[Boatanchors] Question about testing tubes

[Richard Knoppow]

----- Original Message ----- 
From: "Randy and Sherry Guttery" <comcents at bellsouth.net>
To: "Richard Knoppow" <1oldlens1 at ix.netcom.com>
Cc: <boatanchors at mailman.qth.net>; "Rick Poole" 
<wa1rkt at arrl.net>
Sent: Sunday, January 29, 2012 8:52 PM
Subject: Re: [Boatanchors] Question about testing tubes


On 1/29/2012 3:52 PM, Richard Knoppow wrote:

Then again - if the tube is below minimums - I've been known 
to "blast" the cathode - and often salvage a lot of good use 
out of a tube doing so. I've also been known to destroy 
tubes in the process as well - so like all things - YMMV.


best regards...

-- 
randy guttery

     Lots of snipping because I want to address this last 
only.
     The service life of some types of cathodes can be 
extended by cooking the filament at higher than normal 
voltage for a time. I think this applies exclusively to 
thoriated tungsten type filamentary cathodes. The tungsten 
wire has some thorium embedded or aloyed with it. As the 
filament is used the thorium at the surface, which emmits 
most of the electrons, wears away. By running the filament 
at a higher than normal temperature for a time without other 
voltages being present, some of the embedded thorium boils 
to the surface and increases the emmission. Instructions are 
given in a number of places including, I think, Eimac 
literature. Of course this has no effect on plain tungsten 
filaments. Most receiving tubes, use indirect cathodes where 
the filament is only a heater for a sleeve which is coated 
with an emmitting substance. Sometimes the emmission can be 
increased by overheating the cathode but this can also cause 
other problems like outgassing.
     Thoriated tungsten filaments are found mostly in small 
and medium sized transmitting tubes including most of the 
types used in amateur transmitters. Plain tungsten filaments 
are found mostly in older large transmitting tubes because 
they withstand very high plate voltage. Indirect heaters are 
found in receiver tubes because they have enough mass to 
give steady emmission with AC current.


--
Richard Knoppow
Los Angeles
WB6KBL
dickburk at ix.netcom.com