When I change the grid resistor, I see no chage in the ratio between
RF output power and DC input power, this meaning that efficiency
does not change, I would have exprrcted that, going class-C, I would
have got a higher efficiency (more RF power, or lower DC power, or both).

So would I. How do you feed the 829B's screen grid?

The screen grid is fed by a dedicated 200V supply, which is very stable (within
the screen current excursion).

73

Tony I0JX