The 4CX250B is the classic RF Power tetrode valve for VHF and low UHF applications. The base is a special version of the B8F because it has to incorporate the forced air system, a matching ceramic chimney was made for the base, and the control grid was connected to the central metal spigot.
The external anode is surrounded by the large surface area of the folded metal that forms the heat transfer medium. The outer cylinder provides protection and an additional cooling surface.
The base pins and the narrow entrance for the air above the ceramic insulator.
The thin metal tube envelope is 35 mm in diameter and, excluding the B8F base pins, is 47 mm tall.
Reference: Data-sheet. Type 4CX250B was first introduced in 1960. See also1960 adverts.