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[Amps] Tube wear out phenomena

Subject: [Amps] Tube wear out phenomena
From: John Lyles <>
Date: Fri, 25 Dec 2015 15:40:01 -0700
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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.


Date: Tue, 22 Dec 2015 10:01:21 -0500
From: RMorris <>
To: "" <>
Cc: "" <>
Subject: Re: [Amps] Tube wear out phenomena

Some winter reading....

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