8020 high-voltage rectifier
Capable of handling voltages of up to 40 kV, this tube is a real high-voltage rectifier. And it can handle considerable currents, too: up to 100 mA, peak current being much higher - about 1 A (the resulting peak power being 40,000 Watts!). Needless to say, that at these extreme voltages, it needs a perfect high vacuum for sufficient internal insulation without the slightest traces of gas. So, the cathode is a simple direct heated pure tungsten filament rated at 5 V, 6.5 A. The plate is made of a hard-metal with external high-temperature getter coating (the grey center region), like in transmitting tubes.

Some cooling of the top anode cap should be recommended, since this tube has a quite high voltage drop of a few hundred Volts. And with maximum current, a considerable plate dissipation will be released - up to 100 Watts.

All pure-tungsten filaments should be heated only as much as is needed to reach sufficient emission (contrary to all other cathode types, especially thoriated ones, which are the most sensitive to over- and underheating). Even reduction of heater voltage by a few percent will increase tube (filament) life two- or threefold.
 
As with all tubes operating at voltages above 10-15 kV, considerable amounts of X-radiation might be emitted - in rectifier tubes short pulses just at the moments as the tube passes from conducting into cutoff state and the high reverse tension starts to build up between cathode and anode, and during cutoff state at maximum voltage due to field emission effects.

So if you're experimenting with such devices - be careful and take appropriate shielding measures! Needless to say, that these high tensions are absolutely lethal.