The 50B5 is a beam tetrode audio output valve designed for use in equipment with series heater chains. It is an American design and aimed at American mains voltages.
This miniature output valve was based on the octal based 50L6. These 1940's miniature output valves ran very hot in the small envelope. The more successful designs came with the slightly larger B9A based valves.
The output power is up to 1.9 Watts at a total distortion of 9%.
Looking along the grids shows the anode to be rectangular in section. As well as the etched Type information the glass is printed in red/orange with the RCA information.
Above the top mica the control grid supports are fitted with cooling fins. The cathode is a wide flat rectangle.
The thin glass tube envelope is 18 mm in diameter, and excluding the B7G base pins is 60 mm tall.
Reference: Datasheet. Type 50B5 was first introduced in 1946.