I wonder if anyone has ever made a switch mode power supply using a TV horizontal output valve / sweep tube? The way they are meant to be used is not all that different from a power supply, and seeing many of them will tolerate a +6kV pulse on the anode, you could have a duty cycle of >90% and consequently good transformer utilization and lower DC output ripple. Driving the control grid positive say 10V or so during the on-time wouldn't hurt either. Interesting to see what kind of frequency it could run at, given a suitable driver. I expect given the cathode current limits of available valves and using a 340VDC buss things might top out at about 50 watts max. Actually, no reason you couldn't use a voltage doubler and a 680VDC buss and double the available output for a given valve. Of course, this isn't meant to be a practical solution to anything in this day and age, just an experiment to see what could be done.