The 866 is was intended to be used as a high voltage high power rectifier. They are filled with a small amount of mercury in near vacuum. Other than the 2.5V at 5 amps, to heat up the filament, this circuit is outputting less than 40V. Its connected as a "relaxation oscillator", using a 220 ohm resistor and a 2200uf capacitor as the timing elements. Plate current is only 70mA.
WARNING: Emits UV light, and may cause eye injury if viewed too long ! That's the REAL item, not this video.