Looking at the dot pattern, I think that I am seeing a "hollow beam" that has less current in the center... perhaps the center of the cathode is "used up" or it may be ion damaged. I have seen cathodes where the coating in the center has been totally removed by ion bombardment, down to bare nickel.
If that is the case, It will not "wake up" with the application of 8 volts to the heater, but what have you got to loose by cooking it for a few hours?
jr