[AMPS] Filament breakage

[w8jitom@postoffice.worldnet.att.net]

> The problem is that the tubes which have the problem typically do not 
> show signs of abnornal gas.  Out of the 5, kaput 3-500Zs we tested 
> Monday, none appeared to have a gas problem.  However, all of them had 
> bent filament helices  If gas was the cause of the problem, where did the 
> gas vanish to? 
> Rich---

Very simple, the arc itself can getter the gas. That effect is very 
well documented in published literature on gettering tubes, arcing is 
listed as one of the ways a tube is gettered.

The tube's elements give off gasses as a normal part of their life, 
and many things work inside the tube to breakdown those gasses. 

A second cause of arcs is what manufacturers call "barnacles", 
stray sharp whiskers of metal that increase the voltage gradient 
and reduce breakdown voltage. They generally burn out during an arc.

Many 3CX3000's do this during initial turn on. When new 3CX3000 
amplifiers are turned on, almost 50% of the tubes flash over. One 
quick fault and they are OK for years.

Unless you have a very strange tube, the grid is "in the 
way" for an anode to cathode arc. Ground the grid properly, and the 
likelihood the filament is impacted is greatly reduced. Most 3-500Z 
arcs will harmlessly occur between the anode and the large metal 
shield forming the grid support.

73, Tom W8JI 

--
FAQ on WWW:               http://www.contesting.com/ampfaq.html
Submissions:              amps@contesting.com
Administrative requests:  amps-REQUEST@contesting.com
Problems:                 owner-amps@contesting.com