Philips EF91 high gain, high impedance, high slope, screened pentode
The EF91 is a high gain, high impedance, high slope, screened pentode. Mullard describes the EF91 as primarily intended for use as RF amplifiers or mixer valves in television receivers.
The EF91 was almost the universal valve of the early 1950s, as can be seen by the number of manufacturers who made versions of this standard valve. The first reference we have to the EF91 is the review of the 1947 Olympia Show in Wireless World. This envelope has a more rounded shape than EF91. At the time, in the late 1940s, television only operated on VHF. If run hard as in video amplifier service or as a TV sound output valve, the EF91 would give problems but for RF or IF use it was a reliable valve. It could be used from AF to VHF up to about 200 MHz and was found in large numbers in most types of electronic equipment. In radio, it was used for RF and IF amplification, oscillators, and frequency multipliers. The thin glass tube envelope is 17 mm in diameter and, excluding the B7G base pins, is 46 mm tall.