Hardly any tubes are still manufactured. Most off-shore, where we sold the tube assembly lines, or they had their own.
Only metal tubes anymore are the ignitron types etc, some transmitting tubes etc.
Usually the glass version would retrofit the metal, and had better power capability.
But some may not have made it into glass, like the 6H6, I don't recall ever seeing a glass one. Meatl were sometimes considered "industrial" types.
example, the 6L6 (metal) beam power pentode vs 6L6G or 6l6GC, the power capability went up, but you can normally sub them. Once in a while the glass will not fit, or the metal was counted on as a shield (circuit may oscillate with unshielded glass).
I don't think I ever ran into an electronic performance reason for that tube not to work as a glass substitution.
