From: jimt@mullian.ee.mu.oz.au (Jim Trivellas)
Subject: Re: Blue Tubes (FLUORESCENCE or IONIZATION?)
Organization: Electrical and Electronic - University of Melbourne 
Date: Thu, 17 Dec 1992 17:38:51 GMT


On the subject of blue tubes and biasing, I'm surprised that no-one seems
to have consulted the tube circuit "bible", the *RCA Radiotron Handbook*,
(should be out of print, but any engineering library should have one).

Firstly I'd like to point out that incorrect bias settings can only make a 
tube glow red.  When the bias isn't negative enough, then too much DC plate 
current can flow, making the anode (plate) glow red.  Sometimes this can
occur when a power tube is dying, although the bias might still be OK.  In this
case you are observing a space-charge short and this is commonly known as 
"The Red-Plate Special".  If you observe this and the bias is within the
acceptable range for these power tubes, then buy some new tubes immediately. 

There is no way that incorrect bias (within normal operating voltages) can
make a tube glow blue. 

According to "The Book":

******************************************************************************

The vacuum in tubes is produced during manufacture by a combination of vacuum
pumps and is made permanent by the flashing of a small amount of "getter"
material (the silvery deposits inside the glass envelope) which remains in the
bulb ready to combine with any impurities.

Valves coming out of the production line are tested for gas by measuring the
negative grid current under operating conditions.  If a valve has been on the
shelf for too long, then it frequently shows a higher gas current, but this
may usually be reduced to normal by operating the valve under normal conditions
, with a low resistance connection between grid and cathode, for a short period.

When a valve is slightly gassy, it usually shows a blue glow (ionization) 
between cathode and anode.  In EXTREME cases this may extend outside the ends
of the electrodes, but a valve in such condition should be regarded with
suspicion and tested before being used in any equipment, as it might do
serious damage.  A slight crack may permit a small amount of air to enter the
valve, giving rise to a pink/violet glow; this is a sign of immediate end of
valve-life.

Some valves show a FLUORESCENCE on the inside of the bulb, which may fluctuate
when the valve is operating.  This is perfectly HARMLESS and may be 
distinguished from blue glow by its position in the valve.  In occasional 
cases fluorescence may also be observed on the surfaces of the mica supports
inside the valve.

******************************************************************************


Now, I wonder if all this time most people have been observing Fluorescence,
as a result of some high energy electrons getting very close to the anode, 
but somehow managing not to be absorbed, and finding their way to the glass 
bulb? 

The only bluish glow that I have observed (with American and Chinese 6L6s,
and American EL-34s, never with Sovtek EL-84s) was definitely fluorescence.

Cheers,
Jim.

-------------------------------------------------------------------------------
  Jim Trivellas.
  Department of Electrical & Electronic Engineering.
  School of Information Technology & Electrical Engineering.
  The University of Melbourne, Parkville 3052, Victoria, Australia.
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