Found a Sylvania SS/Tube hybrid

[Electronic M]

Perhaps the B+ boost system is dead.....Connect a DC volt meter to the wiper of one of the screen pot with it set at min, and then turn the pot up. It should read something like 350V or more (need to find and check my D12 schematic) with the set on and warmed up, and rise as you turn the control up. If it does not rise the B+ boost is out....Could be from a shorted cap filtering that line, an open boost diode, or a few other things. B+ boost is likely around 1200V so it is likely unsafe to measure it with the set on (unless your meter goes well over 600V), but that voltage should bleed down quickly once the set is turned off.