Bias resistor choices

[TOPLEL]

This is the first stage of a well known fuzz:




That 1.9V DC at the collector would be more healthy if it would be at half supply voltage i guess like this:




So why did the designer choose that 220k value? (there is a slight gain difference between the two settigns like 160x vs. 225x but then there are two extra gain stages after this one so i am not sure if it matters much)

[nocentelli]

I think the "collector at half supply voltage" is not set in stone: I understand it is to maximise avialable swing without clipping, but maybe that was not the designer's intention here. There are many assic, great sounding circuits where the collector is not half supply, e.g. Q1 in the fuzzface, Q3 in the tonebender MKII.

[TOPLEL]

I think the "collector at half supply voltage" is not set in stone: I understand it is to maximise avialable swing without clipping, but maybe that was not the designer's intention here. There are many assic, great sounding circuits where the collector is not half supply, e.g. Q1 in the fuzzface, Q3 in the tonebender MKII.

Maybe the designer wanted asymmetrical clipping?

[R.G.]

This is the first stage of a well known fuzz:
...
That 1.9V DC at the collector would be more healthy if it would be at half supply voltage i guess like this:
...
So why did the designer choose that 220k value? (there is a slight gain difference between the two settigns like 160x vs. 225x but then there are two extra gain stages after this one so i am not sure if it matters much)

Whether the 1.9V at the collector would be more healthy or not depends a lot on what follows it. You mention this being the first stage. If the second stage has a low input impedance and gets its signal as a current, not a voltage, the collector voltage swing on this stage may not be an issue at all.

Which brings up something that is a common problem with electronics beginners: failing to take into account the impedance of the source that feeds the input and the loading of the stage which follows it. Both of these are inevitably part of the circuit, but hardly ever shown. They have an effect. A designer has to know them to understand what really happens, even if it's only to know them well enough to know that they have little effect on the operation of the circuit - which is often true, but false often enough to be crucial.

[TOPLEL]

This is the first stage of a well known fuzz:
...
That 1.9V DC at the collector would be more healthy if it would be at half supply voltage i guess like this:
...
So why did the designer choose that 220k value? (there is a slight gain difference between the two settigns like 160x vs. 225x but then there are two extra gain stages after this one so i am not sure if it matters much)

Whether the 1.9V at the collector would be more healthy or not depends a lot on what follows it. You mention this being the first stage. If the second stage has a low input impedance and gets its signal as a current, not a voltage, the collector voltage swing on this stage may not be an issue at all.

Which brings up something that is a common problem with electronics beginners: failing to take into account the impedance of the source that feeds the input and the loading of the stage which follows it. Both of these are inevitably part of the circuit, but hardly ever shown. They have an effect. A designer has to know them to understand what really happens, even if it's only to know them well enough to know that they have little effect on the operation of the circuit - which is often true, but false often enough to be crucial.

My image shows the DC bias voltages of these stages, i don't think that the previous and following stages could mess with these voltages since they are separated with coupling caps.
Since this stage is a booster stage going into a fuzz face like circuit i thought it would be benefical to bias this stage's collector to half supply voltage.

[R.G.]

My image shows the DC bias voltages of these stages, i don't think that the previous and following stages could mess with these voltages since they are separated with coupling caps.

That's correct. However, that wasn't my point.  My point was that the DC voltages may or may not matter. I'll give some examples below.

Since this stage is a booster stage going into a fuzz face like circuit i thought it would be benefical to bias this stage's collector to half supply voltage.

And yes, it would be if and only if (1) the input signal times the stage gain causes the input stage to clip; this may or may not be true. Guitar signals are often in the range of 100mV. If the actual gain, as opposed to nominal gain, is not too high, it may never clip; and (2) any clipping from misbiasing this stage may have been a desired result in the original design.

The AC gain of a stage, as opposed to the DC voltages, is heavily affected by the loads on it. A fuzz face type circuit may have an input impedance of a few hundred ohms. That means that the collector resistor in this design may be somewhat immaterial as regards AC gain and voltage swing on the output. The input of a fuzz face is largely the impedance of a base-emitter junction with no emitter resistor, and that is guaranteed to be not much more than 0.7V, and more importantly not to swing up or down more than maybe 100mV.

So again, biasing in the middle of the power supply is a good thing, but may or may not be absolutely necessary depending on what it drives. The booster may never get to show of how widely it can swing.