G1 short in a 21FBP22

[IsthmusTV]

Interesting. The short is only there when the heater is on. I watched the reading go from infinite to a low of about 600 Ohms as the tube warmed up. When I switch off the filament, the reading slowly rises back to infinity. It's obviously not a metal-on-metal dead short, but if it's a piece of debris between the cathode and G1 why doesn't it show when the tube is cold? I understand that the elements expand when they're hot, but would that cause the reading to change so gradually? I would think that it would suddenly go from infinity to the shorted reading when things expanded enough to make contact. I'm not sure what to make of this. Meanwhile, the cutoff and emission still test good.