Okay, let's try it this way...

[ayayay!]

I tagged a question onto my existing thread and it didn't get any hits, http://www.diystompboxes.com/smfforum/index.php?topic=74608.0  So I'll do this properly and ask in a new one. 

I built this:  http://www.schematicheaven.com/effects/hendrix_fuzzface.pdf

In the last 5% of the gain pot travel, everything suddenly gets super bright, and I mean in a hard cut-off kind of way.  It sounds good actually, but when dialing back down I lose the brightness and like I said it's SUPER apparent.  Once I hit about 30 ohms on the gain pot, it's all downhill from there.  No smooth taper, it's like a hi-cut button is being pressed. 

Here's what I don't get:  I'm using 2n5088s, which sound great (remember, this is the NPN build...) I tried 2n5089s, MPSA18s & 2n3904s.  They all have vastly different gain structure, but all do the exact same thing with the travel of the gain pot.  So my thought was, "bad pot."  But after replacing the 1K Lin pot, the same thing still occurs.

So then I figured maybe the polarized caps got hit in reverse somehow.  Nope.  Replaced those (2.2uf &22uf) and the same thing still happens. 

What am I missing?

[Roobin]

I had(ve) a similar problem - except I thing mine was gain, not tone. What happens if you use a different value pot? 2K? 5k?

[ayayay!]

Tried a 2K last night.  Everything farted out.  Almost no output.  

To clarify more, the gain is the problem.  It sounds good all around (actually, much better than I anticipated.  I'd play it live if I could get this figured out)  It's just that last 5% where this occurs.  And there's no happy medium with the pot travel.  It's just *boom* brighter. 

[ayayay!]

Taken from the original thread:   

Probably should start a new thread for this...

I'm not too familiar with that circuit, but you could try adding a series resistance (27 ohms??) to the pot (between the pot and Q2,68k,47pF), or increasing the 22uF cap, or a combination of both...

Maybe new MPSA18 are higher gain than they used to be.

Stew.

Well that was my first hunch, but like I said I've tried 4 different trannies, with gains varying from 735 all the way to 125.  And no matter what, this gain pot/tone thing keeps happening at precisely the same spot, even with a replacement pot. 

[Roobin]

What about that 68pF cap and its resistor? This is just me guessing though...

[ayayay!]

Clearly I did something wrong with my layout.  I've got 0 volts on the emitter of Q1.  I don't think that's right.  But, I need to get to bed for the night... Later...

[Boprikov]

Maybe there is nothing wrong with your circuit. A friend of mine has an old silicon Fuzz Face clone made by Schaller. The gain pot behaves just as you described. It suddenly gets brighter near the end of rotation.

I think 0 Volts on the emitter of Q1 is right, as it is connected straight to ground.

[oldschoolcharlie]

Try removing the 47 pF cap.

[tcobretti]

Try removing the 47 pF cap.

I agree; I think this cap is causing the brightness.  You picked a weird variant of the FF to build.  It's got a cap to bleed highs off the input then another the let the highs back in from later in the circuit.  You might try removing both the 47p and 1n to see what happens.

[arma61]

hi there,

did you put it into an enclosure ?, I had my one acting, more or less, like your one, as soon as I boxed it most of the "brightness" at the end of the pot run are gone.

Armando

[ayayay!]

Maybe there is nothing wrong with your circuit. A friend of mine has an old silicon Fuzz Face clone made by Schaller. The gain pot behaves just as you described. It suddenly gets brighter near the end of rotation.

I think 0 Volts on the emitter of Q1 is right, as it is connected straight to ground.

Okay, thanks.  I was thinking about that in bed last night and thought, "Nah, that has to be right."

Try removing the 47 pF cap.

I agree; I think this cap is causing the brightness.  You picked a weird variant of the FF to build.  It's got a cap to bleed highs off the input then another the let the highs back in from later in the circuit.  You might try removing both the 47p and 1n to see what happens.

That's definitely the next place I was going.  I was thinking maybe the 47p, but actually leaning towards the .01 at the output since it's retaining highs, right?  Is my thought flawed on the .01?

hi there,

did you put it into an enclosure ?, I had my one acting, more or less, like your one, as soon as I boxed it most of the "brightness" at the end of the pot run are gone.

Armando

Hmm, come to think of it, no.  It's all wired up, but I never did put the lid on...

