The 813 beam tetrode
The 813 is a beam tetrode, a type of thermionic valve, also known (mainly in the US) as a vacuum tube or electron tube. This wonderful old valve has a graphite anode, or plate, rated at something like 125 watts RMS dissipation.
The diagram above was shamelessly stolen from https://faculty.frostburg.edu/phys/latta/ee/wing813amp/813.html, the photo of the 813 was acquired years ago; source unknown, but both images will be replaced very soon.
This page complies with the W3C XHTML and CSS standards: Valid XHTML 1.0 Strict Valid CSS