Schematic circuit symbol of a triode vacuum tube.  This variant shows the plate (top), grid (center), and cathode (bottom), but the filament which heats the cathode, normally shown as a V shaped element under the cathode, is omitted.  It is a common simplification used in vacuum tube circuit diagrams to omit the filament circuit, or draw it separately in another part of the diagram, to reduce clutter.
Own work
Chetvorno