Fuzz DC voltages with parts value changed

[Gus]

A screenshot of what happens when you change transistors, Bias/feedback resistor value, 2nd stage emitter resistor value(hint as to why I often use a 1K gain and 5K bias in the emitter section of Q2)
Screensot is from a sim of the DC voltages missing input output caps gain control etc.
This should help understand how some values in a FF like circuit interact.
Resistor values are from a schematic I found on the web
The 500K is a potentiometer 0 to 500K





R3 is 0 for the voltages shown at the transistor symbols

[Gus]

R1 and R3 think pinch control also look at other FF like circuits and note the value

Note the current to Q1 base is partly controlled by the bias/feedback resistance of R1 and R3 together. Screenshot voltages are for a 100K, two 2N2222's, current = (.680 VDC- .605VDC)/100k, I did not run the sim again after I changed R3s value thats why the mismatch of voltages to R3 in the screenshot

The voltage at the "top" the emitter resistor R5 also affects the available current to the base because of the voltage drop across R1 and R3 when the voltage at Q2 emitter changes with R5 value

Also note the change with beta/hfe.  You need to select the beta/hfe for this circuit to work correctly.  I would use the transistor beta/hfe and R5 value that gives the range of Q2 collector voltage change you want

So with FF like circuits there are 4 places resistor values adjust bias, both transistor collector resistances , Q2 emitter resistance and the bias/feedback Q2 emitter to Q1 base resistor value

I don't like adjusting Q2 collectors value because I want to select the collector current and output resistance of Q2 for what follows the gain stages.  I pick that and work out the rest.  Yes I know you see posts about changing Q2 collector value but I think that is not a good way to start.

[Gus]

This post is because my posts are to old to modify and I wanted to add some things

I should have named this how to bias your FF like circuit

The screenshot is DC opt of a popular fuzz circuit fragment  I thought people might notice that

In this thread I hoped I showed what changes with different beta/hfe transistors 2N2222 to 3904 to 5089 with the same fixed resistors and the pinch bias control changed.  The voltages are the transistor collector voltages

Also the same set with the Q2 emitter resistor changed

This should help show what direction you can change values of some resistors to adjust you bias for different beta/hfe transistor and /or if the collectors voltages are not what you want what you can change without changing Q2 collector resistors total value

[Gus]

So who can tell what circuit this is from? 
People copy build it.
note the bias is not the 4.5VDC nonsense that the web made standard

[Electric Warrior]

4.8V isn't far off, really. 1.30V Q1C and 4.5 to 5V Q2C voltage are pretty much what a stock silicon Fuzz Face should measure.

[duck_arse]

(wooly mammoth?)

[Kipper4]

I vote for the mammoth two!!!!!!!

[Gus]

4.8V isn't far off, really. 1.30V Q1C and 4.5 to 5V Q2C voltage are pretty much what a stock silicon Fuzz Face should measure.

Look at the 2N3904 voltages with R5 2.2k .98VDC to 3.04VDC
Not the "web standard" 4.5VDC

4.5VDC is not always what you want.

Correct answers for the effect however there is another one with two controls

[Gus]

Built a test circuit.
The 2N2222 measured as simmed. 

Circuit is sensitive to both transistors beta

If you know what the fragment is from note the loading of the 2nd stage in the three circuits that use it.

Interesting selection of parts values.