[Radioroslyn]

Have had crts that are dim when first turned on but can produce a viewable pic after 5 or 10 minutes. I would deem this one weak but usable till a better comes along.
With all vacuum tubes what happens is over it's life cycle of use is that the heat/cathode looses it's ability to produce the original amount of electrons. One way to compensate for this condition is to increase the voltage on the heater.This in turn gets the cathode hotter and dissipate more of the needed electrons.
The down side is that it over taxes the heater and cathode causing their demise to quicken.
This is what the crt brightener does. It's a small step up transformer that takes the 6.3vac going to the crt filament and boosts it up a volt or so.
On your crt tester you may have a button to press to rejuvenate the crt. This applies a HV pulse to some of the elements in the crt to clean them off. This may help some but it seems likely that the main issue is the cathode emission.

Terry