Most amateurs ignore this. The valve will work at 1.2kV, the current can be 200mA and the dissipation 150W air cooled. Removing the anode radiator and replacing with a water jacket allows 200W+ dissipation water cooled.

The anode radiator is easily removed. It either unscrews or is held on by lockscrews. The example on the left is the lock screw type from AEG.

They don't last so long when abused to 150W of 23cms, but they are cheap. Many of mine were rescued from German national service application as badminton shuttlecocks. They were used in a receiver, and became widely available surplus in Europe.