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Re: [Amps] Soft Tubes?

To: W4EF@dellroy.com, amps@contesting.com
Subject: Re: [Amps] Soft Tubes?
From: TexasRF@aol.com
Date: Mon, 28 Mar 2005 20:30:28 EST
List-post: <mailto:amps@contesting.com>
 
In a message dated 3/28/2005 7:27:22 P.M. Central Standard Time,  
W4EF@dellroy.com writes:

Someone  asked me recently if the tubes in their Alpha 78 might
be going soft (they  were measuring poor efficiency on 10 meters).
My response was to inquire  about their measurement technique 
in order to rule out measurement error,  but the question got me to
thinking - are "soft" tubes a common problem in  amateur service? 
It is my understanding that cathode emission will  commonly fall 
off after a few years of 24/7 broadcast service, but this  shouldn't 
be an issue in amateur service since most of us use such low  duty 
cycles (a few hundred hours each year even with heavy  contest
use). So the question is are there other common "soft" failure  
modes that cause graceful degradation of tube output (as opposed 
to  hard failures)? I would be especially interested in hearing if such  
failure modes are relevant to the Eimac 8874's used in the Alpha 78.  

Thanks!

Mike,  W4EF.............................


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And this brings up the question: Why would a soft tube show up on 10m  
operation and not lower frequencies?  
 
73,
k5gw
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