Well something caused the inside of the tube to get very hot, which melted that yoke. It wasn't too much current in the yoke itself, as that would have blown a fuse, and/or melted the windings before the plastic. So it looks to have come from inside the tube. And there are only a certain number of things that could be:
- Internal arcing
- Too much voltage on the heater (from something outside of the tube driving it too hard)
- Internal short driving too much current where it shouldn't have gone
If you don't have a tube tester, you could tone out the heater and then tone out all other grids to each other and the heater, to see if there are any shorts between anything.
Beyond that, you could just power it up while watching the neck and keeping your finger on the power switch, to kill power immediately if something is wrong. This is how I bring up every monitor I rebuild, as there's usually a difference between killing power instantly, versus letting a broken monitor run until it nukes itself. You might still have damage, but you'll have a lot less if you kill the power as soon as you see that something isn't right.
