Commercial practice (I mean commercial transmitters, not commercially
manufactured for amateurs) is often peculiar in this regard, depsite expenditure
on thermal crayons and so on. My experience with tubes of 3-400, 3-500, 4-250,
4-400 ilk (they're all the same basic base and filament structure) is that
blowing the filament seals directly with a small fan pays dividends.
I believe Rich has also said that some force movement of air around the tank
coil has advantages, too - which makes sense if you do a few sums.
I'm away for a few days - at Dayton - so I won't be seeing any e-mails
73
Peter G3RZP
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