[solved] My Ge Fuzz Face does not clean up at all

[j_flanders]

Earlier this year I bought an older, used big red Dunlop Fuzz Face.
I was expecting guitar volume knob clean-up comparable (or even better) to my EHX Double Muff (a Silicon Fuzz Face variant) which has exceptional cleanup.
However, it cleaned up nothing at all. Even with the volume knob on my guitar(single coils) at around 1 it's still fuzzing.
There's less fuzz, but absolutely no sparkly cleans.

I took some voltage readings back then and couldn't immediately notice anything really out of the ordinary.

So today I decided to unsolder the GE transistors and measure them, using the R.G. Keen testing method. (http://www.geofex.com/article_folders/ffselect.htm)

Both transistors are NTE158.
I used a heatsink on the legs when desoldering and let them cool off for half an hour.
I measure / calculate:

Q1: 0,520V (= 210µA leakage)
Q1: 1,875V (actual gain = 187 - 52 = 135)


Q2: 0,480V (= 194µA leakage)
Q2: 1,840 (actual gain is 184 - 48 = 136)

Gain of 135 for Q1 is pretty high. For Q2 it might be somewhat acceptable.

But would that alone cause it to not clean up at all?
Also, is it normal to loose a lot of treble the moment I even slightly turn down the Fuzz knob?
Anything else I should check?

Here's the schematic:
... so, I wrote the above two hours ago, then went looking online for a schematic but every schematic I found for this pedal had errors and some stuff didn't make sense.
I started tracing it myself and was even more confused.
Am I seeing this correctly that it has been set up for negative ground?
Why? You can only power the pedal with a battery anyway.

Does this change the voltages I should aim for when biasing?
Could this have anything to do with the lack of cleanup, or is that purely because of the high gain transistors?


The voltages I wrote down back then (the transistors are out atm):

Q1
C: -0,67V
B: -0,19V
E: -0,05V

Q2:
C: -4,5V
B: -0,67V
E: -0,55V

(I can't remember if those voltages were actually negative. Could well be that I was confused abou them being positive and just added the minus, because... it's a ge fuzz so must be negative)

[MaxPower]

My guess: max out the resistance of tp2 (100k), adjust tp1 to get the q1 collector voltage farther from the supply rail. See if that helps clean up the signal.

Rt1 doesn't make sense to me since it's in parallel with the q1 collector resistance. Might it not be on the base of q1? (See axis face smooth pot). If it is like the axis face, then that's the pot which should clean up the fuzz.

[MaxPower]

D'oh! Apparently rt1 is a thermistor? So disregard the part about the axis face in the previous post.

[j_flanders]

Ok, thanks. Trying to understand:

My guess: max out the resistance of tp2 (100k),

What does increasing resistance there do?
Reduce DC feedback?
Nothing at all for AC when fuzz pot at max.
Less AC feedback with fuzz pot not at max.
How does this help for the clean up?

adjust tp1 to get the q1 collector voltage farther from the supply rail. See if that helps clean up the signal.

Good idea.

Rt1 doesn't make sense to me since it's in parallel with the q1 collector resistance. Might it not be on the base of q1? (See axis face smooth pot). If it is like the axis face, then that's the pot which should clean up the fuzz.

It's a 33k thermistor which also means I can never get the Q1 collector resistor above 33k (no matter where I set the 100k TP1). Not saying it needs to be though. Edit: you found out yourself.

[MaxPower]

I think I have it backwards. Try it with tp2 at minimum resistance. Should give more negative feedback (ac) and reduce ac gain.

[mac]

I bet the thermistor value is near 12k.

Q1 hfe is ok. I use mpsa42 in that range to test Si FF and they clean up very well.

mac

[j_flanders]

I bet the thermistor value is near 12k.

Unless I desolder it I cannot be 100% sure, and even then, but:
-Measuring in circuit gives a value slightly less than 33k (there a 100k tp + 10k resistor in parallel, transistors are out)
-FF usually has a single fixed 33k collector resistor for Q1
-It has 3 orange bands/stripes, which for a regular resistor would mean 33k
-Schematics I find (that contain other errors) also list it as 33k

It seems very likely that it actually is 33k.
If it were 12k I could never read a value above 12k, not? What's your theory behind the 12k?

Great to hear that your 135 range hfe Q1 does not prevent you from cleaning up the fuzz.
Does it do sparkly cleans, or just a reducton in distortion?

[tubegeek]

If it were 12k I could never read a value above 12k, not?

And if there's REALLY a 10K in parallel, you can't get more than 10K. Something's wrong with your trace of the circuit, or the resistor markings.

[j_flanders]

If it were 12k I could never read a value above 12k, not?

And if there's REALLY a 10K in parallel, you can't get more than 10K. Something's wrong with your trace of the circuit, or the resistor markings.

There's a 100k trimpot in series with that 10k...
If I would turn that to 0 than I couldn't get more than 10k like you said. I got around 7 or 8k, which would also further suggests 33k.

