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The VR91 is Valve Receiving number 91 as specified by the UK RAF. After 1941 the then new Common Valve system issued a standard specification for the valve across all government departments and armed services, the new name was the CV1091. However, inside it was still the Mullard EF50. The old stores code of 10E/92 is the usual one and to see the b suffix is unusual. We can find no reason for this suffix.

This amplifier pentode was a general workhorse. It was extensively used in radio and radar during WWII and appeared in great numbers in many stages of early post war television sets. The abundance was due to them having been made by the million - necessary due to a short operating life.

The pins are equi-spaced around the circumference of the pin circle and a locating spigot with key assures both the correct orientation in the socket and guides the valve vertically so the hard pins do not deflect and crack the glass envelope. The high operating frequency of the EF50 was in part achieved by the use of the hard glass foot with base pins directly inserted. The shorter lead length resulted in a smaller lead inductance.

The Crown and A and M for the Air Ministry.

References: Data-sheet & ¹⁰⁴⁰. Type VR91 was first introduced in 1938. See also 1938 adverts.

Pin Connections

Absolute Maximum Operating Conditions^¶

Thanks to Frank Philipse for supplying the above PDF datasheet.