[gary rabbitt]

Hi, the 5Y3 will run hot , just by the nature of it. Same as an incandescent bulb, that's about right. If you have red glowing plates though, you may have other troubles
I have not made any readings, but some of the mini tubes like the 35W4 and 50C5, to me at least, seem to run hotter.

The insulation doesn't have to be too thick, the idea is to dissapate the heat over a wider area, compared to just above the tube.