Fuzz Face Bias

[jcuempire]

I went back and revisited my first DIY project - a GGG silicon fuzz face.  I made some small changes but a big one I made is that I swapped out the original transistors for 5089's.  It sounds fine, but now the lowest setting on the trimpot reads like 8.4 so I can't get anywhere near 4.5.  Would a different value trim be the answer or can I insert another resitor in there somehow?  Or should I just put the old transistors back in?

Thanks

[azrael]

what size trimpot did you use?

I would recommend lowering the gain.
Put a small resistor at the emitter of each transistor. Like maybe 100 ohms on Q1, and 220 ohms at Q2.

[Gus]

this should help

http://www.diystompboxes.com/analogalchemy/emh/emh.html

google, bing etc and search this site lots of FF information

Use the calculator and put in different hfes for each transistor keep one constant and move the other.  Four  values like 100, 200, 500, and 800 might help understand what is going on in the FF or you can sim it in LT spice

10K 100 at Q1 was calculated to control Q1 openloop gain.  This has been in schematics for years.

[jcuempire]

what size trimpot did you use?

I would recommend lowering the gain.
Put a small resistor at the emitter of each transistor. Like maybe 100 ohms on Q1, and 220 ohms at Q2.

It's a 10k trim

I will try what you guys suggest.  Thanks

[jcuempire]

That calculator is pretty cool, by the way.  Thanks

[mac]

With 5089s you should get closer to 4.5v at the collector with a 10k trimmer, since higher hfe means higher collector resistors values.
You'll need 5k or so with lower gain silicons (70-120), and 10k or so with high gain ones (>250)... if all other resistors are the usual values.

Also check my calculators at my gallery

http://www.aronnelson.com/gallery/main.php/v/mac/apps/

mac

[jcuempire]

Thanks, mac.  It's starting to make sense.  One more question, then I'll leave you all alone (for a while).  The plan calls for a 1k fuzz pot, but all I have right now is a 5k.  Will it work?  Or should I just order the correct one?  Thanks

[LucifersTrip]

Thanks, mac.  It's starting to make sense.  One more question, then I'll leave you all alone (for a while).  The plan calls for a 1k fuzz pot, but all I have right now is a 5k.  Will it work?  Or should I just order the correct one?  Thanks

it'll work but have a different tone...you can put a 1.3k resistor in parallel with the 5K pot to make it  1K, but the sweep will be different.

[mac]

If you use a 5k pot you'll need to increase the collector resistor to get 4.5v
You better get a 1k pot.

mac

[skrunk]

If you use a 5k pot you'll need to increase the collector resistor to get 4.5v
You better get a 1k pot.

mac

how is the value of the fuzz pot going to effect the collector voltage?

jcuempire, if you can't get the voltage down, you need a bigger trimmer (more resistance), so just stick a 20k trimmer in there instead of the 10k.
(or a 4.7k resistor between the trimmer and collector will get you there).
and don't worry so much about getting 4.5v, anywhere from 4.5v to 7v usually sounds good to me.
and you'll get more output biasing it at 6.5v say, than 4.5v.

as you said yourself it already sounds fine at 8.4v anyway, so you're not far off.

[jcuempire]

If you use a 5k pot you'll need to increase the collector resistor to get 4.5v
You better get a 1k pot.

Okay.  Now if I understand you, the problem that I originally had - Q2 rocking 8v - is due to the 5k pot?  Or am I oversimplifying it?

[skrunk]

no, it has nothing to do with the fuzz pot, I think mac misunderstood you when you started talking about the fuzz pot.

[jcuempire]

skrunk - I was under the impression that 4.5 was some sort of wall that dare not be breached!  Is it stressing the transistor to pump more through it?  Not that I'll go broke replacing a 10 cent transistor, but this is kind of a revelation.  Actually with the 10k trim, I can get down to ~7.4 so a 20k trim should have a better range.  I'll try that.  Thanks

[skrunk]

skrunk - I was under the impression that 4.5 was some sort of wall that dare not be breached!  Is it stressing the transistor to pump more through it?

no, not at all.
I usually bias mine to between 6-7v.
I've heard of vintage tonebenders being biased at over 8v.
ultimately, your ears will decide what voltage sounds best.

Actually with the 10k trim, I can get down to ~7.4 so a 20k trim should have a better range.  I'll try that.  Thanks

cool! good luck.

[petemoore]

  The value of the potwafer in the gainpot sets DC bias at 1k or whatever the fixed resistance value of the potwafer is.
  IME 5k was too big, and a resistor was put across it somehow to make the wafer/pot division caclulate to more like 1k.
  Once I started with a 200ohm or 500hm pot w/fixed resistor to make about 1k, then I got the right taper from Smallbear electronics and another in a kit.
  I mostly use the gainpots from 2 oclock to full on anyway though, so 'bottom' of the wafer can be stop resistor and this leaves enough for adjustment.
  Tim Escebedos 'Many faces of fuzz' guided many of the alterations to ''the 100k and below'' and other FF derivitave wiring assingments.
  But the volume pot on these guitars are set up as gain knob when FF is interacting with the PU impedance, so the FF gain knob got set and forgot.

[mac]

The voltage drop across whatever emiter pot you use from Q2 emiter to gnd is almost constant, because it is tied to the base of Q1 via the 100k feedback resistor.
If this voltage is almost constant (or have an upper bound), it means that when you increase the resistor or pot, the emiter current ie, which is close to the collector current ic, decreases (V=i.r). On the collector side you have a smaller current so you need to increase the collector resistor to get 4.5v.

Why this happens?
The voltage across the pot is
Ve2=ib1*100k + Veb1
where ib1 is the base current of Q1, and Vbe1 is Q1 base-emiter voltage drop, 0.6v in this case.
You can get an aproximate value for ib1 as follow:
Vcc = ic1*33k + Veb2 + ib1*100k + Vbe1
Noting that ic1 = hFE1*ib1, you can solve for ib1,
ib1 = (Vcc - 2*0.6v)/(33k*hFE1 + 100k)
For a 2n5089, bc549c of hfe:500 you have ib1: 4.7e-7 amp!!!
If you use high gain silicons ib1 is very small, and the voltage drop across the pot is close to Vbe1, a little over 0.6v
Even with 2n3904 of less gain you are close to 0.65v - 0.75v across the pot.

mac

[jcuempire]

I appreciate everyone's input - most of which is way over my head (I was a history major) - and I never fully appreciated what an interesting little device the fuzz face is.  But I have it working.  I replaced the 5089's with 5088's (I had some laying around) for no real reason and I'm having more problems shoehorning it into the 1590A box I had left over, but it makes a pleasing growl and backs off nicely, so I'm pretty pleased.  Maybe I'll try another one soon.  Thanks again

[mac]

Maybe I'll try another one soon.  Thanks again

Try a germanium version, or a low gain silicon at Q1 and a germ at Q2.

mac