The tube current is not jammed at full throttle, which pretty much eliminates the power supply. If “5 power” means 5%, then I’m surprised the tube fires at all, but if that’s 50%, then the current seems low.
It is possible the tube has failed and is no longer resonating in TEM00 mode, which can cause all manner of peculiar problems, because the beam no longer focuses correctly.
This discussion goes over the details and explains how to do the tests:
Pictures of the results on a paper target will be revealing. It should be a nice round spot, not a hole through the paper, so we can examine the details.