Yes Nick it is possible for Thoriated Tungsten cathodes, depending upon how much thorium is left in the interior cathode tungsten filament. IF I remember correctly, with no plate voltage, run the filament 20% above normal for a few minutes. Repeat as necessary. The thorium will "boil" to the surface and emission will increase, if there is thorium left.
The only oxide cathodes I have ever seen improve is in picture tubes. There are usually several stages of a picture tube rejuvenator one can try. The highest stage actually causes sparks inside the tube, ripping off the outer layer of oxide material from the cathode so fresh material is now on the surface. Sometimes it works, sometimes it ripped off all the material, or what was left.
cheers
steve
