It is inherent to the valve. What I read is it is due to flourescence - electrons bombarding the glass seen in power tubes.
A tube that is glowing blue is often wrongly perceived as a defect, however, it’s really just a side effect of a power tube — a fluorescent glow in the blue spectrum. The tube is fine! It actually indicates that the vacuum inside the tube is very good, which is what allows this phenomenon to […]
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It was a slow speed photo with a wide aperture lens. Camera has to be on tripod.
If you have a valve amp you can also do it.
Some amp makers perhaps fit LED under the bases of valves to give extra "glow" to their amps.