Date: Wed, 28 Aug 2013 01:26:57 +0000
From: Charles Harpole <k4vud@hotmail.com>
To: "amps@contesting.com" <amps@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
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