1 tube glowing blue?

I changed the output tubes in an older ARC VT-60 and had the same thing happen. I am curious as well. I had read online that whenever you replace the tubes in an amplifier you should automatically check and adjust your bias (which makes sense). This particular guy had posted the process for a VT-100 ... it was not a quick and simple process. It required the use of a multimeter and in fact, he advised using two meters and it also required interpreting a schematic. It's been a while since I have been down and dirty with the guts of a tube amplifier (25 years?) but the old electronic technician in me read further. The output tubes were fairly simple to adjust but the input tubes required a fair bit of finess. I decided to call in the big dogs and have my local guy do it but I am going to watch and photo the process so the next time I can do it.

In the mean time I have continued to run the amp just to see what happens ... call it morbid curiosity. The amp seems to have mellowed out and I don’t see any more blue glow. I figure at worst I will replace the tubes with a new set which I have already acquired. The sound is fine ... in fact it's very nice with the new tubes in. If had heard anything that scared me I would have taken it in. I bought the little VT-60 just to learn a little before I dive into the more expensive units.

My logic tells me - if the bias was to high on the input tubes they would over drive and I would know it. If they were to high on the output tubes (where I noticed the blue light) it may be more difficult to spot so I have kept the volume down until I can get it in to be checked.

Any advise out there would be appreciated ... books, websites, and technical sources? remember I am a newbie and not an engineer.

Should be OK. Results from certain gasses appearing in the tube (i.e., not complete vacuum)

It's normal, I called SED a while back, they said not to worry.

I have two quads of SED KT-88's, they all have a blue glow, plus they sound great.

Pawlowski6132 is correct,

Red bad. blue ok

There is a good explanation of the blue glow in a FAQ on the Audioasylum Forum. I recommend you read it. Blue glow is good, and pretty as well!

I like Blue Glow.....It looks very beautiful when the lights off. I wish all my tubes glowing Blue :-) !!!!!

Some of the Blue Glow that I have in my amp. are pulseing with the music !

More specifically:

"The glow is caused by the electrons striking the plate, volatizing the surface, and dislodging atoms of nickel. High speed barium-strontium electrons from the filament collide with the atom, tearing off electrons, ionizing them and causing a deep blue color even in a well evacuated tube. The glow is a function of the plate voltage and the number of gas molecules in the envelope. In a good vacuum there are still billions of molecules. The electrons do not cause trouble because they all return to the positively charged plate. The size of the glow also varies with plate voltage and is not the same for each tube: it can vary from not discernable to very obvious even in daylight."

Pawlowski, I couldn't have said it better !

Well, I knew that's what everyone was thinking. I just wanted to throw it out there.

;^)

Thanks for the info. I can now listen without waiting for a POP!

Cool ... I mean ... Hot ...

I did some reading as well, I ended up eating up the better part of a day studying to process of tube amplification. Tubes are facinating to say the least. The use a various materals having variuos effects. I actually ended up on several different sites.

Here is one quick and dirty explanation ... Pawlowski hit it on the head.

http://www.newtube.com/content/aboutus.asp