[turretslug]

There's probably a world history influence here as well- the predecessor EF50 with its large 9-pin base was a Dutch-origin valve, this 9-pin base enabled the development of the improved VHF performance EF54 with no less than 4 cathode connection pins- this feature may have been late in the day by comparison with the RCA 6SG7/6SH7 but was seized with gusto once apparent. Immediately post-war, UK valve procurement would have been dominated by US production/export/availability and the B7g base, apparently developed in the late '30s US for battery portable use, was then exploited for a new generation of compact high-performance VHF valves that were in high demand for military use. There would have been pressure to use this proven investment in B7g production lines, rather than start work on a base with more pins. Hence lots of 6AM6s everywhere in the late '40s, the desire for a separate suppressor connection limiting cathode connection to a single pin. Once post-war investment in valve production got under way, a B9a EF91 successor with duplicate cathode pins would be seen as an early good idea, even necessity.