[Synchrodyne]

A point of interest here is that the EF91 was not mentioned in any of the Philips books of the era about receiving valves. Book IIIA covered the radio Rimlocks, the B7G battery miniatures and the very early radio novals. The EF42 and UF42 were included, but not the EF91. Book IIIB covered the additional FM-AM era valves, mostly novals, including the ECH81 and EF85, but not the EF91. Book IIIC covered the initial range of TV valves, including the EF80, but not the EF91. I also have the impression that the EF91 was probably rare in European FM and TV equipment, and that the EF80, and before it, the E/UF42 used where high-slope VHF pentodes were required.

It would appear then that Philips saw the EF91 as belonging solely to its B7G industrial range, and so did not offer it for domestic receiving applications. Mullard obviously took a different tack with the EF91, although others in the Philips/Mullard B7G industrial range, such as the EAC91 and ECC91, stayed in their places and were seldom used in domestic equipment.

Possibly Mullard was influenced by the competitive landscape in the UK at the beginning of the miniature era, wherein Osram, Brimar and Cossor had all gone the B7G route, and all offered essentially the same high-slope pentode (Osram Z77, thought to be the original, Brimar 8D7 and Cossor 6AM6) for both TV and industrial applications. Thus Mullard felt it necessary to offer the EF91 as well as the EF42 for TV applications. The same was true of Mazda, who I think offered its Z77 clone, the 6F12, as well as its Rimlock 6F13 for TV applications.


Cheers,