The tubes that can be rejuvenated are those with thoriated cathodes. The radioactive thorium emits electrons, which increases emission. On a tube tester, run up the filament voltage a couple of volts over rated - and recheck the tube's emission every hour or so. That boils up thorium atoms to the surface of the cathode material. Best to have only the filament pins connected in the tester during rejuvenation - to protect other tube elements.
The main tube that can be rejuvenated is the '01a. 45s can't. Weak '01a tubes are often a good buy. You tend to pay more for strong ones - which probably have been rejuvenated by the seller.