Thanks for the wonderful response gentlemen.  I'll get back with you late tonight. 

[tcobretti]

Something that just occurred to me is that you could be getting oscillation because of the high gain transistors you are using.  This is a very common problem with the FF.  I know you had it with a bunch of different transistors, but all the transistors you listed are med to high gain.  You might want to try some low gain transistors before you get too far into troubleshooting.

The output cap acts like a high pass filter that only allows the brighter part of the signal to get thru.  I doubt that it is the problem.

[petemoore]

  "1k'' is incorrect taper. Gain 'jumping on' at ~94% is a 'correct' response for a super high gain FF.
  That schematic I believe allowed the assemblage of a large number of 'Fuzzface' circuits which worked with great reliablity [requiring no testing beyond does it: 1. IntenseFuzz? 2. MegaBoost?] the factory gain pot is a 'special correct' taper and value..."factory wired circuit = 'works' = increased production rates = circuit of questionable usability...who cared when no-one knew what a FF sounds like..the guy with the new one. {discontinued}.
  Extreme gain with HG transistors [Hfe's above ~350hfe tend to bias and sound the same as one another], should make sharply clip off signal wave peaks, harsh and high output, tamed some what by the ...HF taming caps.
  The Axis Face is a more popular variant, cleans up @ guitar, better behaved.
  Your circuit's probably biased, otherwise, check bias after every transistor swap.
   You can get a "Fuzzface Gain" pot at Smallbear.
   But in the mean time, that is a pot which varies in R value between lugs 1 &3...you can diddle with resistors next to/across the pot.
  I liked the pot/bias to be low R value when high gain transistors used.
  If too low a value, low gain transistors won't set to high gain circuit tone, I'd add a >100ohm resistor between gain and ground or, tie a resistor across the outer lugs..+/- a little bit from 1k...well you can hear what that last little bit of R is doing when you turn your gain up now.

[ayayay!]

Excellent, thanks Pete.  I didn't know about the special taper pot.  I'm putting together an order soon, so that will be on it.  Great tip on the last 8% travel of the pot.

Y'know, I did know well ahead of time that this would be the least popular FF to build, but for some reason I really wanted to try it.  Mainly because I'm not a big Fuzz guy anyway, and I don't own any PNPs (yet) so I figured a simple build would be fun this time around.  It's not bad, I actually like the sound of it w/ the 2n5088s.  And for some reason God blessed me with a ridiculous amount of patience, so this doesn't bother me.  I love the learning experiences these kinds of small bugs bring.  I'm not frustrated.  This has always been fun for me. 

I'll try the tricks w/ the small-ish resistor across the gain pot.  That may be it. 

This does clean up extremely well (a little too well/abrupt) when rolling off the guitar volume. 

BTW, what's a nice NPN for gain between a 2n3904 and 2n5088?  I'd like to try something in between there.  Thx! 

[ayayay!]

You can get a "Fuzzface Gain" pot at Smallbear.

Link to that one or another dealer?  I can't seem to find it.  Is it the C or W taper? 

[ayayay!]

I'm sure plenty of you wanted this one to die.     But alas, it's still alive.

I've been knocked out w/ bronchitis all week, so today was the first chance I had to get back to this. 

I used a 2k pot for the gain w/a 2k resistor across the input & ground lugs.  It did the trick with the gain "cutting in" very nicely (thanks for the idea Pete!)

I used a 2.2k resistor first, just to see what would happen.  It was great.  Actually they both sounded great.  But with the 2.2k, it was only about unity gain for the total output.  2k got me a little more out of it. 

So basically this 2kpot/2kresistor combo for the gain is cutting the output a LOT.  I really need to just get a 1k reverse taper pot.  Hopefully that will do it for me, and maybe recover a little more output.

One last note, I put a .001uf cap across the volume lugs to retain the high end.  It works very well, but I'm thinking I will try a bigger cap there. 

So it actually sounds good.  To my ears at least.  I'll try to post some soundclips.  I think some of you might be surprised that it can produce some good tones. 

[Gus]

I like this circuit for Si FF types.
http://www.diystompboxes.com/pedals/gusFuzzFace.gif
  Also look at the vox one in R.G.s writeup.  Look at the 3tran/rocket distortion stage.

There is a reason for the change and there have been posts here over the years about the question about the maxed gain issue