Owen Duffy wrote in
:

....
voltage. Efficiency is OutputPower/InputPower. By varying the grid
bias, drive voltage, load impedance and supply voltage for a given
triode, different efficiencies will be found, and the maximum could be
well over 80%.

If you want to explore this approach to design, I have implemented a
spreadsheet which in turn implements the methods described in CPI/Eimac's
"Care and Feeding of Power Tubes" it is described and can be downloaded at
http://www.vk1od.net/RFPATPC/index.htm . The spreadsheet is populated with
the example 4CX20000 in Class C from the above publication.

The spreadsheet calculates the anode current wave form from an
interpolation based on a number of points from the published
characteristics, and calculates the fundamental component of anode current
using an FFT. It then calculates anode efficiency and overall efficiency
given operating parameters.

Owen