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Weird Blue Line on an EL84

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Has anyone out there seen a blue line on the *glass* of an EL84 like this?

It comes on when the tubes power up. In person, it appears to be on the surface of the glass. My other (identical/matched) tube does not have this.

I've heard about tubes that have a blue glow, but this is something different. It's like a thin blue line. Tube seems to work ok though, but wondering if it will fail prematurely and whether I should go back to seller for a replacement.
 

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