Fuzz face and germanium transistor AC128

[Coelhomatias]

Hi, first post in this forum, I built a fuzz face yesterday, simple circuit nothing very fancy, I finished soldering the circuit and tested it, beautiful fuzz I was very excited. A few minutes later I tested it again and the sound was horrible, it only had output when the guitar volume was at max and it made a lot of popping noise. After some debugging I discovered it was the temperature, I heated the solder traces again and that beautiful sound came out. What should I do??

[brianq]

Welcome, it's good etiquette to post a schematic and/or pics of your build to get us on same page. But it sounds to me like your soldering is suspect? Go back & check all your connections

[thermionix]

I think it's bias.  If you put a 100k trimmer in place of the 33k resistor, and maybe a 20k trimmer in place of the 8.2k, you should be able to dial the sound back to the sweet spot at room temperature.  The first trimmer might be enough to get you there.  If you don't have room to leave the trimmers installed, you can take them out and measure their "sweet spot" resistance and replace with a fixed resistor of the nearest value.

[Electric Warrior]

Or we could simply try to guess the ballpark of the resistor if you could measure the voltages for us. 

[Coelhomatias]

Thank you guys I'm going to try the trimmer option and see what I get. Could you please explain me how you reached the conclusion that it was a bias problem?

[R.G.]

Drifting bias is one of the most common issues with germanium transistors. They are very heat sensitive - they make GREAT temperature sensors, in fact, because they drift so much. They're also very leaky. They also have a high failure rate, as most of the germanium devices we have were made with semiconductor processes that are hopelessly primitive compared to today's

They also have a built-in trap for people used to dealing with "normal" power supplies, in that all the polarized capacitors are "backwards" to how we usually think of them, so it's easy to get one or more caps backwards and have them degrade over time. This alone can cause a drifting-worse bias problem.

Germanium circuits are full of pitfalls for us "modern" circuit users. So bias is a likely suspect.