[Milsurplus] [Boatanchors] "Blue Glow" mystery.

[J. Forster]

If the plate voltage drops on key-down, the accelerating voltage may not
be sufficient to excite the glow.

A few hundred eletron-volts are very, very soft x-rays.

-John

====================


>
> ----- 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
>
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