For this months tube Thomas picked up a directly heated transmitting triode, from the RCA 800 series.
Thomas never did any project with this particular tube, nevertheless its very interesting tube, that can be used for audio applications.
Similar to many transmitting triodes 800 series uses thoriated tungsten filament in this case with 7.5V and 3.1A reflecting hefty requirements of the 211 or 845.
800 series were intended to be used with both RF and audio frequencies and sheet show possible Class AB power amplifier use. Its true, that 800 was designed with grid current operation in mind, but it can be used in Class A1 as well.
Link: here