This may be an oversimplification on my part, but it seems to work.
When I am trying to determine the Input (C1), Output (C2), and Tank
Inductance for an amplifier, I consider the circuit to be acting as an
impedance
matching transformer and use the same techniques one would use when trying to
match circuit input and output impedances when designing a colpitts oscillator
tank circuit.
1) First I figure out what the plate impedance should be for full power
output using the standard calculations easily found in Orr and in the Handbook.
2) Then I divide that number by 50 ohms (or whatever my target load
impedance happens to be) to get the impedance transformation ratio I need.
3) Then I divide the plate impedance (Lets call it RPq) by the tank circuit
Q I am trying to obtain.
4) Then I solve for C1 based on C1 having that value of impedance (equal to
RPq) at the frequency of operation.
5) Whatever capacitance value I get for C1 gets multiplied by the impedance
transformation ratio number I obtained earlier, and becomes the capacitance
value for C2.
6) Then I calculate the value of C1 and C2 in series. (usually just a
little less than the original value for C1 and in most cases you can just use
the
value of C1). Lets call this new value Cr.
7) The last thing I do is use the resonance formula and solve for the value
of L by using the value derived for Cr at the frequency of operation.
I have used this procedure successfully a number of times now. Maybe 4 or 5
times with different tubes and different impedances, and it has always
gotten me into the 'ballpark' as far as tuning was concerned.
I am certainly not a math major, and don't totally understand how this
works, but it seems to work for me and is a fairly easy and quick calculation.
Regards,
Dennis O.
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