[Mondial]

The early 1920's UV-200 and UX-200 detector tubes had about 50 Microns of argon gas added which increased their sensitivity to weak signals.

Hard vacuum rectifiers like the UX-280 should have no gas included and any glow between the tube elements is not normal.

A lot depends on the intensity of the glow in the rectifier. If it is very dim, then you probably can continue using the tube on a limited basis. If the intensity increases to the point where it is bright and extends into the empty space surrounding the elements, then stop using the tube immediately as it may arc over. In any case, continued use of a gassy tube will eventually result in destruction of the cathode and loss of emission, due to ion bombardment of the active surface.