Setting transistor bias - general questions

[tracer03]

So, I'm going for my 2nd stripboard build (first was a Bazz Fuss) - and I've decided on the Lovepedal Silicon Fuzzmaster.

I've got all the parts ready, but in reading all the comments I saw that folks were talking about setting the transistor bias correctly to get a good fuzztone. Since I am pretty green when it comes to building, I wanted to make sure I could get a successful build - and so I started digging for info on what that entails.

Unfortunately, my search-fu was weak (or I was just too sleepy - it *was* after midnght) - and I couldn't find reasonably simple answers to the questions I have.

1. I've never set bias before, so is there a straightforward/step-by-step guide somewhere? Tutorial perhaps?

2. Assuming that you 'rock it before you box it', should you breadboard the build, set the bias values (if necessary) and then start soldering?

3. In circuits that are posted here - is it necessary to set the bias each time? Or are the 'stock' values shown on the stripboard layouts good enough?

Thanks in advance.

[nocentelli]

It has a trimmer on the board, so I would suggest soldering it all together (including pots) and tweaking the trimmer once you have established it makes some sound but before you box it.

It is a variation of the classic fuzzface, and these sometimes have a trimmer on the Q2 collector since this has a significant influence on the sound. Also, the gain control in this particular variation of the fuzzface actually changes the bias (by altering the resistance between emitter and power), so I wonder if the trimmer is there so you can dial in a "compromise" bias point that sounds good throughout the whole travel of the gain pot. A collector voltage of 4.5v is often suggested for the npn FF: For this one, you would be measuring the voltage at Q2's emitter - you will get a variety of interesting and useable sounds above and below this voltage, try setting it by ear in between playing the guitar to check for sustain and gating and adjusting the drive pot between minimum and maximum.

It is not usually necessary to tweak the voltages of diy effects if the layout/schematic is verified - a well designed biasing scheme will allow a wide variety of different transistors to be used with no adjustment to the voltage. However, there are a few designs where tweaking is unavoidable: e.g. single JFET stages often require a trimmer between the drain and +9v to adjust the drain voltage to around half supply voltage to maximise the headroom and ensure the device is operating correctly: Since JFETs vary wildly in their specs, it is often difficult to predict what value resistor would be appropriate.

In these circumstances, it is usually clear from the layout (i.e. there will be a trimmer or a note on the layout saying that it needs to be tweaked, or "drain resistor 3k to 50k" etc).