John Kaesehagen tells us : The Resnatron was built in the 1940s, it produced up to 60 kW at 300-600MHz. It was significant in that it was vacuum pumped while in use !!! It was able to cover a broad frequency range.....and was supposed to be tuneable in the field......by dismantling and fitting different parts !!! Some of the required soldering had to withstand vacuum and coolant pressures !! Imagine doing that in a rush on the battle field !