[AMPS] Emission from Thoria Filaments

km1h@juno.com km1h@juno.com
Mon, 04 May 1998 19:53:35 EDT

On Thu, 30 Apr 98 00:15:50 -0800 Rich Measures <measures@vc.net> writes:

>As it was explained to me, when Econco Broadcast Services 
>rebuilds a thoriated-tungsten filament tube, No thorium is used to 
>restore emission -- no matter how many times the tube is rebuilt.  
>Acetylene gas (C^2 H^2) is the sole agent.  When tungsten/1.5%-thorium 
>alloy  is heated to c.1800 deg K in the presence of acetylene, carbon 
>atoms from the acetylene molecule bond with tungsten atoms to form a 
>tungsten-carbide layer at the surface, leaving hydrogen gas byproduct. 
>During recarburization, a continual input of acetylene brings in a new 
>supply of carbon atoms and forces out hydrogen.  
>-  - How does one determine how much tungsten carbide has been 
> .  Tungsten is a better electrical conductor than tungsten-carbide, 
>one can tell how thick the tungsten-carbide layer is by measuring the 
>decrease in filament current as the process takes place.  When the 
>layer's thickness is determined to be sufficient, acetylene flow is 
>stopped, the tube is pumped to a high vacuum and resealed.   With 
>control of fil. potential, emissive life is typically c.20k hours 

I had heard that the quality way to rebuild a tube included a new
filament and grid structure and was about 50-75% the cost of a new tube.
The less than reputable rebuilders used the acetylene method....and a few
charged the full rebuild price although others were honest and charged
Perhaps someone will explain the true story.

BTW, in case anyone did not see my last message I had to go away on a
family matter for a week. 
Hope I didn't miss anything of value.

73  Carl  KM1H

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