Biasing transistors

[dmk2113]

Quick question - I'm currently building a tonebender mk ii clone and I was hoping someone could help guide me as to how exactly to go about measuring collector voltage for biasing. I have Q3 hooked up to a 10K trim pot and I started out by adjusting it by ear until I had removed most of the hiss and was pleased with the tone that I got, but when I measure the voltage at the collector I get something absurdly high like 8.2V - until I run a signal through it, at which point it drops to about 4.5 until the signal dies.

This is a very elementary question, but when measuring the voltage, am I supposed to measuring the voltage with or without an audio signal across the circuit?

Honestly the thing sounds pretty good so I'm not overly concerned, but I just wanted to be careful and double check - since maybe I could be making it sound even better.

Thanks!

[smallsnd/bigsnd]

measure without a signal.
honestly, just use your ears. biasing something "correctly" may or may not give you the best sound because of a number of reasons... differences in component tolerances, leakage/hfe of your transistors, personal taste and not to mention the most important thing - since you are building it for yourself, you want it optimized for your own rig... that's why i feel like you should just go with what sounds good to you. don't be too concerned with what's "the best"

[Scruffie]

measure without a signal.
honestly, just use your ears. biasing something "correctly" may or may not give you the best sound because of a number of reasons... differences in component tolerances, leakage/hfe of your transistors, personal taste and not to mention the most important thing - since you are building it for yourself, you want it optimized for your own rig... that's why i feel like you should just go with what sounds good to you. don't be too concerned with what's "the best"

Good point although a ball park range can be handy to get it in the right sort of area.

Incidentally 8.2V isn't absurdly high... there's alot of misinformation around and if you want to stick close to the original, you don't want to bias down to 4.5V, that works great for fuzz faces but around 7.2V is good for Tone Benders in my opinion and from measurements done on original units by the owner of D*A*M.

[dmk2113]

Thanks -- yeah, I figured that I should just go by ear -- I've never been a purist/originalist, but I'm slowly getting more and more technical with my pedal building and I want to make sure I'm doing things right. The fact that I'm using 2 AC128s and an AC188 is also not to be ignored (I am thinking about buying a set of OC75s when I'm home next, but for now I'm just using what I have around).

At any rate, I'm also attaching a picture of the thing just to show off, I guess -- I kept it simple, but I think the paint job came out nicely!

[Jero]

Looks good, love that hammered texture!