You should never run a beam power tube - or a pentode - with screen volts and no plate volts. That over dissipates the screen grid.
That is very, very true.
Kenwood has a screen on/off switch, and besides removing the screen voltage, the switch applies about -100vdc to the screen. You can pick off the -100vdc from your grid supply. I install a screen switch on my Tempo radios, they didn't have any either. It helps a lot doing neutralizing.