Calculating Tank Q, Tune C (C1), and Optimal Anode Load Resistance (RL)

Tank Q, the reactance of C1, and the optimal anode load resistance for linear operation (RL) are inter-related. Tank Q is defined as the capacitive reactance of C1 (XC1) at the frequency of operation, divided into RL ---i.e., Q = RL/XC1......and XC1= RL /Q. Note: C1 includes the anode (output) capacitance (Ca) of the amplifier tube. At 29MHz, Ca may be a sizeable fraction of C1.

RL = Esupply/2*IAn where IAn is the average anode current in amperes.

(Note: There is some variation in the constant in the denominator of the RL formula. For tubes with minimal anode-cathode potential at peak anode-current, like the 8877, a constant of 1.6 should give more accurate results. However, for tetrodes like the 8171, which use a high screen potential (reduces anode AC peak-V), a constant of 2 seems to be more accurate.

Thus, for a tube operating from 2500v @ 1A, whose anode capacitance (Ca) is 10pF:

RL = 2500v/2*1A = 1250 ohms.

Calculating C1

For a Q of 12.5, XC1=1250 ohms/12.5 = 100 ohms.

The needed tune capacitance, C1 = 1/(2*Pi*f*XC1). For 14MHz, C1 = 1/(6.28*14*106Hz*100 ohms) = 113.7pF. However, since part of C1 is comprised of Ca, the net tune C is 113.7pF -10pF = 103.7pF. At 28MHz, the tune C would be roughly: 57pF -10pF = 47pF. At 280MHz, 10pF has about 100 ohms of X, so, for a Q of 12.5, Ca furnishes 100% of C1, so no tune C can be used.