Cecil, W5DXP wrote:
"What is the linear source impedance of a class-C amp?"

A conjugate match is necessary for maximum power transfer.

A Class C amplifier is not inherently linear. That is disasterous for an
already modulated AM signal but it is of no importance to an FM signal.
As Richard Fry points out, no tank circuit is required. A low-pass
filter to suppress all harmonics is all that is needed for a clean
signal. No tuning is required of a tank at the operating frequency.

A tank circuit is not selective enough to prevent intermod anyway. When
I worked the morning shift at 790 KHz in Houston, and I fired up the
transmitter, I would hear 740 KHz`audio coming out of the monitor
speaker. They started programming earlier than we did. They were 15
miles away. Our transmitting antenna made a dandy receiving antenna. The
received 740 KHz modulated our final amplifier and we rebroadcast it
although at a level much lower than our own modulation. Any one
listening to 790 KHz who turned up the volume to hear the 740 KHz audio
got their ears knocked off when our modulation started. We had high
level plate modulation of the final amplifier for our own signal. For
the 740 KHz signal the level of modulation was much lower, millivolts
not kilovolts. Either program was cleanly modulated on our carrier. The
only difference was the enormous difference in modulation levels.

Best regards, Richard Harrison, KB5WZI