The 5B/251M construction dates this valve at the commencement of the process that ended with the all glass miniature valves by the late 1940s. The pins are short and set into a glass disc base and are placed equi-spaced around the pin circle circumference. The seal is concealed within the spigot. The grids are made of thin wire on the usual notched rods. The cathode tube is flattened to an oval, the valve is a beam tetrode. The anode is brought out to the top cap.

In use other members of this series are power valves for audio or transmitter RF to the HF bands. The 5B/254M is a miniature version of the 807.

The wide glass tube envelope is 28 mm in diameter and, excluding the B8B base pins 85 mm tall.

This exhibit was last updated on 18 February 2007