fuzz face build trouble

[Eugene Martone]

Hi. I recently buildt a bc108 fuzz face based on the schematic on this site:
http://fuzzcentral.ssguitar.com/fuzzface.php" onclick="window.open(this.href);return false;

the bypass switch is wired as suggested here:
http://www.diystompboxes.com/smfforum/i ... _next=prev" onclick="window.open(this.href);return false;

(i just thought it was clever)
there's a led with a 1 k resistor between it and B+ and all the ground points are soldered together and to common ground.

Everything seems to work the way it should, except that it cuts out if I play a little soft or turn the volume down just a little. Anyone have any idea what might be wrong, what to search for? I suppose it might be the input cap, but I'm not that well versed in electronics.

Any help much appreciated

[zaphod777]

Replace the 8k2 coming off of Q2 with a 10k-20k trim pot. The bias may be a bit off. Adjust by ear.

[Eugene Martone]

thanks I'll give that a shot

[HTH]

Replace the 8k2 coming off of Q2 with a 10k-20k trim pot. The bias may be a bit off. Adjust by ear.

^ +1

also measure your voltages and compare them with a known-good source - I think the General Guitar Gadgets website lists voltages for pedals.

you'll want around 1/2 of your supply voltage at the collector of the second transistor (i.e. around 4.5v).

[Eugene Martone]

Solved. The tranny (33K) on Q1 collector turned out to be a 3K3 one, changed it to the right value and changed the 8K2 on Q2 to a 5K6 resistor. This got the voltage on Q2 collector to 4,7 and the box is working perfectly.

[Twistingcrow]

Solved. The tranny (33K) on Q1 collector turned out to be a 3K3 one, changed it to the right value and changed the 8K2 on Q2 to a 5K6 resistor. This got the voltage on Q2 collector to 4,7 and the box is working perfectly.

Great! I'm planning a fuzz build and that's exactly what I need to know: I need a 0.5V bias on Q1 collector and 4.5V bias on Q2 collector. It's a PNP germanium fuzz. How do I do to get those voltages? Trimpots? What do I need to measure and how? Can't find that info anywhere...

Thanks!