"While valves will work with only 9 volts on the plate they are high
impedance devices and so you will not get any significant power."

Depends upon what you mean significant. I just need a couple of milliwatts
to go a mile. He has a little cmos oscillator going into a final transistor
and keys the final transistor. The cmos oscillator runs constantly at a few
milliwatts. I can hear that, even with the final disabled, so it is simply
the blowby of a few mw oscillator.

Build a tube transmitter with power supply and keyer into an Altoids tin
seems significant.

Colin K7FM