polifemo wrote:
With a 470k resistor on Q1's collector the circuit produces much less fuzz compared to 33k-47k, and doesn't sound very good at all.
I did not have time to mess with the feed back loop resistor, which I feel is a must if using such a large resistor in Q1!
I feel that anything between 33k - 100k works just fine (did not try anything lower than 33k) with the lowest values producing the most Fuzzy tones.
This makes sense. If we "guesstimate" the input impedance of the Q2 to be around 100k, then Q1's fixed collector load (normally the 33k resistor) really needs to be below this value for the circuit to function. I would expect some lower gain if you lower the value.
Your 33k-100k range is probably about all that is practical... even with messing with the other resistors. You'd have to raise the 1k fuzz pot to raise Q2's input impedance to try and get around the limitation.
I've seen 27k many times for this circuit.
Seems like the actual voltage in Q1 doesn't matter that much (with this particular transistor: 2N3440, with a hfe at about 60, it will sit at about 2v with 33k and about 1.8v with 47k) and it seems to be of more importance to stay within the 33k - 47k range on Q1's collector and then use whatever is needed in Q2 in order to get the "4.5v" or whatever one goes for.
Q1's voltages are stabilized by the feedback loop. Start at the base and trace the dc path "back" to the collector. When tracing dc, consider all capacitors OPEN, or disconnected. You'll go from base, to 100k resistor, to Q2's emitter. Follow through Q2's emitter to base, and you're back at Q1, now at the collector.
Because of this loop, Q1's voltages tend to stay put.
Why? Imagine a small increase in voltage at the collector... this goes through Q2's emitter, through the 100k, and creates a small increase at Q1's base. The base
is locked to the collector.
Still a bit curious regarding how much of the differences in sound between GE/SI is due to the fact that a GE transistor in Q1 will operate at about 0.5v whilst a SI transistor usually sits at 1.2v.
The straight voltage differences are due to the physics of Ge junctions vs. Si junctions. These transistors are called "bipolar junction transistors" or BJTs for short. They are made on the same idea that Si and Ge diodes are made. An elementary view of a transistor is two diodes connected back to back. You should always visualize the base to emitter junction as a diode. The base to collector diode is also a diode, but for normal operation it is
not so helpful to visualize this as a diode.
If you apply voltage to a diode, starting a 0.001V and increasing slowly, the diode does not conduct much at all. Keep going, and eventually the diode "turns on" and conduct's heavily. It's "resistance" changes from very high to very low suddenly.
This "forward voltage drop" which every diode has is usually 0.100V-0.300V for Ge diodes, and usually around 0.600 for Si diodes.
If you've got 2 Ge's with 0.250V drops, and 2 Si's with 0.600V drops, add the two together and you get your voltages : 0.5V for Ge, and 1.2V for Si.
Follow the loop again. Q1 from base to emitter
must be 1 diode drop. Q1's base will always be around 0.100-0.300V for Ge, and around 0.600 for Si. Keep going back up the loop, and you get the Q2's base emitter diode drop. This means Q2's base, which is also Q1's collector must be another diode drop higher. Make sense?
Why the Ge circuits and Si circuits
sound different is a wholly greater can of worms, and I don't think we've settled on a scientific answer yet. I would guess it's part of the reason why so many people keep building the circuit all these years later.