Hi, a very rough approximation, would be that the outputpower is
RL*I^2, where I=75V/10.
I've roughly put the coupling factor k between the transformers equal to 1, but it should rather be somwere 0.5<k<1.
Output impedance of transistor (for BJT at least it is always lesser than 50 ohm ) is always low I guess. I hope It is the same as for MOSFET. I don't have my reference beside me...
As far as for the input power, you can do a rough estimate that your source impedance is perfectly matched to the PA input impedance.(R_source=R_in)
So it should become So if you have U_source your U_in then becomes U_source/2 and your I_in is U_source/2R, so your P_in will become
U_source^2/4*R.
But you asked for the gain : P_out/P_in, so you must decide how high input signal is ENOUGH.
Could somebody pick up the ball ?.....
regards,
StoppTidigare