Fuzz Face: Can't bias with Ge transistors

[kirs]

I built a basic NPN Fuzz Face circuit which works 100% with one exception.  I got it up and running with 2N2222 Si transistors and was able to bias Vc @ Q2 to 4.5v no problem.  Sounds great but I meant to to add a pair of AC187s for the final build.   For some reason with both AC187s in place Vc only biases from 9v - ~8v via the 10K trimmer on the Q2 collector.   If I put a 2N2222 in Q1 I can again bias to 4.5v but with either Ge transistor in Q1 it does not work and sounds terrible.

Any ideas?  I am sure my circuit is 100% as I have it both on a breadboard and perfboard + pedal.  I'd chalk it up to bad transistors but either one in Q1 is a dud.

Layout below.   R2 is a 10K trimmer (diylc 3 did not have a trimmer component when I did this).  R6 and C4 are to filter DC adapter noise.



Thanks.

[smallbearelec]

Hi--

With the "standard" bias resistors, the circuit is expecting that Q1 will have a gain of 75-100 and Q2 maybe 110 to 140, and that neither will leak much. Probably the 2N2222s actually fall somewhere in those buckets, or close enough. The AC187 may be too leaky to bias correctly without adjusting the other resistors.

Test the AC187s for gain and leakage according to my notes in here:

http://www.smallbearelec.com/HowTos/FuzzFaceFAQ/FFFAQ.htm

or at  GEOFEX.com.

Silicon devices don't leak appreciably. Good germanium devices can leak up to 300 microamps or so before they are too temperamental for the Fuzz Face. But then you should save them to try in an FZ-1A:

http://www.smallbearelec.com/Projects/FZ-1A/FZ-1A.htm

Regards
SD

[kirs]

Thanks, great info.  Looks like I bought the wrong transistors.  Live and learn.

Got leakage of 993 microamps on one with 1206 on the other.   120 and 94 hFE, respectively.   The silicon transistors measure no leakage at all with hFE of 233.

I'll grab a matched pair from you when I put together an order for next month's project.  Thanks again for the info.

[smallbearelec]

Your germaniums are Not wasted! They will be fine for Q2 and Q3 in an FZ-1A, which is a really super fuzz, different sounding from an FF. But for the last 10 years everybody has been totally focused on the low-leakage parts needed for the FF and the RM. Hold those AC187 in reserve. You will want a low-leakage/relatively low-gain part for Q1 in the FZ-1A, and that is easy to find and not expensive.

An all-silicon FF with relatively high-gain parts sounds a little harsh to my ears. There are some tweaks and substitutions in this article:

http://www.smallbearelec.com/HowTos/BreadboardSiFF/BreadboardSiFF.htm

that you might enjoy trying. Happy Construction!

SD