Re: Klystron
BBC Droitwich transmitter was built to a custom design in the 1930s, the current Marconi one dates from the mid-80s. IIRC it uses a matched pair of valves which can handle the 500kW output and have not been made for many years and the BBC now has the whole remaining stock. The only practical approach to keeping it running would be a whole new transmitter, which is of dubious value considering how little interest there is in analogue LW (198kHz) these days.