Fuzz Face help needed - are my trannies knackered or what?

[DuncanM]

I thought I'd do it right - Smallbear selected Germanium jobbies with a Tonepad PCB, how hard can that be?

Plug in the values that Steve suggested - and it's way out of bias...

Steves suggested values are:
R3- 33k
R4 - 680R
R5 - 8.2k
R6 - 120k

These values give (using a negative earth arrangement)
Q1 E - 9.37v
     B - 9.24v
     C - 9.32v     

Q2 E - 9.33v
     B - 9.31v     
     C - 0.3v dropping to 0.17v as it warms up.....

I've checked my soldering is OK (no bridges etc.), the trannies are socketed and un-abused, fresh battery, right resistors in the right places (triple checked with a DVM....)

This morning I did the leakage and Hfe tests to check -
Q1 (2N404) gives an Hfe of between 90 and 100 with leakage of less than 80uA.   
Q2 (2N1305) gives an Hfe between 120 and 130 with leakage of less than 180uA   
So they SEEM OK..?.!!.? If somewhat lower than the 112 and 160 Hfe on the bags though (not a bad thing).

What do I do now?   

[AL]

What schematic and layout are you using? There are about a billion FF schems floating around and Tonepad has two layouts.

But, if you're FF isn't biased... Replace the resistor from Q1 base - Q2 emitter with a pot and do the same for the resistor coming off Q2 Collector. That will allow you to adjust the bias. You can either use a trimpot and leave it in or, once you get it biased, measure the pot and replace it with a resistor.

AL

[DuncanM]

Thanks for the response Al.

The short answer to my own question is - NO they're not knackered...
I traced the problem to a bad electrolytic, well, I removed it and the circuit started to behave...
Also changing to a positive earth scheme got rid of the oscillations (I was doing a LOT of "searching" yesterday.) and changing the value of the input capacitor rolled off some of the overly heavy bass.

So, after fiddling with trim pots in the collectors of Q1 and Q2 I've settled on the stock values as being most appropriate !!!   
It's quite muffled, so I think a buffer in front will be included in the final boxed version. I tried a BOSS pedal in front of it on bypass and it improved the high end dramatically. (I know it's not traditional and having the low impedance input thing is supposedly more "mojo" but it sounds more usable to me without the mud...).

So on to the fun part - cramming it all into a 1950B box and decorating it....... 

[AL]

Glad you found the problem. All FF's seem to sound different. Weird huh? If it's too muddy for you try changing the value of the input cap.

I would also recommend the GEOFX "Technology of.." series. There is a GEO link at the top of the page. The "Technology of.." link is then at the top left. Click on the Fuzz Face and it will break down the entire pedal for you.

You could also put an input cap blend control on the front end ala Oskar Bruil or Joe Gagan's Easy Face. http://www.generalguitargadgets.com/diagrams/ezface_blend_cap.jpg

AL

[DuncanM]

Thanks, The input cap blend thing looks interesting.

I DID do a lot of homework before starting this but it's only when you start having problems that you find out about things like the potential oscillations you can get from a negative earth layout... (something that I didn't know about before I started...).

I've currently gone back to the neg earth layout with a BFC (big freaking capacitor) across the supply rails - so I can use a neg earth high impedance buffer on the front end. This sounds pretty good to my ears like that BUT produces an annoying hum when you turn the guitar volume down.
More work is in order before I'm satisfied.

[zjokka]

thanks for posting, this, I should check my electrolyts too, I measured battery voltage at ALL transistor pins in my PNP negative ground fuzz face.

my fuzz did work at this point, all the stranger. same for yours?

But also had that same hum, but only with the volume all the way down. When I opened it up just a little bit. I didn't think much of it. In a pedal, just have the bypass on when you want to turn down. This is hum that will never interfere with playing of course.

there should be a solution though
ZJ

[DuncanM]

Despite what I read on Jack Orman's site about positive and negative grounds, I have resigned myself to the fact that my FF is only going to be stable with a positive ground layout.
The big decoupling capacitor certainly helps with a negative ground layout, but I think that for me, the easiest solution is to buffer the input with a PNP transistor emitter follower and the positive ground layout and see what that does...   

