As the tube heats up the glass and metal pins expand and if they expand at different rates the seal between them will open up allowing air to enter the tube. Having the pins come through the glass was a real technological achievement and significantly simplified the manufacturing process.
A closer examination of the tube (see picture at the beginning of this article) reveals that there are actually two amplifier sections within the glass envelope – the 12AX7 is twin triode and each section has its own cathode, grid and anode. Only the anodes can be seen though because the anode shrouds the grid, cathode and heater, which are situated within it. The alphanumeric name printed on the side of glass envelope, ’12AX7′ is not just a model number, it contains quite a bit of information about the tube. The ’12′ indicates that the heater (the part that glows orange when the tube is powered up) requires 12 volts to operate; the ‘A’ indicates the tube is an amplifying component; the ‘X’ identifies the tube’s electrical characteristics, that is the gain factor, plate (anode) resistance, etc; and the ’7′ the number of active pins.
Early versions of the 12AX7 the heater could only operate in parallel filament circuits (tube guitar amps run the heaters in parallel anyway) however the 12AX7A operates in series or parallel filament circuits. Also, 12AX7s manufactured after the mid 1950’s feature improved construction with the helically wound heater invented by Sylvania by Gehrke, Huntington, and Granger. Their patent (US2677782-A 2,867,032) highlights longer tube life as major benefit of the invention, however the helical heater also minimises AC heater supply noise in the tube’s output.