I remember, a few years back, someone had asked if there was such a thing as a 12V filament version of the 6X5GT, well, it turns out that there is,
though these may be difficult to find. 5838 has a 12V, 600 mA filament, 5839 has 26.5V, 255 mA filament, and the 5852 has a 6.3V, 1.2A filament.
I'd suspect that these tubes would also work in a 6AX5GA's capabilities.
http://frank.pocnet.net/sheets/131/5/5839.pdf
Like all of the Bendix Red Bank tubes, these were developed for extreme military and industrial uses,
including being launched on an early version of a cruise missile.
And, if you ever have a need to operate them at 80,000 feet altitude, these are the ones for you!
/ed B