[R-390] Tubes glow blue

[mikea]

On Mon, Sep 19, 2005 at 10:06:56AM -0400, Roy Morgan wrote:
> At 11:24 AM 9/17/2005, Scott Bauer wrote:
> >.. If a tube is glowing blue ( besides OA2)
> >it should be replaced. ...
> 
> Baloney.
> 
> Beware, beware!  There are two (three, actually, or more) kinds of blue or 
> other color of glow in tubes:
> 
> 1) The thing is normal and operating properly. It is likely a beam power 
> tube and the electrons that don't get glommed by the plate whizz on by and 
> hit the inside of the envelope. The *glass* glows.  This condition is 
> recognizable because the glow is ON the INSIDE of the glass envelope.
> 
> This is not harmful or indicative of any fault or weakness in the tube.
> 
> 2) The tube has some gas inside (when it should not have much at all).  The 
> electrons whizzing toward the plate hit the gas particles and the gas 
> glows. This condition is recognizable because the glow in AMONG the tube 
> elements, NOT on the inside surface of the glass.
> 
> This IS indicative of a fault or weakness in the tube. However, if you run 
> the tube for a while, the heat *may* make the getter material glomm onto 
> the gas, reducing or eliminating the gas and the glow goes away.
> 
> Certain relatively new tubes (such as made by the Viac company for 
> audiophiles) have special getters attached to the plate, which plates run 
> so hot as to heat up the getter device and activate it further to glomm 
> more gas.  These unusual getter devices may be washer- or ring- shaped 
> things that are seldom seen in old tubes.

Roy's otherwise-excellent post unaccountably omits mention of one 
particularly-to-be-avoided form of glow in tubes.

When a metal 6L6 starts glowing visibly, then you have a serious 
problem and need to investigate it *IMMEDIATELY*. That's especially 
true if it's glowing blue. I've never seen one get past glowing red,
and then only for a fairly short time. 

;=), wth tongue firmly in cheek, in case you hadn't noticed.

He's right about beam power tubes, too: a properly-operating beam 
power tube can show a really pretty blue glow just inside, or even in,
the glass. I don't know how fast the e- move in a 6L6, and hadn't 
thought it would be fast enough to stimulate Cerenkov radiation, but
it sure is the right color. 

-- 
Mike Andrews, W5EGO
mikea at mikea.ath.cx
Tired old sysadmin