[Arran]

I can't tell from that picture whether the screen grid is glowing or not. A lot of directly heated output tubes tend to look like that but upon closer examination it's simply a reflection from the heater. If the screen is glowing then Ron's explanation makes sense if that's what's happening, I had the idea that it might be caused by powering up the radio without a speaker. Without the speaker field connected the power supply is almost running with no load aside from the power output stage, I can't remember whether the output transformer is speaker or chassis mounted on a model 70. If the output transformer is on the speaker then the power supply would be running flat out with no load, and the screen would be getting 400-500 volts or so, that will make it glow!
Best Regards
Arran