[zjokka]

Duncan,

I've put my negative ground FF aside for now, but will like you soon redo in postive ground on breadboard. I still don't understand why this issued isn't pursued further. Positive ground is inconvenient for me to integrate in my setup, I suppose a lot face the same dillemma.

I read all the stuff before I set up Orman's schematic on breadboard. I had none of the symptoms described in other threads apart from big hum at guitar volume 0 when fuzz engaged. I put the extra cap between emitters from the start. The fuzz in itself was great, some noise but that's normal before boxing up, I suppose...

The treble loss you talk about, is that at guitar volume maxxed you are talking about?

Really curious the postive ground would be more stable (what would that mean exactly? controllable? consistent? ) or at least sound better.

let me know - will do the same
zJ

[DuncanM]

Hi Zjokka, like you -  I went with the negative ground option because I thought it would work better with my board...

I've been doing a lot of searching about this problem and it seems to have been ongoing for years. It seems that positive ground versions don't have problems...
When I converted mine back to positive ground it worked perfectly straight away - back to negative ground and it squeals....
The solution seems to be a HUGE capacitor to decouple the battery (I used a 1000uF I found in my bits box). In my case the oscillations and instability went away as soon as I did that.

However, to my ears, my FF sounds best with a buffered pedal in front of it - my TS9 in bypass for instance.
Apparently, part of the 'charm' of the FF is its low impedance input  which gives that smooth fuzz - to me it sounds like a blanket has been thrown over that amp...
With a buffer in front it sounds brighter and cleans up evenly as you back off the guitar's volume. (without the buffer it seems a bit 'all or nothing' to me).

So I tried a standard FET buffer with the negative ground layout - I get a nasty hum when I back off the guitar's volume.
I tried a PNP transistor emitter follower in positive ground layout - I get a nasty hum when I back off the guitar's volume.
I built a clone of the TS9's input buffer - I get a nasty hum when I back off the guitar's volume.
I put the TS9 before the FF and it behaves perfectly....

I'm putting it aside until next week.

However, I'm having serious doubts about the claim that the positive and negative grounds are equivalent circuits. I also doubt the explanation that the battery's internal resistance is now in the emitter side of Q1 rather than the collector side.

(Just looking at the two circuits - in the positive ground configuration the guitar pickup is between the base and emitter of Q1, in the negative ground configuration the guitar pickup is between the base and collector of Q1 via a 33k resistor...
Unfortunately I don't have time to think about this until after the weekend.) 

[freax]

Hi to all !

I have finished the convertion to PNP NG too but when the volume goes to 0 it hums !

Does the 1000uf capacitor solves the problem ? From + to - in the wires that comes from the battery ?

There is other way ?

[]'s

[DuncanM]

It might help - you should do it anyway.... 100uF is probably enough.
I'm afraid I became somewhat disillusioned with the Fuzz Face.
I don't like the muffled sound of the low impedance input and the way it misbehaves when wired for negative ground, so it's been in the drawer for the last couple of months. Maybe it's time to revisit the thing - buffer the input etc..

I've HAVE been meaning to write an explanation as to why negative ground PNP circuits are prone to oscillations and other problems.
Despite what some people say, they are NOT equivalent.
Maybe next week if anyone is interested...

[freax]

I'm interested ! :-)
Seriously, i think is important (for all) if there is some document about this on internet.

I will test tomorrow, but about the buffer, do you mean if i put my boss tuner before my fuzz face, it will work without these problems ?

[]'s

[brett]

BUT produces an annoying hum when you turn the guitar volume down

Mine too.  And Axis Faces, too.  It's not just a PNP ttransistor in a neg ground circuit problem.
cheers

[formerMember1]

With a buffer in front it sounds brighter and cleans up evenly as you back off the guitar's volume. (without the buffer it seems a bit 'all or nothing' to me).

What !?!?!  how?  A buffer in front of the fuzz, should stop it from cleaning up...

just curious, what guitar/amp are you using?

