[ARC5] Strange problem with racked transmitters

[Scott Johnson]

The blue glow (outside the elements) doesn't necessarily equate to a lower
pressure,( although it usually occurs at higher Va, which requires a tube
with a harder vacuum), and is caused by the electrons hitting the glass,
dislodging Si atoms and ionizing them.  The reduction of the SiO2 in the
glass is what causes the "browning" of the glass in a tube that has been
"ridden hard" (Often seen in guitar amp output tubes and transmitting tubes,
especially 4-400s in the modulator. The brown color is elemental silicon.
The oxygen that is liberated by this process is promptly gettered.  If this
process goes on too long, the getter will be consumed, and the tube will
begin its slide into oblivion.  This is probably best exemplified by an old
brown, worn out horizontal output tube from a large color TV set.

Scott W7SVJ

-----Original Message-----
From: ARC5 [mailto:arc5-bounces at mailman.qth.net] On Behalf Of Kenneth G.
Gordon
Sent: Sunday, June 28, 2015 6:43 PM
To: ARC5
Subject: Re: [ARC5] Strange problem with racked transmitters

On 28 Jun 2015 at 16:17, Dennis DuVall wrote:

> The blue glow you are  seeing is probably due to electron bombardment 
> of the glass tube envelope(s), not gas, and is harmless.

The difference between the two types of "blue glow" is quite obvious once
you have experienced both: generally, if the blue glow is INSIDE the tube,
between the plate element and the filament, then it is gas.

If the blue glow is OUTSIDE the plate, and near or up against the glass, it
is what Dennis described, and this usually indicates an unusually good tube
with a very hard vacuum.

Ken W7EKB
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