Can't max out my Fuzz faces

[dennism]

I have built several Fuzz Face clones over the past few months and for the most part I have been happy with them.  The only issue has been that with all of the clones (using the standard commonly accepted schematic), when I turn both volume and fuzz knobs up to 10, I get hissing and radio interference.  If I back either knob off even a tiny touch, then everything is OK and the pedal sounds great.  This has been universally true for at least 3 pedals, so I am perplexed.  Is there a fix for this?  I'm thinking it's maybe just as simple as a resistor or cap on the output, but I have no idea what value? 

I greatly appreciate any help.  This is a great forum!

[GibsonGM]

Assuming there are no errors in construction.....Maybe a very small cap on the input lug to ground, like 50 to 100pF, could help.  Seems like you may be amplifying radio interference you're receiving from your environment!  This cap will shunt high freq's to ground, where the RF lies.

Another possible cause could be ground loops...ground wires acting like antennas inside the box.  If you're using a metal box and proper grounding, this shouldn't be happening tho!  I always get RF interference when my stuff is OUT of an enclosure (bad radio area), but it is gone when in the box. 
Hope this gives you a place to start 

[mac]

I've recently posted this:

http://www.diystompboxes.com/smfforum/index.php?topic=54081.0

BTW, I have two TV stations in the same block... I'm so happy! If I close my eyes I can see see the discovery channel without the need of a tv set. Maybe I should implant myself a 100pf cap to gnd in my butt.

mac

[petemoore]

  Adjust the gain in the high frequencies like GM said or some other way.
  Reduce gain setting.
  Lower transistor gain.
  ~10k series @ input might 'just help.

[Skreddy]

I've always noticed that my Fuzzface clones get REALLY hissy and sometime oscillate with the gain dimed.  So what I do is put a 100 ohm resistor in series with the fuzz cap (the 22uf cap).  This is like preventing it from going all the way to 10, by just a teeny bit, and it works fine.

[dennism]

I appreciate all the replies.  I will have to work on the transistor gain, however, those things are expensive and I only have a few to choose from. 

Skreddy, your tip is exactly what I was thinking might work except that I didn't know what size resistor and where to put it.  I'm going to try what you suggested.

Thanks to all who have replied so far.

[petemoore]

I will have to work on the transistor gain, however, those things are expensive and I only have a few to choose from.
  not to say transistor options aren't helpful, but HF can be rolled off in a number of ways, FF goes so far, then oscillates, the way it is there.

[Skreddy]

Yes.  For example, there's the 100k resistor which is a negative feedback loop.  Parallel that with a 47pf cap, and you'll reduce very high frequencies (probably above the audio spectrum for the most part).  Raise that to a 100pf cap if it's not enough.  Go even further to 220pf or even 470pf; stop when you get the high end rolloff you want.

[mac]

My Si FF (2N2219A, hfe: 110 & 117) has 3 roll off caps:
• 47pf from Q1 B->C
• 100pf from Q2 B->C
• 0.047uf across the 390R, as in the axis face.

That keep the highs just right for me.

mac

[petemoore]

  The FF 'edges'..
  They can be gone under, up to, and over.
  They can move and be moved.
  Go too far over an edge, and the FF will tell you.
  That's what's cool about it, the gain/frequency thing, set up just 'so, altering where the signal is in relation to the edges is possible and it works great, it can also easily get out of hand.
  That method works well mac:
  My Si FF (2N2219A, hfe: 110 & 117) has 3 roll off caps:
• 47pf from Q1 B->C
• 100pf from Q2 B->C
• 0.047uf across the 390R, as in the axis face.
  I did FF something like:
• 27pf from Q1 B->C
• smalle cap  from Q1 B to ground
• 0.047uf across the 1k, as in the axis face.
  And:
  little cap at input or in circuit
  a 1uf cap / 20k pot between output and ground [see Rocket] as HF control.


 
   

[mac]

One thing I did recently was to put small inductances in series with the 100k fb resistor and/or from Q1 emiter to gnd.
The ones I used are about the size of a 1/4w resistor, 10 Ohm dc and having a (grey, red, black) code I guess they are 82uH.
When used on Q1 emiter they reduce the final gain. Can't tell for sure if it also rolls off some highs because it could be the diminished gain what gives the feeling of less highs.
In series with the 100k only the bass content goes back to Q1. This is negative feedback so bass is reduced. Using some caps across the 100k or from the inductance to gnd, splitting the 100k in two, putting some resistors to gnd, etc, makes a selective neg feedback filter.
I had best results in the fb loop with a small one-wiring choke.

mac

[Ronsonic]

"So I says 'Doctor it hurts whenever I go like this.' He tells me 'so don't go like that.'"

I expect slightly stupid behavior from devices as brilliantly primitive as the fuzz face. True they don't all have exactly your problem, but then I don't know a single one that is really at it's best with the fuzz knob all the way up. The traditional setting is to turn up the fuzz all the way and back off until it isn't too spitty, spatzy, sputtery etc and then adjust the volume to taste.

If you just gotta do something stick a small resistor in the hot side of the fuzz pot.

Ron

[tcobretti]

Is this a Si of Ge FF?

[R.G.]

There are two issues with RF - RF pickup and RF oscillation.

You kill RF pickup by shunting the most sensitive point - the input - to ground with a small ceramic cap and inserting a series resistance in the line. The resistor can be 100 ohms to 1K, and goes right at the input of the first active device. The cap, about 10pF to 47pF, goes to ground from the input of the first active device.

RF oscillation can be killed by dropping the frequency response of the circuit or by reducing the feedback. Reducing feedback is not generally acceptable, so most people use a small cap in negative feedback around a high-gain active device. In your case, Q1 and Q2 both qualify.

A small ceramic cap, up to about 100pF, works on Q1 C-B and on C2E-Q1 base.