[DuncanM]

just curious, what guitar/amp are you using?

I'm using 2 strats, a tele, a Les Paul and a LP junior through a Fender Bassman reissue.

What !?!?!  how?  A buffer in front of the fuzz, should stop it from cleaning up...

Well, to my ears, my unbuffered FF sounds like I've got the tone control rolled back all the way on my guitar - no brightness at all.
This is fine if you're playing "Summer Breeze" by The Isley Brothers but not for much else.... (showing my age there...) 

Rolling the volume back DOES result in less distortion and a brightening in tone but it's still not really what I consider "usable".

Put a unity gain buffer in front of it (like my TS9 bypassed) and you get lovely bright, agressive fuzzyness which still "cleans up" when the guitar volume is backed off without any change in tonality (Yes, I know this is sacrilege...). Much more useful for me - you can always roll off the brightness but it's much more difficult to add brightness to a dull signal.

So far, the fuzz face is the one pedal that I'm disappointed with. It MAY have authenticly correct low input impedance mojo, but I don't understand why some people rate it so highly - mine sucks.
In fact I'd call it the very definition of "tone sucking" (and we all hate THAT, don't we...).   

[petemoore]

Put a unity gain buffer in front of it (like my TS9 bypassed) and you get lovely bright, agressive fuzzyness which still "cleans up" when the guitar volume is backed off without any change in tonality (Yes, I know this is sacrilege...). Much more useful for me - you can always roll off the brightness but it's much more difficult to add brightness to a dull signal.
  This leads me to surmize you like a harder edged distortion than your FF is producing by itself as built.
  Besides looking elsewhere...
  Here's ideas:
  try higher gain Q1 or other transistors in it
  Use the Buffer if needed
  stick a 100ohm - 220ohm resistor between gainpot and gnd.
  put a boosting pedal in front of it use the boost level as another 'gain'
  put a booster behind it and use the FF's volume as another 'gain' knob.
  Of course you've reported that noisy thing, and they can be noisy.
  Try a Dist+, or OD 250, diode to ground type clipper?

[formerMember1]

thanks for replying duncan m,

I am still wondering how the sound cleans up with a buffer in front, but I guess for your setup it does somehow. 
I used to use a ts9 infront of a dunlop fuzzface, to make it better, but I was going for a different sound, and it didn't clean up. So i stopped doing that...

oh well, good luck. 

[freax]

The pedal is working OK now but I won't do the 100uF capacitor because I don't have the knoledge to do that and I may burn something. Only if I find a gif with this mod on google or something.

But I'm writing to confirm that with my tuner before the fuzz the hum almost desapear.
It won't do that but I listen a continuous "SCHHHHH..." when the volume is at 0 and/or the tone control at 0.
The volume is aceptable because we won't do that  but it's anoying for the tone control of the guitar that is usefull to shape the sound 

It might help - you should do it anyway.... 100uF is probably enough.
I'm afraid I became somewhat disillusioned with the Fuzz Face.
I don't like the muffled sound of the low impedance input and the way it misbehaves when wired for negative ground, so it's been in the drawer for the last couple of months. Maybe it's time to revisit the thing - buffer the input etc..

I've HAVE been meaning to write an explanation as to why negative ground PNP circuits are prone to oscillations and other problems.
Despite what some people say, they are NOT equivalent.
Maybe next week if anyone is interested...

[mac]

I avoid using electrlytic caps whenever I can. In my Ge FF I put three 6.8uF and a 2.2uF polyester. They are big, need a lot of board space, but they sound better and have no aging issues. A bigger board and box worth it.


mac

[DuncanM]

I avoid using electrlytic caps whenever I can. In my Ge FF I put three 6.8uF and a 2.2uF polyester. They are big, need a lot of board space, but they sound better and have no aging issues. A bigger board and box worth it.

I can understand that some people might hear a difference with signal passing through electrolytic caps, but I fail to see how using Polyester caps to decouple the power supply, rather than electrolytics, could have any influence on the sound...