Fuzz face bias problem

[MickJ]

Hi all, greetings from tropical Darwin Australia. I have recently breadboarded my first pedal (total novice) a germanium FF clone and, after only one false start with no output at all,  lo and behold it works! I didn't quite sound like Jimi H but I thought no problem, ge trannies in the tropical heat etc and set about adjusting the bias at Q2 collector. Swapped  the 8.2k for higher, then higher, then ever higher values , added a trimpot and finally dialled in about 4.7v with about 42k of resistance, which I thought was very high. The circuit sounds ok although has a bit of wobble (oscillation?).
All components aside from the 8.2k resistor are standard FF and the transistors are AC128 with hfe of about 80 and 105. Not sure of leakage. Also I'm pretty sure everything is wired up correctly.
I guess my guestion is, is the need for a  much bigger resistor value possibly due to a fault somewhere else in the circuit and, if I can dial in an appropriate bias v and the pedal sounds not too bad (although I think it could sound better), then is this a problem?
Any help/ideas/advice would be greatly  appreciated as I am very much a cold, scared, alone little child when it comes to pedal building.

[MickJ]

I think I posted this in the wrong place. Just found where it should have been. Oops. Don't know how to move it.

[duck_arse]

Mick - can't help you much with the ff problem, but if you click "report to moderator", and then type in some old .... something, about moving the thread, the gods might stir and see you right.

welcome to the forum. I don't believe the part about cold, by the way, not in darwin.

[MickJ]

Thanks mate, I'll give that a go. You're right about not being cold. Almost midnight and about 29 degrees ( C not F)

[duck_arse]

you seen the temps in sydney recently? weatherman told us it was going to get hotter overnight tonight.

[italianguy63]

Hi all, greetings from tropical Darwin Australia. I have recently breadboarded my first pedal (total novice) a germanium FF clone and, after only one false start with no output at all,  lo and behold it works! I didn't quite sound like Jimi H but I thought no problem, ge trannies in the tropical heat etc and set about adjusting the bias at Q2 collector. Swapped  the 8.2k for higher, then higher, then ever higher values , added a trimpot and finally dialled in about 4.7v with about 42k of resistance, which I thought was very high. The circuit sounds ok although has a bit of wobble (oscillation?).
All components aside from the 8.2k resistor are standard FF and the transistors are AC128 with hfe of about 80 and 105. Not sure of leakage. Also I'm pretty sure everything is wired up correctly.
I guess my guestion is, is the need for a  much bigger resistor value possibly due to a fault somewhere else in the circuit and, if I can dial in an appropriate bias v and the pedal sounds not too bad (although I think it could sound better), then is this a problem?
Any help/ideas/advice would be greatly  appreciated as I am very much a cold, scared, alone little child when it comes to pedal building.

Mick--

I have seen that happen before.  When it happened to me, the trannies were just very high gain.  High Hfe's.  Also, the high gain combo like that does make the fuzz do some strange (and neat) "wobbley" things.  You might recheck the Hfe's...  MC

[MickJ]

Thanks for the rply MC, I thought that it may have had something to do with the transistors. I bought them as a FF pair and also tested them myself in one of those little component testers that you can get on ebay these days, which are supposedly pretty good. Both came in close to the hfe values that I bought them for. Maybe I have to set up Mr Keens transistor tester to make sure. The circuit sounds ok so I think I'm going to solder it up but with sockets so I can swap the trannies over later. Thanks for your help.  Mike

[Electric Warrior]

How are your voltages with stock values?
If Q2's collector voltage is off, so is usually Q1's. You can adjust both at once by tweaking Q1's collector resistor.

[MickJ]

Aha!I'll give that a shot. Thanks Mr Warrior. What should Q1 be?

[LightSoundGeometry]

http://www.geofex.com/article_folders/fuzzface/fftech.htm

[MickJ]

Nice one. Thanks

[Electric Warrior]

Aha!I'll give that a shot. Thanks Mr Warrior. What should Q1 be?

It depends a lot on the transistor's leakage. I'd expect a good sounding set of germaniums to bias anywhere between 0.15 and 0.7V. That would put Q2's collector voltage  between 8.5 and 4.5V. There's quite a range in which this circuit can sound good.

But then again voltages in that range don't guarantee a good sound. It depends a lot on the matching (or mismatching) of the transistors.

[Ruptor]

I don't understand why people are so hung up on the Hfe of the transistors. Is it because you want to stick to the resistor values of the original circuit? The gain is set by the ratio of the emitter and collector resistors so doesn't it make more sense to set them for the optimum bias? Then every circuit built would have slightly different resistors but all sound the same and people could change them from the optimum to give what sound they prefer.

[Electric Warrior]

Hfe doesn't even have all that much influence on the voltages with germaniums. Leakage is far more important. And you can easily make up for lack of leakage or too much of it by tweaking Q1's collector resistor.

As there is quite a range of voltages in which this circuit sounds good, I don't think that all that much tweaking is required. If you need to mess with more than Q1's collector resistor, your transistor selection is probably not very good.

I'm under the impression that a lot of people go for certain hfes and leakages and then mess with the collector resistors of Q2 or both transistors to bias them exactly like someone on the internet recommended ages ago. They're doing everything at once.

[MickJ]

Hi all, thanks for your helpful replies. I think I found the problem. I set up Mr Keens transistor tester and found that the the leakage on both was huge-readings of just under 2v and 2.7v (both with the switch off ). So time to track down soome new (old) ones. Thanks for your help 

[Electric Warrior]

That is quite a lot, indeed. Have you tried making up for it by using a smaller resistor for Q1C?

[duck_arse]

just for laffs, Mick, chuck in any silicon PNP, if you have one, see what readings you get in yr leak-o-meter.

[MickJ]

Yep,  thought I might have wired it wrong so tried 2 silicon pnps and got zero readings. Does tweaking Q1 collector resistor help with leakage?

[Electric Warrior]

With too much leakage Q1C measures very low and Q2C very high. Using a smaller value resistor on Q1's collector will bring up Q1C and bring down Q2C. So yes, that should help.