I stumbled across this fuzz design...

[zachomega]

http://www.euthymia.org/DIY/

While searching for the TA-28 fuzz manual (which I still haven't found), I came across this RCA fuzz design.  Any ideas for transistors? 

-Zach Omega

[MartyMart]

Q1/Q3 NPN - try 3904's or "Keens second law" 2n5088's
Q2 - PNP 2N3906

MM.

[brett]

Hi
Q1 is ok for a 2N3904 or similar, but the others will produce too much gain with modern transistors (there's no feedback via emitter resistors).  Try a BD139 or 2N2369A for the NPN and a BD140 for the PNP.  Alternatively, you could add a 100 ohm resistor on the emitters of Q2 and Q3, but this often has a slightly negative effect on the tone because it centres the bias point.
cheers

[zachomega]

I did a google search and found a lot of people having problems putting this circuit together and making it work. 

I breadboarded the thing and found the voltages to be all out of whack.  I used a 2n404 with an hfe of approximately 140-145 for Q2 and a bc 547 with an hfe of 250 for q3.  I didn't bother wiring up Q1 since it is just an emitter follower. 

This was just a breadboard to get voltages.  I didn't wire up any of the surrounding circuitry to test it with audio. 

Without an emitter resistor on Q2, it is basically an open switch.  The voltages I read from Q2 were B - 8.96v, E - 9.18v, C - 9.1v.

I played with putting a 470 ohm resistor on the emitter of Q2 and it put the Collector voltage at nearly half of the supply voltage right away. 

Then I wired up Q3.  Once again, it was like an open switch functioning.  I suppose the 47k resistor could be increased or an emitter resistor could be added to help stabilize Q3.  I suspect a little of both will be needed since putting a small emitter resistor (100 ohm) on Q3 barely had any effect and increasing the emitter resistor too much will play havoc with the gain. 

That also leads to the question of should the emitter resistors be bypassed since they weren't in the original schematic? 

-Zach Omega