakage.  It's Q2 I'm having trouble with...none of my devices have the right gain range.  In my PNP build, I used an AC125 with 115 about Hfe and leakage of 250mA, and the circuit sounded great.  But none of my NPN's have that.

On the other hand, I have several 2N1308's that are in that gain range, but which have much lower leakage.  If memory serves, having very low leakage on Q2 in this circuit is okay if Q1/Q3 are sufficiently leaky.  Am I right about that?  Could I pop in a 2N1308 at, say, 120 with under 100mA leakage and have the thing work properly?

[antonis]

an AC127 Hfe 65 with 450mA leakage, and an AC176 Hfe 90 with 500mA leakage. 

That's not leakage.. That's flood..!!

[iainpunk]

I can not confirm that it works, but you could try the first and third stages NPN, and "flip around'' the middle gain stage and put in a PNP gain stage?

Maybe try it out on breadboard first

Cheers

[MordechaiBenZev]

an AC127 Hfe 65 with 450mA leakage, and an AC176 Hfe 90 with 500mA leakage. 

That's not leakage.. That's flood..!!

Yeah, it's a lot but in the Mark I circuit, high leakage yields an amazing sound.  In my PNP build (OC75, 2 AC125s) my Q1/Q3 were around 400ma and Q2 was 300, and it sounds incredible -- like, amazingly beautiful.

[iainpunk]

an AC127 Hfe 65 with 450mA leakage, and an AC176 Hfe 90 with 500mA leakage. 

That's not leakage.. That's flood..!!

Yeah, it's a lot but in the Mark I circuit, high leakage yields an amazing sound.  In my PNP build (OC75, 2 AC125s) my Q1/Q3 were around 400ma and Q2 was 300, and it sounds incredible -- like, amazingly beautiful.

i believe he means that you are a order of magnitude off, that its supposed to be leakage in uA, not mA

cheers

[MordechaiBenZev]

DERP!

Yes, of course, uA.

With that in mind -- anybody have suggestions about using a low leakage device in the Q2 position?

[antonis]

It's now time to lend you "μ" which is the proper unit mark for micro..

[danfrank]

In mine (PNP), #1 was high leakage, also #3. #2 was the highest gain (110)  of the three with low to moderate leakage 100-140ua, IIRC. I like the way it sounds, about 1/3 of the attack knob I have gating and the rest is thick fuzz. On mine, when the attack knob gets to solid fuzz, the sound doesn't change much with further knob turning. Mine is sensitive to temperature fluctuations, so it likes a studio environment

[MordechaiBenZev]

This is helpful info for me Dan, thanks.  I the build I'm doing, Q1 will be 65 Hfe and 350uA and Q3 will be about 95 Hfe with similar leakage.

I have two options for Q2, both of which sit around 120-125 Hfe: a moderate leakage device at about 250uA, and a *very* low leakage device at about 60uA. I've put a 470K resistor on the base of Q2. 

Would the lower leakage Q2 function as well in this circuit as the moderate leakage device (and maybe impart a touch less temperature sensitivity as a result)?

[Bret608]

If you have 470k on the Q2 base along with a 47k (more likely 50k) fuzz pot, then you should be able to use something less leaky in Q2. That worked on a build I did anyway. My Q1 and Q3 leakages were similar to what you've got as well. I'd say try both of those 2n1308s you have and see which you like better! This circuit is so picky about transistors even when the gain and leakage specs look "right", so you might as well swap around a bit to see what sounds best to you.

[MordechaiBenZev]

Thanks, I will give that a shot!  And the info for Q1/Q3 changed from my first post as the ones I now have with slightly lower leakage seemed to be closer to what most people have reported about what works well in this circuit.  But I'm guessing the 400/500uA leakage devices could still probably work well in the first and third spots if they were all I had to work with.

[Bret608]

Lower leakage in Q1 will still work well. I think I had something like 250uA in that spot. On Q3, it's interesting...that spot can play a big role in how gated or sustaining the fuzz is. More leakage will mean more sustain/less gating. luciferstrip had a great post some years back on this forum on how to tune each position in this circuit, but it always takes me a while to dig up again! 

[iainpunk]

a way of faking some leakage, you could place a high resistance between the Collector and the Base of Q3. im talking about 1MegaOhm or more

cheers