Is this normal in a fuzz face?

[Ptron]

With the fuzz control all the way up, and the guitar volume turned down the output is reduced to a fizzle that comes in and out, like a radio with dying battery.

[Stephen]

Asuming the circuit is right..BAD transistors

[petemoore]

  I question bias.

[Ptron]

Well, here are the quiescent voltages(all neg):

Q1: b - .071   Q2: b - .174
     c - .174          c - 8.32
     e - 0              e - .103

I don't know what's normal for these things.

What's with the .07ish be voltage drop? I thought ge transistors were supposed to have something in the neighborhood of .2-.3V.

As far as the fizzly sound with the fuzz all the way up and the the guitar turned down: I thought with the odd assymetries of the the design (and the inconsistancies of germaniums) that this might be normal... or at least not unusual.

[mac]

Vc2 should be -4.5V. If you are using the standard 8.2K you need to replace it with a higher resistor to bring Vc2 down to -4.5V. Put a 20K trimpot and adjust Vc2.
About Vbe, I've never had a Ge with 0.2V voltage drop across the base and the emitter, always less than 0.13V. I don't know if it is normal, maybe my cheap chinese DDM is interacting with the Ge.


mac

[Ptron]

Thanks Mac (always wanted to say that). I thought maybe fuzzfaces were supposed to have radically assymetrical clipping. It actually sounds really good unless you use the settings I described above. This is a Arbiter fuzzface by the way. It has the MKT275 transistors that I see are mentioned in a couple other threads on here.

[Ptron]

In contrast, I'm examining a Jimi Hendrix, pedal. You know, one of those fuzzface lookalikes. I traced out the circuit and it is very similar to the fuzzface, the most important differences being that it's got silicons (Q1 is a 2N3904 and Q2 is an MPSA18), the resitor values are different, and it's negative ground. Vc2 is 1.32V quiescent. This thing sounds like utter crap. I'll try mucking with the resistor values on both pedals to get Vc2 near 4.5V and post the results.

[brett]

Hi

Q1: b - .071   Q2: b - .174
      c - .174          c - 8.32
      e - 0              e - .103

Those base-emitter differences are very low (0.07 volts instead of 0.25 volts).

Also, are you using NPN transistors?  If they are PNP, you need to use a special circuit for negative ground (as indicated by your voltages).  Tonepad and ggg's have neg ground circuits for PNP devices (they reverse the orientation of the transistors).
cheers

[petemoore]

Tonepad and ggg's have neg ground circuits for PNP devices (they reverse the orientation of the transistors).
  The thing is, PNP Neg Gnd. FF circuits have a track record of
  Having problems fixed by converting to PNP Neg Gnd.
  I would get a Fuzz Face working before attempting to figure out what made PNP Neg Gnd FF circuits not tick.

[mac]

Ptron,
1. Try the Axis face, at http://fuzzcentral.tripod.com
It is a silicon FF that sounds very good. The trick is to use low gain Si hfe: 70 -120, and some caps to reduce Si fizziness.
I built one with a couple of 2n2219A and it sounds very good. Not exactly the Axis Face, but very similar. I suggest avoiding the 2n2369 because the pick some radio interference. Use a couple of BD139s.

2. Beware of NKTs And ACs. Many of them are just crap. I wrote some post and replies suggesting using other Ge like 2sb172, 175, 177 and others japanese Ge. Reliable, available (at least in my country), low cost, great sounding.

Brett,
As I mentioned above all my Ge have Vbe less than 0.10 - 0.13V. And I could not measure Vce=Vcb+Vbe. I suspect that my DDM is not good and it is interacting a lot with the transistors. What do you think?


mac

[Ptron]

Do germs go bad or are they usually bad to begin with? This fuzzface is stock, the way it came out of the factory.

[brett]

Hi
In my experience, germanium devices are usually made bad or made good.  Though you can break them during soldering, too.

As I mentioned above all my Ge have Vbe less than 0.10 - 0.13V. And I could not measure Vce=Vcb+Vbe. I suspect that my DDM is not good and it is interacting a lot with the transistors. What do you think?

Sounds unusual.  Vbe is measured over a small resistance (20kohms), so the DMM should have minimal effect.  Even cheap DMMs (ie $10) should have 1M input impedance.

You can test your DMM by placing 2 x 1M resistors in series between the + and - of a 9V battery.  If the measured voltage where the resistors join isn't within 1.5 volts of Vbattery/2, then the DMM is fairly weak.  A good DMM will read to within 0.25 V of Vbattery/2.
cheers

[mac]

Thx brett. I need to buy a good DDM.
I did what you wrote. It read 3.1V across one resistor and 3.06V on the other. Battery was 9.2V 


mac

[brett]

Hi
Actually, that's not as bad as it could be.  Your DMM has 1M input impedance (common for budget DMMs).  My backup DMM is the same.  It'll read voltages OK across anything up to 300k or 400k resistance, but obviously not 1M.  You measured 3.1V because 3.1V is approximately 9V x 1M/(1M + 1M +1M). The "extra" or "third" 1M resistance is the meter.
cheers

[Ptron]

I took out the transistors and measured the gain with a cheap DMM.

Q1 hfe: 80
Q2 hfe: 47

I put a 100k trimpot in place Q2's Rc and even with it maxed, Vc is still in the -7V range. Doesn't this seem a little extreme even with the very low gain of Q2?