[willienillie]

So the thermistor is to keep the bias somewhat stable in changing temperatures?  Neat idea if it actually works.

I think R9, C4, C6 will probably keep this unit from ever behaving exactly like a regular fuzz face.

[j_flanders]

So the thermistor is to keep the bias somewhat stable in changing temperatures?  Neat idea if it actually works.

Yes, but no idea if it actually works.

I think R9, C4, C6 will probably keep this unit from ever behaving exactly like a regular fuzz face.

I'd think, to AC/audio signals it's as if they're not there, similar to having the emitter connected to ground directly and for DC there's an additional 100 Ohm (stabilising?) emitter resistor. I'm not sure how that would profoundly affect the behaviour.

[j_flanders]

I finally managed to get this fixed. There were a couple of problems, some of which I caused myself.

This is a run through of everything I did (wrong), starting from when I got the pedal earlier this year.

I bought the pedal mostly for the enclosure and planned to put my own FF circuit inside, as these pedals often get not so good reviews.
But I thought, let's try it anyway, who knows I got a good one. The seller said it sounded awesome.

It sounded a little scratchy and didn't cleanup and sounded kinda gated. I'm thinking, yep reviewers may be right but let's try a fresh battery.
After trying a couple, I notice the wire to the battery snap is barely hanging on, so I solder in a fresh, new, higher quality battery snap.

I notice very little difference. I do notice however there's some static, scratchy noise when screwing on the bottom plate. Maybe the hot lug of one of the jacks is touching the bottom plate? I bend it inwards, but no difference.

I take voltages on the transistors and they don't make sense in anyway. (they were not the voltages I posted earlier)

So, on to rebiasing.
Firstly, I mark the original settings on the pcb, and put a tiny black dot on the pcb right where the arrow of the trimpot points to.
This is where I go wrong. These arrows are so small, almost looking like plus signs instead of arrows and on TP1 I mistakenly mark where the back of the arrow is instead of the point of the errow (180° wrong)

I try the bias voltages I find online, use them as a starting point and then deviate from those, trying to make it sound better. It doesn't help one bit.

I put the pedal away and start looking for new Ge's and other components to make my own circuit as originally planned.

Fast forward to this week. Before ordering new Ge's I decide to unsolder the stock transistors and measure them for gain and leakage and see how bad they really are. They're not too bad at all actually. Hmm, so what gives?

Today I soldered them back in and went over the circuit, soldering and wiring again with a magnifying glass.
I see that someone (maybe me) scorched the wire to the output jack. A tiny part of the wire is bare, and worse, that wire is bent in a way that it makes contact to the bottom plate. I put some tape over it for now. Without a doubt that's where the static came from while srewing on the bottom plate.

While doing that, the wire to an outer lug of the volume pot just falls off. It check the other two and another is broken off as well and simply making contact to the pot because it pushes against the lug.

I resolder all wires, put in a battery and bias the transistors somewhere in the ballpark of the 'general' voltages you find online.

I get less scratchy noises, less gating, no more static when screwing on the bottom plate.
But... still no clean up.

I decide to go by logic(?) set the feedback resistor trimpot to its minimum value for more feedback, less distortion. According to the mark on the pcb this is also where it was originally.
Next I turn the trimpot on the collector of Q1 and low and behold at some point there's perfect cleanup. Its position? 180° opposite of what I marked originally. Meaning? It's back where it was stock...my mark was wrong.

In the end it seems all my problems were caused by loose wires/bad solder joints and bad contacts and even (slightly) grounding the output.

It sounds awesome and nice and smooth and really does have excellent cleanup. Just great!

One final note. I measured the voltages on the transistors again and their values are back to what I remember them now. They still make no sense to me. This is probably because of that negative ground?

I get these voltages:

Q1
C: -0,35V (instead of -0,7V)
B: -0,16V (instead of -0,2V)
E: -0,05 (instead of 0V but there's a 220r on my emitter)

Q2:
C: -6,45V (instead of -4,5V)
B: -0,35V (instead of -0,7V)
E: -0,25 (instead of -0,5V)

Anyway, sorry for the long post. Thanks for the replies and suggestions.

[MaxPower]

Good deal. May have to give it a try myself.

[PRR]

> Q1 C: -0,35V (instead of -0,7V)

You appear to be measuring to the POSitive battery terminal.

This is unconventional. However it does make it easier to compare with positive-ground circuit notes.

[j_flanders]

> Q1 C: -0,35V (instead of -0,7V)

You appear to be measuring to the POSitive battery terminal.

This is unconventional. However it does make it easier to compare with positive-ground circuit notes.

I do. Otherwise I get values between 8V and Vcc which could seem to make even less sense (to me at least)
Tbh, I don't know how to measure them properly.
Measuring against 'ground' I get:

(Old) Battery: 8,64V

Q1
C: 8,26V
B: 8,48V
E: 8,59V

Q2
C: 2,27V
B: 8,26V
E: 8,37V

(or the same values but negative, if I reverse the DMM leads)