well obviously if you exceed the h-c rating you could cause a h-c
breakdown.

If you float the filament at 150 V, make sure you don't have any other
tubes connected to the same filament winding and make sure the
transformer filament winding is rated for 150 V.

But I don't understand why the cathode has to be at 150 V above ground
for a cathode follower, isn't the cathode a few volts above the grid
and the grid is near ground?

Of course what I really don't understand is why you want to do this at
all , but lets not go there.

Mark