Noob - biasing, how and why

[glesconz]

Hi, I am building this pedal, a Colorsound One Knob fuzz with Big Muff Tonestack, and have never bothered to bias transistors before..how do I go about doing it and why is it done? Am I correct in that you replace certain resistors with trimpots and trim until certain voltages are reached and use those resistor values? If so, what resistors do you tweak, and what voltages do you look for? Here is the link from where I got the circuit, the transistors are supposed to be BC182  but I am using BC546's.

http://www.diystompboxes.com/smfforum/index.php?topic=53097.0;prev_next=next

Also, I want to make a Rangemaster clone, being Germanium, is it worthwhile to bias that transistor if they drift due to temperature?

Thanks for your help,

Glenn

[JKowalski]

All transistors have to be biased or they don't work at all. If the signal is not biased correctly the signal cannot be amplified correctly.

A good design will not need tweaking of bias after the circuit is made (unless you want to mess with the operation that way on purpose).

The trimpot method you mentioned is the uninformed way to do it. To do it properly you would analyze the circuit and choose appropriate resistor values after a few calculations.

http://en.wikipedia.org/wiki/Bipolar_transistor_biasing

[LucifersTrip]

Hi, I am building this pedal, a Colorsound One Knob fuzz with Big Muff Tonestack, and have never bothered to bias transistors before..how do I go about doing it and why is it done? Am I correct in that you replace certain resistors with trimpots and trim until certain voltages are reached and use those resistor values?

You're confusing biasing with tweaking the bias to your taste. As JKowalski wrote, the designers of the circuit should have already biased the transistors to work well. You can find a calculator here:
http://diystompboxes.com/analogalchemy/emh/emh.html

What you probably want to do is tweak the bias to your taste. In fuzz face variants, the common resistor to tweak is off the collector of the final transistor. In this case the 10K off of Q3's collector [replace with 20k pot]....though, you will change the bias by changing any resistor and the sound will probably change, also...just experiment. Try everything.

The voltage you're referring to is generally 1/2 the supply voltage (9/2 = 4.5). You will be measuring Q3's collector with a multimeter (black to ground, red to Q3's collector). Turn the 20K pot till you get near 4.5 and you'll hopefully get a good tone.

Rangemaster:
http://www.geofex.com/Article_Folders/Rangemaster/atboost.pdf

good luck

[glesconz]

Hi, thanks guys, the wiki link was a bit over my head but thanks all the same. And if Im always yelling "Oh God!" when i cant understand something, then i guess it should be no surprise to get help from Lucifer...thanks, man! the calculator looks cool and thats explained quite a bit as to regards biasing and biasing to taste...

I cant believe Im the only one that uses this board that doesnt understand about biasing, so many others must do what i do, just build up a circuit and use it how it sounds when powered up, without tweaking. Perhaps an article, Biasing 101, on what resistors to fiddle with etc in Real Dumb Speak might be of help...

Thanks!
Glenn