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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