I’ve been spending far too much time reading scans of old issues of Wireless World.
Here’s a small PA tube from a 1969 Wireless World advert that may be of interest to QRO ops.
Heater supply: 12v @ 660A (six hundred and sixty)
When driven with 3kW of RF and with 19kV on the plate and some 32.5A of plate current you could get 460kW (four hundred and sixty kilowatts) of RF out. There was the small matter of 155kW plate dissipation to handle, but that is a small matter of plumbing.
I always felt that a the mark of a good PA was that it had a greater heater dissipation than the “legal limit”.
What a joyous concept 
For contrast, I well remember reading in an issue of “Short Wave Magazine” (sometime back in the paleolithic!) about a guy who succeeded in working across the North Sea (into PA?) on Top Band with a few mW from a home made point-contact transistor!
Brian
Of course, you’d want a decent antenna system to go with it…
The photo just shows part of the array, worth googling “duga-3” if you haven’t come across it before.
Adrian
G4AZS
You’d probably want to key something early in the chain rather than the cathode of the PA. With 32A flowing you’d need big contacts!
Imagine trying to series modulate it 
Imagine getting that into your backpack… no room for lunch.
