I read the link below; my comments come from discussions with the direct
tube engineers and application engineers with RCA, Eimac. E2V, and
Thales (Thomson). I was at Thales factory for a week last month, and had
very detailed discussions with tube designer about filament voltage
management for a particular tube.
From the link you provided, it appears that W8JI's advice is posted to
counter advice from another website (AG6K) and I would never want to
jump into the mire between these hams. It may be that the filament of
the 3-500Z must never be operated xxx percent below nominal due to
'poisoning' of emission. I cannot comment either way as I have not asked
these questions of the manufacturers, and honestly, the only Eimac
engineer who would have explained this in detail was Reid Brandon, who
is no longer at CPI. Because that web article is so lengthy and pointed,
it is hard to read if the information is from Care and Feeding, from
direct conversations, from here-say, or from long time experience with a
particular tube like 3-500Z.
With indirectly heated cathodes, it has always been known that proper
temperature is necessary for long life. But we are talking about
carburized thoriated tungsten filaments now.
Because the tubes I use for work are costly ($40,000 - $360,000 each
depending on the transmitter, freq, power), I am speaking from my own
experience running and protecting these tubes, and have been doing so
for 25 years for some of them.
One cannot imagine that some principles don't scale to smaller tubes. In
general, if you can operate with the filament at reduced voltage (once
you have operated at the nominal level for so many hours) below the
datasheet value, the tube will not suffer, but should have longer life.
If you go too far, usually much below nominal (10-20%), not a few
percent, you will eventually run into trouble. Take it all in stride and
be careful when adjusting filaments too high and too low.
73
John
K5PRO
Date: Tue, 22 Dec 2015 10:01:21 -0500
From: RMorris <robrk@nidhog.net>
To: "jtml@vla.com" <jtml@vla.com>
Cc: "amps@contesting.com" <amps@contesting.com>
Subject: Re: [Amps] Tube wear out phenomena
Some winter reading....
http://www.w8ji.com/qst_the_care_and_feeding_of_a_3-500zg_filament.htm
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