why does only one 300b tube glow Blue at the top?

Greetings all,
I'm brand new to tubes and just got a pair of 300b SET monoblocks. For my 300b tubes (ONE in each monoblock), I've got what is supposed to be NEW electro-harmonix tubes. When the amps play, all seems and sounds OK.
But ONE of the tubes glows BLUE at the top of the tube. The other one, on the other amp, is not doing this.
Could any of you enlighten me and tell me why this might be happening?
Ever seen it before?
Thanks and happy listening.
I also have a 300B amp and am on my second set of power tubes. All four burned blue. You might try www.upscaleaudio.com for advice and they are quite knowledgeable about tubes.
Thanks Clueless,
BTW, how did you put that link (blue glow) in your response?
I don't know about the present but in the "Old days" when a tube had a blue glow it meant that it contained gas and the gas was excited so it glowed. It usually meant the tube was bad based on the "Gas test." You use to be able to go to about any drug store and test tubes for shorts, gas. etc. Of course times have changed. Tubes were not suppose to have any gas in them.