The HL22 is a high voltage control triode with a maximum dissipation of 4 W. The data-sheet shows the use in a stabilised EHT supply where the controlled element is an HR9 grid controlled rectifier that acts as both series pass element and half wave rectifier.
Looking onto the grid one can see the mesh and bell structure. It is though a planar triode has been stretched to accommodate the high voltage and the long pinch speaks also of insulation rather than high frequency use. In fact the design is the very opposite of high frequency design principles.
Looking into the bell of the grid, the oxide coated cathode is seen as a white rod under the mesh. The other small white structure is a mystery.
The wide glass tube envelope is 29 mm in diameter, and excluding the IO base pins is 111 mm tall.
Reference: Data-sheet. Type HL22 was first introduced in 1938. See also1938 adverts.
