Here is a bunch of information about the 6BN6 FM limiter/detector tube.
data sheet: http://tdsl.duncanamps.com/link.php?target=282562A7
design and operation part 1: http://www.r-type.org/pdfs/6bn6a.pdf
design and operation part 2: http://www.r-type.org/pdfs/6bn6b.pdf
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Then replace nospam with fastmail. Lastly, replace com with us.