In this case the grid resistor, connected between the rf-bypassed
negative port of the bias supply and the cold end of the grid choke has
to provide the difference between the protective bias (above mentioned
-33V) and the class C bias specified in the valve data sheet. Simple
Ohm's law can be applied. If the grid current is say 2mA and the desired
grid bias is -63V, i.e. a difference of 30V, the control grid resistor
needs to have 15kOhm.

73, Eddi ._._.

Yes, Ohm's law is OK, but the my issue was that whatever "desired grid bias" I
take, the final stage efficiency does not change, So, changing the grid resistor
makes almost no difference, whilst I would have expected that biasing the tube
in the class-C region would yield more RF power that when it operates in class B
(i.e. with grid resistor = 0 ohm)

73

Tony I0JX