[AMPS] Another Stupid Question

[jstrohm@texas.net]

OK, only stupid because I never asked it.  Yet.

What determines the capacitance value for a plate-blocking capacitor?  By this I mean the capacitor (usually a doorknob cap) between the plate choke and anode connection, and the start of the tank circuit / pi network.

Obviously, this cap has to resist DC and RF voltages present, be capable of passing the RF current present, and resist heat and other environmental conditions present.

What determines the capacitance?

In reviewing a couple dozen amp schematics this morning, I saw a range of 500 to 2000 pF for HF and VHF amps.  The only excursion from this range was an .047 uF (4700 pF) in a 6146 final, which ran at significantly lower voltage and power levels in HF.

I don't need a detailed mathematical explanation -- just wonder if this was an empirical "we always did it this way" design decision, or if there's an electrical basis, such as time constant or reactance or a contribution to the tank circuit elements, or a physical basis such as reactance vs. physical size of the device in the circuit (e.g., parasitic inductance on the structure of the cap vs. allowable value of capacitance).

Jim N6OTQ

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