Just noticed your info on web site re checking out a used(?) amp. http://www.guitarsite.com/guitar_FAQ2.htm#AMPBUY I quote your info below. Two very important pieces of information that are misleading (and need clarification) regards tube appearances when operating, are as follow:

1) BLUE GLOW- blue glow is NOT normal when it is within the stucture of the elements. Blue glow within the elements is gas due to vacuum loss and/or element contamination, which is definitely NOT OK-under any circumstances.

Blue glow on the inside glass surface is OK. This is fluorescence.

2) Orange glow is a matter of degree and does NOT NECESSARILY indicate a "dangerously under-biased" condition. A lot of tubes are designed to have lightly glowing plates when operating close to max ratings. A positive statement as to whether or not an "orange glow" is always to be avoided can NOT be made. Only experience and knowledge of tube types and operating conditions can help make that determination. A BRIGHT orange glow surely is a bad thing, indicating POSSIBLE under-bias, but ALSO could be a load problem and NOT have an under-bias condition.

Just thought you might like to know that I am not just spouting off to run you down. Definitely hope you take this as intended, as it NOT critcism. And for background, I spent 44 years with RCA and GE as a design engineer, all of which were either directly or application-wise related to vacuum tubes-from sub-miniature audio tubes to 1 megawatt klystrons. I also spent 10 years on the national committee regulating tube standardization (JEDEC).

I have decided to see what the web has to offer on the subject. I am attaching two published articles from excellent sources, labelled A) and B), but first some elaboration on my original comments: (you can then draw your own conclusions)

My observation of the web "experts" on this subject seem to fall into three classes: 1) Real experts 2) Self-proclaimed "experts" 3) Self-proclaimed "experts" which copy from others, usually with no credits.

The long and short of it (by my judgment), no matter what anyone says, blue glow within the interelectrode stucture of a non-gas filled receiving/audio tube should be interpreted as a tube needing replacing. Granted, the amount of glow is a relative indicator of "how bad is bad", but I would not consider it normal.

There are "experts" on the web proclaiming that a tube with blue glow within the elements indicates a really "tight" tube (high vacuum). I suppose this is a case of a little knowledge being dangerous, for just the OPPOSITE is true. A tight tube has no gas to ionize in the first place.

A glow on the GLASS, or for example, on a mica spacer might very well be construed as being a healthy tube (vacuum-wise). Also a tube with a VERY slight amount of inter-electrode glow might work just fine-for a while at least.

A) Blue Glow -- WHAT CAUSES IT? (by Svetlana)

Glass tubes have visible glow inside them. Most audio types use oxide-coated cathodes, which glow a cheery warm orange color. And thoriated-filament tubes, such as the SV811 and SV572 triodes, show both a white-hot glow from their filaments and (in some amplifiers) a slight orange glow from their plates. 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 Svetlana glass power tubes show FLUORESCENCE GLOW. This is a very deep blue color. It can appear wherever the electrons from the cathode can strike a solid object. It is caused by minor impurities, such as cobalt, in the object. The fast-moving electrons strike the impurity molecules, excite them, and produce photons of light of a characteristic 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. If you are unsure if these special tubes are used in your amplifier, consult with an experienced technican before replacing them.
ALSO NOTE: these light sources cannot be seen in metal-ceramic tubes, because their parts are opaque. As we said above, it is difficult to tell if a ceramic tube has become gassy. Usually, in a large radio transmitter, a gassy tube will arc over internally. (This does not damage the transmitter. It has protective circuits.) The equipment operating manual should give more information on this.

B) "RADIOTRONICS" Technical Bulletin No. 39 - 24 February 1936
Radiotronics was published by the Amalgamated Wireless Valve Co. of Australia using technologies derived from RCA USA, Marconi, Telefunken and others.

FLUORESCENCE and BLUE GLOW
With modern valves a blue colour is frequently observed (either steady or flickering) on portions of the bulb wall; this is a phenomenum known as "fluorescence" and is somewhat akin to X-ray fluorescence and is due to electron bombardment of the glass. The colour of this glow and its intensity depend on the nature of the glass, the voltages employed, and the design of the valve.
It has no deleterious effect, however, and actually is a sign of an extremely high vacuum. It is particularly prominent with type 42, but may also be observed on other types, such as 6A7, 6C6, 6D6, but since in these types a black coating is used inside the bulb the fluorescence is rarley noticed.
Blue glow is a glow between electrodes caused by ionised gas. It is never observed in a really hard valve, but there are certain types in which a very slight glow may be observed which, although indicating a very small amount of gas, is not deleterious to the valve; for example this sometimes applies to type 50. When any appreciable glow is observed inside the plate, the valve is definitely defective, due to gas, and a negative grid current will be observed if a micro-ammeter is placed in the grid return circuit. A valve reading less than 1 micro-ampere for each 10 milliamperes of anode current is quite satisfactory, and even two to three times this amount of negative grid current is usually quite permissible. When a valve becomes very gassy, the blue glow frequently extends in the shape of streamers radiating from the ends of the anode, and the valve in this condition is completely unusable. In high vacuum rectifiers used at voltages not exceeding 400 volts, a small amount of blue glow is not detrimental and may be rather beneficial. High voltage rectifiers, on the other hand, must have an absolute minimum of gas. Occasionally fluorescence may be seen on the inside surface of the anode or grid; this fluorescence may be easily distinguished from blue glow by the fact that it is a thin film and does not spread through the space between the electrodes.
All types of fluorescence are completely harmless."

info submitted by Alan, a GNW reader

The original article in question is below, and on GuitarSite at: http://www.guitarsite.com/guitar_FAQ2.htm#AMPBUY

WHAT TO LOOK FOR WHEN BUYING AN AMP

First, obviously, check the speaker. Give the cone a gentle poke to make sure it's firm and not deteriorating. Turn the amp on and check to make sure there's no orange plate glow on the power tubes (blue glow is OK). Don't confuse this with the normal orange glow of the tube's heater filament. Orange glow indicates that the tubes are dangerously underbiased--this isn't necessarily a failure, but would have to be addressed right away, as it might burn a tube or transformer quickly.

Pull all the tubes and look at the tube sockets to see if any of them look cracked or burned. You probably won't be able to do this, but it won't hurt to ask the seller if he'll pull the chassis and let you have a look around inside. The most obvious thing to check for is burned or cracked resistors. Again, this wouldn't necessarily indicate a problem with the amp, but rather that the components are simply worn out.

All about Amps... http://www.guitarsite.com/amps.htm