Back in the late 60's, there was an article in 73 magazine about ARC and the
Command series. I remember they said that the 12A6 was developed for it. I've a
feeling that they started with the receivers - the BC453 LF set first, if I
recall correctly, because it was originally used by the airlines for flying
the old 'radio range' system.
One slightly interesting point was that the VHF ARC5 used 717A doorknob tubes
on octal bases, rather than 954/955/956 acorns, while the IF used 6SH7 rather
than vari mu 6SG7, even though AGC was applied. I suspect the SCR522 receiver,
the BC624, was rather better.
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