adding a bias to the silicon fuzz face

[estch71]

I just built this and have been searching for a way to add a 10k bias pot to the schematic to, basically, bias the pedal.  Any advice is greatly appreciated.  Also, as I'm new to pedal building, a "dummies" version of placing the pot would be amazing.

[LucifersTrip]

you remove r3 and connect the middle lug of the pot and either side to the exact same 2 spots where r3 was.

though, you will probably want a 1-2K limiter/cutoff in parallel with one of the pot lugs so you don't have 0K there.

others use a 50-100K pot instead of r2.

[Buzz]

Something like this.... don't put the 8k2 resistor in ( r3 )



You might hang around for a more knowledgeable dude to OK it, I'm just a beginner too, but I can't see why that wouldn't work.

[Buzz]

10k pot is linear... 10kB

[Thecomedian]

Yes, that works. You also want to have two of those lugs shorted together with a wire soldered in. if you would normally have one of the lugs going to ground, you instead short that lug to wiper. It's a safety measure for the circuit.

You may also find that the value is either too small and doesn't give enough difference, or that it is too large and makes the unit sound awful or become weak.

What I did for a fuzz face was try a 4.5 volt voltage on collector by lowering the resistor by an amount that is suggested by smallbearelec to make the fuzz face more "focused" and less "weak". It worked, but then I also swapped in a trimmer and went up to 20k on collector resistance, and I found that made a nice sounding distortion, while 5-5.6k also provided a good and different sounding distortion.

like the other guy said, putting on a 1 to 2k limiting resistor in series with the pot is a good idea.

[Buzz]

Thanks Comedian updated the "artwork". The little rectangle with squiggles in it represesents a 1k resistor.

[estch71]

Thanks much!  This pedal has the potential to sound great, although at high gain it squeals and sounds like a dying battery, which I know is somewhat favorable (maybe?). 

[induction]

I did something similar to Thecomedian, but I allow the bias to drop as well rise.  I replaced R3 with a 2.7K resistor in series with a 5K pot wired as a variable resistor.  This gives me a bias range of 2.8V to 6.8V on the Q2 collector.  Low biases sound gated and sputtery.  High biases sound hard-edged, more distortion than fuzz.  Right in the middle of the pot rotation is classic fuzz face territory.  The volume also increases with the bias.  Not drastic but definitely noticeable. 

The best thing to do is to experiment with the range of resistances, though.  I fine tuned mine for the voltage range I wanted, but my resistance values won't necessarily give the same voltages on another circuit with different transistors (I used piggybacked 2N3904's).  Using a DMM is useful for this, but you can also do it by ear.

Generally speaking, you want the whole pot range to be useable (1K minimum might be too low, or might not).  If you accidentally bump the pot onstage, a slight tweak in the fuzz quality is better than the sound cutting out completely.

Good luck.

[GGBB]

at high gain it squeals and sounds like a dying battery, which I know is somewhat favorable (maybe?). 

A smallish cap - ~100pF - between Q1 base and collector will tame that.  Experiment with the value - more capacitance = less squeal.

[Gus]

Here is a sim for higher gain FF like circuits
note the placement of the bias control
PCB, perf etc. and wiring layout matter

[Buzz]

Thanks much!  This pedal has the potential to sound great, although at high gain it squeals and sounds like a dying battery, which I know is somewhat favorable (maybe?).  

That squeal can still happen in 'boutique' style fuzz faces. I don't think it matters. AFAIK it happens when you turn both fuzz and volume up to max at the same time. If you back off the volume pot ( in my experience ) the squeal stops... even though you still have the fuzz pot maxed. So in the long run I think it gives you more available sounds from the same box. Just don't max them both out simultaneously then all is cool.

[Gus]

The "squeal" can be caused by layout and wiring.  
Layout and grounding can be very important.
The power supply matters.

[Buzz]

Hi Gus.

Serious question from a new guy... this vero layout was my first fuzz face. It was never great, even biased to 4.5v on Q2 Collector.

Ended up snipping out the board and replacing it with a Hornet circuit of my own vero layout. I used the small bear trans and suggested resistor values. Now it rocks.

Would you improve the given layout? How and Why?

( power supply irrelevant... battery only )

[Buzz]

I've got another "why".... Why does connecting the unused lug of a pot to the wiper make it safer?

( as a variable resistor )

[Gus]

A 9VDC battery is not a perfect voltage source.  One thing to try is to add a 100uf or greater cap 9VDC to ground this can "help" the battery a little.  The gain stages of a circuit can interact via the power supply.  The more perfect the supply the less interaction.  I have hear an effect get squealing due to a battery.

Layout and grounding is another area of study.  It can get deep.

Some simple rules
Wires at 90 degrees have less coupling than wires at other angles.  Parallel can cause more coupling.

You can have signal inversion and phase shift between signal wires in the build.  This can be a positive feedback path.

Shorter wires are often better.

Wipers can bounce and can lose contact with the resistive element, so if the circuit is sensitive to the resistance becoming an open circuit having the wiper to the lug sets the failed condition to the resistance of the potentiometer value.

[LucifersTrip]

I've got another "why".... Why does connecting the unused lug of a pot to the wiper make it safer?

( as a variable resistor )

it doesn't make it safer. it just keeps some type of connection there if the pot actually fails...instead of having an open circuit.

btw, re-reading my original post, I realized I actually wrote parallel instead of series. It should of course be:

"limiter/cutoff in series with one of the pot lugs so you don't have 0K there."