Quote:
Originally Posted by Mike. Watterson
The USA 6AM6 (EF91) and 6AK6 (EF95) are probably the originals and seem to be Jan 1949 and 1942 (though registered in May 1943).
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I think as already stated, the 6AM6 was not the original of the EF91. Rather it was the American designation for the Z77 et al, dating from 1947, the registration having been done by Cossor. Once this was done, Brimar switched to the 6AM6 moniker, having previously used 8D3.
I am not aware that the 6AM6 was ever used in American equipment. The VHF pentode in the initial (1945) American miniature domestic receiving range was the 6AG5, drawn from the existing WWII range. Early updates were the 6BC5 (c.1949) and the 6CB6 (1950), still on the B7G base.
The early history of B7G valves is well-covered in these two RCA articles:
Battery Miniatures from RCA Review 194004.pdf Miniature Tubes in War and Peace from RCA Review 194706.pdf
I don’t know when the EF91 became obsolete, but as already noted, it was still nominated for new applications in the early 1960s, presumably in applications where the EF80 or other later valves offered no advantages, or perhaps even where the EF91 was directionally better. Neither had the B7G base become obsolete in British equipment by that time. For example, a look at the valve lines-up for Eddystone HF receivers that were new in the 1960s shows a B7G majority, B9A minority in each case. I am not sure that the available information really supports the notion that the EF91 would inevitably be replaced by a noval. In fact there was not a direct noval replacement. The EF91 and the EF80 certainly overlapped, but were enough different that they co-existed for most of the latter part of the valve era, and anyway, as already observed, the EF80 was conceived as a (mostly) improved derivative of the EF42. That the latter was of the Rimlock type meant that
it was inevitably replaced by a noval.
Cheers,