How to Set Bias

[joelorigo]

I just finished building a Sunflower, my first Fuzz Face type pedal build. As I am still a beginner, I am wondering how to actually set the bias to the recommend value. I have this multimeter:

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[Mcknib]

I think that meter has a 1M ohm input impedance so won't give the most accurate readings

You want a meter with at least 10M ohm input impedance

Aside from that to set bias you'd adjust the bias trimmer so you get around 4.5v on Q2 collector or simply do it by ear to where it sounds good to you

I personally always do it by ear

With this pedal you've also got the sundial pot to re bias as required, with PNP transistors being affected by temperature it may sound different in some situations so you can adjust it on the fly using that

I'm not 100% but I'd turn the sundial knob completely CCW before biasing with the bias trimmer

 

[joelorigo]

I could give it a by ear try.

But I actually want to know what to set the multimeter to, and where I put the leads. From what I've been able to learn I put the positive on the Q2 collector and the negative on the output jack ground, correct?

 

[Gordo]

Correct, or the black lead to any convenient ground point. If you have an alligator clip kicking around you can clip it to the black lead and a ground point to free up a hand to turn the trimmer. Set the meter to D.C. Volts. Touch the red probe to the 9v pad to see what your power is (and to make sure your setup right). Ideally you want the collector to be about half that voltage. Move the red probe over to the trans you're trying to bias and twiddle away.

As mentioned, ultimately you'll dial it in by ear but the reading will give you something to come back to, and a good sanity check.

I wouldn't be too concerned with the meter being good enough, you should be fine. It's only rock and roll.

 

[joelorigo]

OK, for the first step I got a reading of about 9.5, which seem right as I am using a 9V battery. Then for the transistor I adjusted the trimmer to about 4.8 (close enough for rock n roll?).

But, does the position of the "sundial," the bias knob on top of the pedal matter when measuring?

 

[Mcknib]

Sorry I hadn't realised you were asking how to set your meter up to bias

Looking at what the man himself says about the sundial knob maybe it doesn't matter too much setting it in the middle looks to be the best, so that you can turn it up or down as required

We factory set the SUNDIAL so the face is vertical at our shop temperature (70 degrees or so depending on if it's Winter or Summer!). You can set it by ear, just turn it up until the buzziness goes away as much as you like.

You may find you need to turn it down (CCW) at higher temperatures, and up (CW) at lower temperatures. This BIAS adjustment allows it to work well at any temperature.

http://www.analogman.com/manuals/PDFs%20(may%20be%20older)/sunface.pdf

 

[joelorigo]

Thanks! I learned some new things!