On Sat, 12 Jun 2010 06:13:15 -0700 (PDT), Richard Fry
wrote:

It has been theorized that a circuit consisting of a Class C vacuum-
tube r-f amplifier using a tuned tank circuit in its output network
provides an operational “non-dissipative source resistance” of 50 ohms
for energy present at the output connector of the transmitter.

From "Care and Feeding of Power Grid Tubes:"

"The first step in designing the output circuit is to specify the
resonant load impedance of the tube, the loaded Q of the circuit and
the desired output impedance of the network. The resonant plate load
impedance for the 4065A is determined by dividing the plate peak RF
voltage swing by the plate peak fundamental RF current.
[work shows Resonant load impedance = 7600 Ohms]
"if it is assumed that the output impedance of the network is to be 50
Ohms, and the loaded Q is to be 15, the output tuned circuit may now
be designed. The output impedance of 50 Ohms will match into a
properly terminated 50 Ohm transmission line."

This document provides plenty of math and charts for the design of
efficient output networks (finals' plate load).

73's
Richard Clark, KB7QHC