how to bias fuzz in a Hipster?

[octa805]

Howdy!! I've built a few pedals and fuzzes even but never biased one. Looks relatively simple, just measure the voltage on the Q2 emitter and get it to about 4.5V? that right?

uh, dumb question, how do you do that? I mean, I know how to measure voltage but where in this case would I put my probes?

also, seems like when I just crank my bias trimpot completely counterclockwise it sounds great. anything towards the other way just sounds slowly less volume and crappy until you get nothing.

I'm using BC108's in there now.

thanks for the help!

[PimpMyTone]

Put your red probe on the collector of Q2 ( not the emitter )  and the black one to ground (I put mine on the sleeve of the stereo jack ) .

[raulduke]

Officially bias to 4.5V.

I would say tweak it to where you like the sound the best though.

Higher bias voltages result in a more open, distorted tone, and lower voltages result in a more compressed, splatty, buzzy tone (eg. EJ Fuzz Face). This is all my opinion of course 

[octa805]

thanks simon. that's so easy it hurts 

fully cranked sounds the most fun to me so far but maybe I'll swap them out for 109s.

[octa805]

Oooh! Like the BC109s now!

Can you make the bias trimpot external just by subbing it for a full size pot?

If so, would any 25k pot work? A or B? Thanks!

[raulduke]

Yep; just sub a pot for the the trimpot. Makes a very handy external control!

You can use a 25KA (log) and it will work, but using a 25KB (linear) is a much better choice, as you will be able to sweep through the different bias voltages more easily.

[midwayfair]

Yep; just sub a pot for the the trimpot. Makes a very handy external control!

You can use a 25KA (log) and it will work, but using a 25KB (linear) is a much better choice, as you will be able to sweep through the different bias voltages more easily.

There's a much better way to do this external control. Although you can bias most Si transistors as low as 2v and still get sound, or as high as 7v and still get *a little* distortion from Q2, it's much better to limit the range of an external bias pot to the range between about 3v and 7v.

You should sweep the pot until it hits the lowest voltage and measure it, then figure out the resistor in series needed to prevent it from reaching that spot. Sweep the pot the other way and measure the voltage until you hit about 7v. Ideally, that value minus the series resistor is the value you want the pot to be. A resistor across the pot will get you there if it's not a convenient value like 5, 10, 20, or 25K.

This is a little more work, but your reward is an external bias knob that has no dead spots and is thus useful. Especially when you start getting a couple dozen fuzz pedals floating around you don't want to have to remember that this one's not broken, the knob is just in the wrong place.

Of course, people use the Fuzz Factory live, and that thing changes if you look at the knobs the wrong way ...

My feeling, though, is that an external bias is not worth it on a pure silicon fuzz. Silicon does not have the temperature drift issues of germanium, and even though you can get the pedal to sound DIFFERENT with varying bias settings, eventually you are likely to notice that the bias just sounds right at one particular place, and then you've got an extra knob on the outside to bump out of that setting. Not to mention the extra expense of a pot + knob. (It's different if you do the AM thing with the little bolt knob, which can't be changed accidentally.)

[octa805]

thank you both. appreciate it.

In this case, I'm stuffing the hipster in a poorly thot out 1590A so the thought of making the bias external is that it would be the only way I could actually get at it. As it stands, due to my lack of planning and a couple larger than expected caps on my PCB I'll have to wedge it in there in such a way as to make the bias pot inaccessible moving forward.