Unusual halo from electron tube

Dear knowledgeable radio people,

At present I am restoring an old cinema amplifier of which I have not been able to find any information at all (Strong Ljudsystem, Amerikansk ljudteknik AB, STOCKHOLM, Modell ND20). I have managed to get it to work and produce good sound. However, a strange phenomen has arisen: One of the two tubes (6V6) at the power stage gives a halo of an unusual coulor: blue, not red.

Has anyone got an explanation for this?

Yours sincerely/Stefan Karlson

Attachments:

• 6V6 in Strong amplifier (82 KB)

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Dear Stefan,

as you know, power tubes are operated with a high voltage; in the case of power amplifiers there might be several hundred volts. The 6V6 is specified for plate voltages up to 315 Volts.

When electrons from the cathode, accelerated by a plate voltage like that, hit the glass envelope of the tube, they can cause fluorescence. I´ve seen this blue fluorescence on amplifiers using the EL34 as well as on RF transmitting tubes. If it is fluorescence, there´s nothing to worry about.

There might be something else in it, though: If both tubes are of equal type, and are equally loaded, both should show the same degree of fluorescence - for the moment, I suppose your amp is push-pull. If under push-pull conditions olny one tube has a halo, try swapping the tubes. If the halo stays with the particular tube, check the tube for residual gas (some better tube testers have a vacuum check). If the vacuum is going bad, the halo could be a glow discharge which will in time destroy the cathode of your tube, as highly accelerated gas ions (positive charge!) hit the active surface. Then there is no way but to exchange the tube.

If the glow is not tied to the particular tube, but also appears on a different tube in the same tube socket, there might be imbalance in the operating conditions of the two tubes, and the fault is in the circuit.

Yours,

Nikolaus Löwe

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Hello Stefan,
 

if you make a Google-Search about "blue glow in electron tubes", you will find many explanations, only to show some:

 

http://www.jacmusic.com/html/articles/blueglow/blueglow.htm

 

http://www.triodeel.com/tubefaq.html#blueglow

 

http://www.webace.com.au/~electron/tubes/blueglo.htm

 

Best Regards,

Jacob

 

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Dear Nicolaus and Jacob,

 I thank you both for your enlighting (!) answers. Time will tell what will happen. I just hope the blue glow will stay on and the amp still produce good sound, since I feel the glow adds to the experience.

Yours sincerely

Stefan

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Hello Stefan,

by my experience this blue glow lasts for a long time, probably as long as the tube lives on.

It makes fun, to watch the blue glow, since it varies with the output of the tube. In some cases it behaves as similar as a magic eye tube.

Best Regards, Jacob

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