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Additional EF91 Information

R J Sutherland, November, 2013.
    
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It's often overlooked that CV specifications are not exclusively military; some are civilian, the qualification authority being the 'General Post Office' (as it then was). Its principal interest was in long-life and reliable valves, not least those used for repeater service 'down a hole' somewhere. One of the major UK suppliers to the UK telephone sector was STC, the British arm of Western Electric aka. Ma Bell, with its own label rather than via Brimar, its 'consumer' arm.

Starting from the CV4014, skip back to the CV4002, which is its wire-ended direct equivalent, presumably intended for use in severe environments where reliability of connection was as important as valve integrity. (And probably no coincidence that this variant takes precedence of place in the 4xxx series!) The early CV2xxx series is mainly a GPO cluster, and the very first of these, the CV2000, is described in its official data sheet as 'the CV4002 fitted with part no. 1/DPL/596', this evidently being the same kind of base wafer with spigot as used with the CV2001 except that in this case it simply supports the existing wires, whereas the base wafer of the CV2001 essentially acts as a socket/converter for a conventionally-pinned B7G valve. So the CV2001 could be deemed a 'half-way house', whereas the CV2000 is the 'real deal'!

But this is not the end of the trail of this versatile valve type. The CV2031 has the exact same wired/wafer termination as the CV2000, and its data sheet states 'The CV2031 is a special version of the CV2000 for use in Post Office CEL6A line amplifiers' This it seems refers to 'Coaxial Equipment, Line 6A', so one may reasonably surmise that this unusual base was specifically designed to fit GPO repeater equipment. The data sheet goes on to refer to the CV5377 for all electrical characteristics, so this is clearly the pinned direct equivalent. Its data sheet in turn states that it is a 'long-life version of the CV4014 with low cathode resistance'. And there the trail ends. I presume the latter property refers to the usual cutoff-tolerant cathode which doesn't deterioriate after periods spent in a quiescent state, as eg. with computer valves. No lifetime is explicitly stated as such, but the acceptance tests for this version extend to 3000h.

Quite a span. The EF91 in one form or another seems to have made a significant contribution virtually throughout the entire post-WW2 valve era!

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