Here are a few references I have bookmarked in my electron tube folder...
Black heat is normally ruinous to tubes - ymmv -
I ruined a really nice 3CX1200 when the carefully adjusted filament voltage
right at the lower limit of mfr's rating got further decreased by a sneaky
power company trick of dropping their voltage a few percent during the post
11PM to 5AM low usage time period - when I was transmitting at full-bore,
war-emergency power on topband...
Twere me, I would lower the filament voltage at rest by 3% which gives a huge
increase in filament life without undue risk of poisoning the oxide, and then
pop it up to full rating for a short period, perhaps 2 seconds, before the 1
second operation...
http://www.cpii.com/docs/related/22/AB18.pdf
http://www.w8ji.com/vacuum_tubes_and_vaccum_tube_failures.htm
http://www.archive.org/stream/VacuumTubeDesign/Rca1940VacuumTubeDesign_djvu.txt
denny / k8do
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