What Is Tube Glow

From 104267.234–(at)–ompuServe.COM Tue May 14 13:11:56 CDT 1996
From: cynic <104267.234--(at)--ompuServe.COM>
Newsgroups: rec.audio.tubes
Subject: Re: Blue glow in output tubes
Date: 14 May 1996 06:44:27 GMT

I am amazed at the ignorance of so many of the posts here.

Blue glow is caused by one of two things:
a) fluorescence due to electrons striking an object. Often, stray
electrons will escape past the plate (esp. in cheaper Chinese
tubes) and strike the glass envelope, producing fluorescence.
This kind of glow is harmless. If you can look thru the slots in
the plate of a 6550 or EL34, you will probably see the inside
surface of the metal plate glowing a similar blue. This too is normal.
b) gas ionization. BAD. Many older tubes, such as type 50 triodes, have
soft vacuums and can sometimes show a bit of this. Cheaply-made
tubes (most Japanese and Chinese types, for example) were poorly
evacuated at the factory and can have gas glow inside their elements.
ANd occasionally, power tubes can develop a slight leak thru a glass-metal
pin seal, allowing a tiny bit of gas in. Ionization due to air
is nitrogen and oxygen, which gives a purple-pink glow. It is
distinctive and very different from fluorescence.

I know this because of years of experience, and years of doing
research on tubes in engineering libraries. There is an excellent
discussion of this in the 1939 Sylvania tube manual.
—

I HATE COMPUSERVE
(but they are cheaper than Netcom)