Depends on the machine. Not all of them are cathode tuned.
Depends on the technology being used. Cathode control is just one way.
Well, on the CT: A anode supply created up to 60 KV, The Cathode supply created down to
-60 KV. Total supply current was up to 300 mA to the tube during a run for a max worse case thick skull, and well over that during testing/tube conditioning. PSUs could run adjustable constant current or constant voltage, and change values more then 1024 times per rotation. Yes, the cathode was regulated as well. So up to 120 KV, 300 mA, and rotation times of .4 second. There was even more magic in the form of deflection plates for multiple anode spots. Don't forget the anode was spinning, on the order of 10K rpm, driven by a six phase drive.
The unit even sensed and adjusted contrast within scan lines, to reduce dose. There is also special magic in the PSU to quench arcs in the tube.
Of course no radiologist ever maxes these values out. They are there for emergencies.
Steve