NPN Negative Ground Fuzz Questions

[Easywider]

Using A General Guitar Gadgets Dallas Arbiter Board , & Want to wire it up with a LED & D/C jack . Theres a Layout & wiring diagram on their site for a PNP Transistor Negative ground version . Would I be able to use that board layout & wire it up that way , then use my BC108's instead ? just making sure..... Also , my transistors have gains in the 200's , would those make a nice silicon Fuzz , or are the gains too low ? Also using 250K volume pot & a 2K Linear . Is this O.K as well ?

http://www.generalguitargadgets.com/index.php?%20option=content&task=view&id=154&Itemid=18354

Thanks ,

Chris

[RGW]

No, that wouldn't work. You would have to still reverse the polarity to the actual components to get it to function with NPN transistors.

All wiring a PNP circuit up with a "negative ground" set up is doing is allowing you to isolate your enclosure, jacks, etc. from the PCB's "positive ground" so you can power it off of a common voltage source supplying multiple pedals and not short the power supply itself via your 1/4" cables. That, and it makes it convenient to use Boss style 9VDC negative center pin power supplies 

As far as transistors, even when using silicon transistors I like to find low gain ones measuring under 150hfe.

take care,
Robbie

[R.G.]

Using A General Guitar Gadgets Dallas Arbiter Board , & Want to wire it up with a LED & D/C jack . Theres a Layout & wiring diagram on their site for a PNP Transistor Negative ground version . Would I be able to use that board layout & wire it up that way , then use my BC108's instead ? just making sure....

It won't work that way. You need to use an NPN/negative ground version. This is obtained from the positive ground PNP version by reversing the polarity of all polarized devices (transistors, diodes, and capcitors) and the battery.

All wiring a PNP circuit up with a "negative ground" set up is doing is allowing you to isolate your enclosure, jacks, etc. from the PCB's "positive ground" so you can power it off of a common voltage source supplying multiple pedals and not short the power supply itself via your 1/4" cables. That, and it makes it convenient to use Boss style 9VDC negative center pin power supplies

You left out "and giving yourself a fair chance of it having intractible oscillation problems."

[Easywider]

Thanks to all for the advice ! Wondered the difference in a 250k audio & 500k , 1k linear & a 2k for the silicon fuzzface . what should i use , don't want things to be too bright sounding .

Thanks ,

Chris

[tcobretti]

I have played with the output vol pot value a fair amount, and it is a weird situation.  I find that larger values are actually darker because they produce more output volume which drives your amp harder.  Lower values push the amp less and let the highs thru.  Roger Mayer uses 47kA output pots to keep the pedal from pushing too hard.  However, depending on your amp, you may want that darker sound. So, you should experiment, but remember that you can turn the knob down most of the way and it sounds very similar to the lower value; I ended up using a 500k in mine (for purely cosmetic reasons) but I run it at about 10 o'clock to get that brighter sound.

Another weird trick you can try is to solder lugs 2 and 3 of a 500k->1M trimpot across lugs 1 and 3 of your volume pot (Unless I am doing it wrong - a 1M trimpot and your 250k pot would give you a max resistance of 200k, whereas a 500k trimpot and your 250k would give a max of 167k).  This way, as you decrease the value of your trimpot, you decrease the value of your volume pot.  Now, see if you can hear the difference.  If you find a sound you like, you can measure the total resistance across the outside lugs of the volume pot, then buy a pot close to that value.

I have never been able to hear the difference on the fuzz pot, but I think that's cause I run it dimed all the time.

[foxfire]

i don't know it applies here, but i've read/been told that for example using  250k pots in your guitar rather than 500k pots will give you a slightly darker sound because the 250k pot will bleed more high end to ground. like i said i don't know if it applies here?

[Easywider]

Thanks to All for the advice . I went ahead & used the standard values for the pots & just had another question . whats a good HFE for silicon transistors in this circuit . Using BC108 workalikes from Smallbear , 263 in Q 1 , & 293 in Q2 . Seems to sound pretty nice , But just wondering if i should score a matched pair from Banzai , classic silicon transistor set , if it would sound better .

thanks ,

Chris

[tcobretti]

It certainly makes a difference, but a matched set isn't necessarily the way to go.  You might check out the Axis Face over at fuzzcentral.ssguitar.com.  It is one of the really great Si FFs, and he has a whole article about the choices he made on values.

He uses very low HFE transistors to get a more Ge sound, but if memory serves, original Si FFs came with BC109s.  As well, I built a FF variant, my Mondo Fuzz, that uses MPSA18s and sounds almost like a Big Muff.

The best thing you can do is socket the transistors and try some different combos.

[Easywider]

Travis , Thanks again for the advice , Very cool ! ! Going to check out the Axis face on fuzz central !

[Easywider]

Another question , what could I do to get more fuzz out of this circuit ? Would the transistors play a big part in this , or could something else be done . Also whats the difference between a BC109 & a BC109C .

Thanks ,

Chris

[tcobretti]

"Fuzz" is a relative term. If you want more gain, you can use higher HFE transistors, but you will run into issues with oscillation.  The best way I have seen to fix the oscillation in a FF is a small cap (50-100pf) across the collector and base of Q1.  Another classic trick is a large cap (47u or greater) from +9v to ground. You might actually want to take a look at my Mondo Fuzz if you want a high gain FF.  It addresses the oscillation issues as well as the muddiness that is inherent in a very high gain FF.  It sounds less like a FF and has its own character.

http://aronnelson.com/gallery/TCobrettis-Stuff/FF_Mondo

If you want it fuzzier, you might try a larger input cap, which will allow more signal in to push the transistors a bit harder.  You could also put a 500k-100k trimpot in place of the resistor from +9v to the collector of Q2 and dial it to where the transistor is slightly misbiased.

Another thing you could try is a pair of diodes facing opposite directions going from the collector of Q2 to ground, or even from the base of Q1 to ground.

BTW, I haven't tried some of this stuff, so let me know if one of them works out.

If you want a really nasty, gritty 60s fuzz, the FF is not really the right circuit.  It's more of a heavy fuzzy distortion.  To me, the more nasty fuzzes fall into two categories: the Fuzzrite, Orpheus, and their ilk of two transistor very splattery fuzzes, and the Univox Superfuzz, Foxx Tone Machine type of nasty octave fuzz.  You can find soundclips of all these if you search.

[tcobretti]

If memory serves, I believe the BC109C has slightly higher HFE than the BC109.