[Amps] Tube wear out phenomena

[John Lyles]

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 at nidhog.net>
> To: "jtml at vla.com" <jtml at vla.com>
> Cc: "amps at contesting.com" <amps at 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
>