[Amps] Any new tube research?

[Dan Mills]

Agreed that FETs on a reasonable DC rail will likely become increasingly
popular, but you forgot the major component count from your list of
boxes: Solid state amps use low Q output networks requiring output
filters, unlike the glass fets. 
At KW+ levels these will not be small and will themselves need cooling
without being any thing like as concentrated a heatsource as the fet
itself. 

Also water cooling can be a little subtle sometimes (ask the laser
guys), in particular if the radiators are outside you can have a nasty
condensation problem on the amplifier proper if it is in a warm humid
shack and the water is cold. 
This one causes fits in the laser business with condensation on the
series passbanks having a bad habit (in some designs) or dripping onto
the low voltage control circuitry with occasionally smoky results. 

You also want to do something to ensure turbulent flow behind the
transistor as otherwise the actual thermal resistance can be MUCH higher
then you would expect, stuffing a spring into the pipe is effective. 

Regards, Dan, 2E0CHE.