Grasping at straws now.
Not sure if that rig uses a feed thru cap to get drive into the pa grid. If there is one, and it opened, you would lose drive.
PA screen voltage could be too low for tube to conduct. Check the screen rheostat motor to see if it is working, check screen rheostat for an open, check to see if screen is turned down and only needs to be raised. Look for an open to the screen in a location past the rheostat.
Aha, another possibility - not sure if that rig has an "underdrive relay", but if it does and it didn't engage, there wouldn't be any drive present.