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 base with the pins set in ceramic. The central locating spigot is also the control grid connection. Note the connection of pin 1 to the metal ring, the pin is used at lower frequencies but high frequencies the g2 connection can be made to the ring and thus the valve can be used within a co-axial cavity.
The anode connects to the top cap, outer ring and radiator fins.
In normal use the special chimney directs a blast of air from beneath the chassis past g2 and out through the anode fins. Close inspection reveals that there are three rows of folded fins. The centre row is offset to maximise the air contact with the metal surface.
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.