Fuzz Face 69 clone over saturated

[matsmorot]

Hello pedal heads!

So I recently built my first pedal, a Fuzz Face 69 clone from musikding.de.
Schematics: https://www.musikding.de/docs/musikding/face69/69erschalt.pdf

The kit's Ge transistors are AC125 and AC125U, supposedly with 80-89 and 110-119 hFE values.

I was happy to see it worked right away, but not very impressed by the sound. Unless I have the fuzz knob set very low it is overly saturated. And when set it low the sound is quite boring.

I have tried switching the transistors, since I didn't take note of which one had which hFE value, and also tweaked the trim pot, but to no apparent avail.

Could the transistors be bad, or have I perhaps done something wrong?

Maybe easier to understand with some audio, so I'll try recording a clip this weekend.

(I'm using a Gibson LP Studio 2004)

[matmosphere]

Do you have a multimeter? Did you bias the transistors using the trim pot? That is where I'd start.

Make sure you have all the resistors in the correct places.

Beyond that, sound clips would be useful. But imo Fuzz Faces can be pretty saturated sounding.

[matsmorot]

I do have a multimeter but I did not bias the transistors, simply because that wasn't mentioned in the kit's description. You live and learn.
But I'll try that as well this weekend then.

Thanks for the suggestions!

[matsmorot]

OK, so now I have checked the resistors placements and color codes, and they are OK.
I'm reading 7.69 V on the Q2 collector, which would explain (?) the over saturated sound I guess.
Unfortunately the voltage doesn't change when I change the value of the 10K trim pot or the bias knob.
I have measured a difference in resistance in the trim pot itself, so it appears to be working.
The soldering looks OK I think (for a beginner). But if something would be wrong with it, there would be no sound at all right?

[matsmorot]

OK, wow... After measuring it sounds even worse. Before it was usable to some extent. Now it's just crap. 

Here's a video clip of what it sounds like now:
https://youtu.be/V7V5lKlDkqk

[midwayfair]

That's not saturation, it's just misbiased. And if it changed when you measured it but you didn't touch the trimpot, you probably have some bad solder joints.

Take measurements on all three pins of both transistors after you reflow your solder joints and then adjust the trimpot to something lower on the Q2 collector.

[matsmorot]

Did another measure now, and I don't know where that prior reading of 7.69 volts went. Now I'm getting these values:

10K trim pot only differs 0.06V at max. Sounds very low to me.
Q2-collector: 9.21V
Q2-base: 0.33V
Q2-emitter: 0.34V

Have not yet done any reflowing of solder joints. I'm guessing I would start by just reflowing Q2, or should I reflow everything on the PCB? 

[matsmorot]

Hello again...

So I started scratching the PCB where the solder joints were close, and now I get around 5.4V on Q2 collector. And now the voltage changes when I adjust the trim pot. Progress, but it still sounds crappy.

I noticed that the 10K RV1 (trim pot) on max actually only measures around 7K ohm. So, would a 20K on RV1 perhaps get me to the magic 4.5V?

[matsmorot]

Aaaaand he's back! 

This is just weird. After having left the pedal for a while (off) I fired away with the measuring again, and now I got around 8V on Q2c. And the voltage is dropping, ever so slowly. About -0.1V in 15 minutes.

Is this just more evidence of bad solder joints?

[gordo]

That's where I'd start.  There's really not enough on the board to get too complicated.  Make sure all connections and solder pads are stable and well soldered and take it from there.

[matsmorot]

So, there was nothing wrong with my soldering. At least not totally.
I found the solution in a YouTube video of an assembly of the same pedal, where the guy pointed (no talking) at the trim pot marked C1K as the contour knob. I used the C1K for fuzz...

I would have wanted this information in the kit's description:
B1K: 1 kOhm rev log
C1K: 1 kOhm lin

Maybe I should have understood which one was going where, but I didn't, and there were nothing explaining that to me.
I based my choice on the description of the potentiometer in the web shop, which says "rev log C":
https://www.musikding.de/Alpha-Potentiometer-16mm-1k-rev-log_1

So now I measure 4.7 V on Q2c, which I'm quite happy with for now.

Thank you for your replies!

[gordo]

Excellent!

[midwayfair]

So, there was nothing wrong with my soldering. At least not totally.
I found the solution in a YouTube video of an assembly of the same pedal, where the guy pointed (no talking) at the trim pot marked C1K as the contour knob. I used the C1K for fuzz...

I would have wanted this information in the kit's description:
B1K: 1 kOhm rev log
C1K: 1 kOhm lin

Maybe I should have understood which one was going where, but I didn't, and there were nothing explaining that to me.
I based my choice on the description of the potentiometer in the web shop, which says "rev log C":
https://www.musikding.de/Alpha-Potentiometer-16mm-1k-rev-log_1

So now I measure 4.7 V on Q2c, which I'm quite happy with for now.

Thank you for your replies!

The taper shouldn't change your biasing, though, which was clearly very far off from what it's supposed to be. It does change where on the pot's travel that the fuzz starts to sound fuzzy, and using a B taper will bunch everything up. A C1K pot is ideal for the fuzz control on a fuzz face but all the original units have a B1K.

If you take a look at the schematic, the pot forms a constant resistance to ground of 1K for the transistor. The wiper is attached to a capacitor, and as that capacitor is moved up toward the emitter of Q2, the AC gain increases -- but the DC voltage does NOT change.

Swapping that pot for a different taper of the same value won't change the biasing.

The contour pot does have an effect on the biasing, since it increases the resistance between the 9V supply and the collector of Q2. Is it possible that you're changing its setting while you're fiddling around with the pedal and that's why your bias is inconsistent?

You should also be aware that germanium transistors are temperature sensitive.

[lars]

In the future, it's good to hammer out all the quirks of a Fuzz Face circuit on a breadboard first, before soldering anything into a PCB. The biggest mistake a lot of people make with a Fuzz Face is just using the "traditional values" for components. Fuzz Face's are so finicky, even with silicon resistors, that you sometimes have to modify the values of just about every component to get the sound you're after.
Trimpots give a false sense of control.
The Hfe values and type of the transistor are the least of your concerns (there are a ton of different options that work great in that circuit). It has more to do with the ratio of gain between the two transistors, and a fine balance between the biasing between the two, since they're so interactive.
Breadboard. Breadboard. Breadboard.

[somnif]

In the future, it's good to hammer out all the quirks of a Fuzz Face circuit on a breadboard first, before soldering anything into a PCB. The biggest mistake a lot of people make with a Fuzz Face is just using the "traditional values" for components. Fuzz Face's are so finicky, even with silicon resistors, that you sometimes have to modify the values of just about every component to get the sound you're after.
Trimpots give a false sense of control.
The Hfe values and type of the transistor are the least of your concerns (there are a ton of different options that work great in that circuit). It has more to do with the ratio of gain between the two transistors, and a fine balance between the biasing between the two, since they're so interactive.
Breadboard. Breadboard. Breadboard.

And even with a "perfect" circuit, old school fuzzes are temperamental beasts at best. They don't like active pickups, they don't like being after buffers, they don't like being after wahs, they don't play well with some amps, they get grumpy about temperature changes, they're finicky about volume and tone settings on either side....

If it weren't for the fact that they sound amazing when everything clicks I don't think anyone would bother anymore!