Ive been asked this many many times and people think that it means their tubes are needing to be replaced. Not always true and so I thought i'd talk about it here.
Glass tubes like the EL34 , 6L6 (5881) and 6550 have visible glow inside them. You will usually see the glow as a cheery warm orange color and this is from the heater filament. In some amplifiers a slight orange glow from their plates is visible. The Plate is the outer metal casing inside the glass tube. All of these are normal effects. Some newcomers to the tube audio world have also noticed that some of their tubes emit a bluish-colored glow. There are TWO causes for this glow in audio power tubes; one of them is normal and harmless, the other occurs only in a bad audio tube.
#1 Most glass power tubes show a fluorescence glow inside them. This is a very deep blue color. This is usually observed on the interior of the plate, on the surface of the mica spacers, or on the inside of the glass envelope. THIS GLOW IS HARMLESS. It is normal and does not indicate a tube failure. Enjoy it. Many people feel it improves the appearance of the tube while in operation.
#2 Occasionally a tube will develop a small leak. When air gets into the tube, AND when the high plate voltage is applied, the air molecules can ionize. The glow of ionized air is quite different from the fluorescence glow above--ionized air is a strong purple color, almost pink. This color usually appears INSIDE the plate of the tube (though not always). It does not cling to surfaces, like fluorescence, but appears in the spaces BETWEEN elements. A tube showing this glow should be replaced right away, since the gas can cause the plate current to run away and (possibly) damage the amplifier.
PLEASE NOTE: some older hi-fi and guitar amplifiers, and a very few modern amplifiers, use special tubes that DEPEND on ionized gas for their normal operation.
-Some amps use mercury vapor rectifiers, such as types 83, 816, 866 or 872. These tubes glow a strong blue-purple color in normal use. They turn AC power into DC to run the other tubes.
-And occasionally, vintage and modern amplifiers use gas-discharge regulator tubes, such as types 0A2, 0B2, 0C2, 0A3, 0B3, 0C3 or 0D3.
These tubes rely on ionized gas to control a voltage tightly, and normally glow either blue-purple or pink when in normal operation. Please consult with an experienced technican before replacing them.
Have fun --
Edited by proaudio - Jul 29 2008 at 12:45am