How Can I Bias a Push-Pull Fuzz?

[jdduffield]

Hi,

I built this fuzz circuit tonight and am wondering how to change the bias on it. It just isn’t quite fuzzy enough, sounds a bit choked and Velcroy, spitty. Here is the schematic.
For Q1 & Q2 I’ve got hFE values of 354. For Q3 I’ve got an hFE value of 320. Also, I had to sub the 500k resistors with 470k. See photo.

 

[jwin615]

What is the voltage you are seeing from between ground and the base of Q2 and Q3?

 

[jdduffield]

What is the voltage you are seeing from between ground and the base of Q2 and Q3?

Q2: 4.0V
Q3: 4.3V

But… Q1 base is only showing 1.2V

 

[Aleph Null]

Actually...

...at the risk of being pedantic, it's not a fuzz; it's an octave up. If you swap out the 1M resistor for a red LED, you basically have the Lizard Queen. You could also add clipping diodes to ground before the volume pot.

 

[JohnnyCreepy]

How it sounds with neck PU and from 12th fret upwards? Is the octave effect more apparent?

 

[jdduffield]

How it sounds with neck PU and from 12th fret upwards? Is the octave effect more apparent?

Yes, the octave effect is more noticeable up there.

 

[gonzo]

It's a fairly clean octave pedal. I've been using it for a decade plus. It glorious into a dirty amp or with a drive pedal. You can switch a leg off (emitter or collector) the last transistor to make it a full on fuzz.

 

[DDad]

I think the 500K resistors connected to the bases of Q2 and Q3 set the bias. You could put 1M pots there and adjust to ear. Then read the value of the pots out of circuit and replace with the same value resistor. When you connect the pots, just use one end lug and the wiper lug, don't wire the other end.

 

[jwin615]

Q2: 4.0V
Q3: 4.3V

But… Q1 base is only showing 1.2V

Looks good. You could futz with the 500k/470ks. I think increasing them would clean it up, to a point, too much after a certain point.
Can go up to 0.7Vish.

I think the 500K resistors connected to the bases of Q2 and Q3 set the bias. You could put 1M pots there and adjust to ear. Then read the value of the pots out of circuit and replace with the same value resistor. When you connect the pots, just use one end lug and the wiper lug, don't wire the other end.

May be fun to try a 1M dual pot in series with the 470s
At least on the breadboard

 

[jdduffield]

Thanks! This gives me some good ideas to try.

 

[drew.spriggs]

Make the 10k resistor to the power rail a 50k trimpot.

Pro tip: you can also add a 1k pot between the power rail and the emitter of Q3 and you've got a nice little octave control pot.