I brought my trusty Sencore CR70 to the ARCI Radiofest earlier today and helped oldradio99 test the 15GP22.
At first we had good filament glow but no emissions or cutoff on any gun. After about 10 minutes the filament current drop dropped because one of the three filaments burned out. A short while later the draw dropped to 0 as all the filaments stopped drawing current. It seems that indeed it had gone gassy.
So that's a shame but at least it's in good condition and maybe can be rebuilt one day.
I stayed there until late afternoon chatting with fellow collectors (ElectronicM and Wiscojim mostly), and had a great time. Unfortunately though, I left my CR70 in Wiscojim's van

I'll try to swing by tomorrow and pick it up otherwise I'm sure I'll get it back eventually...