[Amps] why a tube fails ?

[Jim Thomson]

Date: Wed, 28 Aug 2013 01:26:57 +0000
From: Charles Harpole <k4vud at hotmail.com>
To: "amps at contesting.com" <amps at contesting.com>
Subject: [Amps] why a tube fails ?

Some time ago I asked WHY DOES A TUBE FAIL?
Or What happens when a tube fails or becomes "soft"?

##  Dunno at the sub atomic level.  What kind of tube are you
talking about, the indirectly heated  8877.... or the directly heated 
3-500Z ?  

##  You can get a bit of info from some of the tube rebuilders... like
Econco, etc.   They can re-thoriate the tungsten fil with acetylene gas. 
The story I got was  tubes can be re-built almost indefinitely.  Well
the  Eimac tubes can be rebuilt, like the 3CX-3000A7  etc, which use
directly heated fils.  Most indirectly heated tubes cant be re-built. 
Svetlana + chinese directly heated tubes tubes cant be rebuilt. 

##  If using directly heated tubes, their useful life can be greatly extended
by reducing the fil V....AFTER 200 hrs.   The emission increases on a new or
rebuilt tube for the 1st 200 hrs, then hits a peak.   At that point,the fil V can be
reduced til the PO  just begins to drop off by a few watts...knee effect.
Then the fil V is increased by .1 volt.    Then you keep an eye on it after that .



Jim  VE7RF