This is essentially a Rangemaster (which is nothing but a
common emitter amplifier with voltage divider biasing). RG has good instructions for biasing Rangemasters
here. It involves tweaking the 68k resistor (Rb1) as well as the trimpot (Re).
Here's the relevant section, but you might consider reading the whole document if you're interested in learning more. (For example, he recommends a Vc between 6.8V and 7.1V.)
RG wrote
I recommend you do the actual tuning this way:
1. Build up your circuit by one of the methods shown later. Leave the Rb1 and Re resistors out, attaching a 100K and a 10K pot respectively on long wires
2. Use a DMM and set these pots to the nominal 68K and 3.9K respectively, and make a temporary mark on the pot and shaft to note the pot rotation at these values.
3. Apply battery voltage and measure the collector voltage to ground. You would like to see –6.6 to –7.2V with a fresh battery (9.0 to 9.3V).
4. If the collector is too high (the transistor is too “off”) increase the Rb1 value. If you go very near the highest resistance before you get to –7V, reset the Rb1 pot to the 68K mark and inch the Re value down a bit, then try tuning the Rb1 value again.
5. If you go near the low end of the Rb1 resistor before you get to –7V, increase the Re value slightly.
6. Once you get to –7V on the collector, turn it off , remove the pots, being careful not to turn the shafts. Measure the resistances and solder in the nearest standard value fixed resistors.
7. If Rb1 is below 27K or above 82K, or if Re is below 2.7K or above 5.1K, your transistor either has the wrong gain or is too leaky to use, you’ll have to use another one.
He also recommends tweaking the input cap if you change the biasing resistors, but the range control might make that less necessary in this build.
I'd do this procedure on a breadboard, myself.
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