There is something special about the Mercury Vapour rectifier blueish glow, that not only set the mood, but somehow creates timeless, captivating atmosphere. 

Thomas Mayer latest 211 drives 211 mono blocks uses Mercury Vapour rectifiers. By default Mercury vapour tubes need to go through special procedure on the initial use and needs to be preheated in absence of high voltage for about 30 minutes.

Mercury vaporizes and condenses at the glass, then settles at the bottom. In this way its ensured, that mercury droplets between filament and plate, that could potentially creating a short are properly set. 

Then, after 6AU4 is heated, current starts to flow magical blue glow comes on. 

Article: link.