[Boatanchors] "Blue Glow" mystery.

[Richard Knoppow]

----- Original Message ----- 
From: "David Stinson" <arc5 at ix.netcom.com>
To: <milsurplus at mailman.qth.net>;
<boatanchors at mailman.qth.net>
Sent: Monday, April 21, 2014 7:57 AM
Subject: [Boatanchors] "Blue Glow" mystery.


> Here's a mystery for you.
>
> A VT-25 in a Power Amplifier stage.
> Key-up idle.
> Ep = 500V.
> Eg = -110V (cut-off)
> Cathodes at ground.
> No plate current, of course.
>
> Yet.... on the glass envelope is a small blue glow!
> And it disappears at key-down...
>
> Ghosts?  Goblins??

    The dark blue glow that seems to be right at the glass
is caused by x-rays from the tube.  Its commonly seen in
tubes like audio output tubes with fairly high voltage on
the plates. Some older tubes had a black coating on the
inside of the glass to stop it.  It does no harm.  I am not
sure why it disappears when the tube is drawing current.  I
think a web search may find an explanation of the physics.
     Note that this is a different kind of glow than is 
caused by gas in the tube. That usually takes the form of a 
cloud between the elements or throughout the tube and is 
usually a lighter color. This is a dark blue and is actually 
the glass itself glowing.

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