The 813 is a high voltage, low current beam tetrode with a thoriated tungsten filament. The 813 is one of the largest valves used in HF amateur radio. Classic amplifier designs from the late 1950s would squeeze 1,000 Watts PEP from a single device for SSB use. In class AB2 the input power for AM or CW would be 500 Watts.
Like many vintage valves the 813 has been used to build audio amplifiers. A single ended and triode connected 813 would run successfully from an HT of 800-1,000 Volts. An utterly lethal voltage.
The valve base would be mounted on spacers so that a blower could be fitted under the chassis and a stream of forced air cooling would pass over the envelope.
The substantial anode seen end-on. The anode is held between ceramic spacers.
Face view of the anode. The base shell has a locating pip. The base pins are UX7 but originally a substantial and special base would have been employed. This base would have a skirt with a gap for the locating pin to slide into.
The classic envelope is 63 mm in diameter, and excluding the UX7 base pins is 170 mm tall.
Reference: Data-sheet Type 813 was first introduced in 1954.