First build - Twin face bias issue (PNP side)

[pi.cast]

Hello fellow pedal builders,
New member here, and I’ve just completed my first build, a twin face/fuzz face clone.

I’ve used AC125 germanium transistors (Q1=80 Hfe, Q2=140 Hfe) for the PNP side and BC108 silicon transistors for the NPN side.

I was able to bias the NPN side correctly to +4,5v using the 10k trimpot, but the closest value I can get on the PNP side is -6,2v.

By doing some forum search it seems like others ran into the same problems because of leaky transistors, but mine have very low leakage (<130 uA) and I’ve also tried to swap a set of AC128 and GT402V without being able to solve the problem.

Am I overlooking something?

Any help is appreciated

Thanks!

 

[kurtis1]

Man, I ran into the exact same issue with needing way too much resistance to bias my ac128's and I tried 5 different matched sets from 2 different suppliers.

A couple of very helpful users here helped me troubleshoot the issue. I ended up getting some p28's from ebay out of the Ukraine and they sound absolutely amazing.

Look at my post history on the lengthy troubleshooting thread.

Edit:
I should add that even with my lower leakage ac128's, I still had the same issue with needing too much resistance to bias them

 

[fig]

Try pairing something like a 2n2222a with that BC108

 

[Coda]

How does it sound, though? The numbers don't always have to be spot on...

 

[pi.cast]

Man, I ran into the exact same issue with needing way too much resistance to bias my ac128's and I tried 5 different matched sets from 2 different suppliers.

A couple of very helpful users here helped me troubleshoot the issue. I ended up getting some p28's from ebay out of the Ukraine and they sound absolutely amazing.

Look at my post history on the lengthy troubleshooting thread.

Edit:
I should add that even with my lower leakage ac128's, I still had the same issue with needing too much resistance to bias them

Thanks, I just encountered your post before reading your answer! I've already ordered some p28's but it's going to be a while before I can test them.
They are also coming from Ukraine.

Try pairing something like a 2n2222a with that BC108

Thanks but the silicon / NPN side is fine. The issue is on the germanium side

How does it sound, though? The numbers don't always have to be spot on...

Well, it's the first fuzz face circuit that I try, so I'm not 100% sure about how it should sound but so far I've been very unimpressed.
The germanium side sounds really muddy, and both PNP and NPN sides are very noisy and, with volume and fuzz controls maxed out, they hiss like crazy.

I've tested all the groundings and the components values and everything seems okay.

I've decided to build this pedal because it had just a few components and a simple circuit, but in reality, it is finicky as hell

 

[fig]

Thanks but the silicon / NPN side is fine. The issue is on the germanium side

Oh bother! I got it wrong way round.
What are the leakages?
You could try some lower hFE sets to see if it's easier to dial in.
Cheers!

 

[Coda]

Thanks, I just encountered your post before reading your answer! I've already ordered some p28's but it's going to be a while before I can test them.
They are also coming from Ukraine.



Thanks but the silicon / NPN side is fine. The issue is on the germanium side

Well, it's the first fuzz face circuit that I try, so I'm not 100% sure about how it should sound but so far I've been very unimpressed.
The germanium side sounds really muddy, and both PNP and NPN sides are very noisy and, with volume and fuzz controls maxed out, they hiss like crazy.

I've tested all the groundings and the components values and everything seems okay.

I've decided to build this pedal because it had just a few components and a simple circuit, but in reality, it is finicky as hell

Fuzz Face circuits are never simple. They also never sound like you think they should. Most people (or at least me) think of Hendrix when it comes to a Fuzz Face. Of course, Hendrix used his as a boost of sorts; into an already maxed Marshall. When I built my Twin Face (BC108/AC124) I was unimpressed. The Ge side was ok, but too dark for humbuckers. Sometimes I have to leave the pedal on for 20 mins or so before the Ge-side behaves. And when the temp is above 75 or so, it sputters out. The Si side, meanwhile, is crisper, clearer, and definitely more fuzzy/hairy. It is also very noisy.

If it wasn't for Hendrix, my opinion is that the Fuzz Face would have faded away, and no one would really care. In my opinion, if you want a great fuzz face tone, build a Vox Tone Bender (Italy)...

 

[pi.cast]

Fuzz Face circuits are never simple. They also never sound like you think they should. Most people (or at least me) think of Hendrix when it comes to a Fuzz Face. Of course, Hendrix used his as a boost of sorts; into an already maxed Marshall. When I built my Twin Face (BC108/AC124) I was unimpressed. The Ge side was ok, but too dark for humbuckers. Sometimes I have to leave the pedal on for 20 mins or so before the Ge-side behaves. And when the temp is above 75 or so, it sputters out. The Si side, meanwhile, is crisper, clearer, and definitely more fuzzy/hairy. It is also very noisy.

If it wasn't for Hendrix, my opinion is that the Fuzz Face would have faded away, and no one would really care. In my opinion, if you want a great fuzz face tone, build a Vox Tone Bender (Italy)...

I've tried it in an already overdriven Plexi style marshall, and a custom shop strat, so ideal conditions in theory.
I'll for sure try another build, but I do want to find out how this one works and behaves.

I'm now thinking to build another one on a breadboard and try different components.

I'll try different transistors on the germanium side, and I've also read that many people install some small ceramic capacitors between the base and collector of Q1 and Q2 to filter unwanted HF noise.

Also, I was testing the pedal with a cheap boss power supply, I'll switch to battery operated.

 

[Coda]

I've tried it in an already overdriven Plexi style marshall, and a custom shop strat, so ideal conditions in theory.
I'll for sure try another build, but I do want to find out how this one works and behaves.

I'm now thinking to build another one on a breadboard and try different components.

I'll try different transistors on the germanium side, and I've also read that many people install some small ceramic capacitors between the base and collector of Q1 and Q2 to filter unwanted HF noise.

Also, I was testing the pedal with a cheap boss power supply, I'll switch to battery operated.

This should help, if you haven't seen it already...

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