One military transmitter used QY4-250's (which are very similar to
4-250A's, but without the metal base ring) and thus not that different to
4-400's, but only blew from the top - a big (8inch diameter fan) blowing
straight down on the anode seals. This did lead to filament seal problems,
though, with the usual problem of the socket contact giving up.
I use that a method with a pair of QY4-250's, but with a muffin fan blowing
on each tube socket from below - i.e. two muffin fans. It seems to have
worked well for several years. It is also relatively quiet.
73
Peter G3RZP
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