if the tube is wore out, you might not get a decent picture out of it, but there is something you can try, you can turn the drive gain up on the guns. basically you push more color out to the screen. the small circuit board that is attached to the neck of the tube should have the adjustments.
what you want to do is display a color gradient on the screen:

what you will notice is that the colors will be okay on the right side but get very dark to black (probably before the halfway point) you want to EVENLY turn up the DRIVE AND/OR the GAIN settings to bring the color further across the screen.
you want them to be even (red green and blue) if they are uneven, your whites will be tinted. if you run out of adjustment (maxed out) on one color before the others you will have to turn the others down to match the lowest.
this MAY give you some additional low level color brightness at the cost of reduced color separation at high color levels, although this is unnoticeable in 99.9% of cases.
your other option is having the tube rejuvenated. some people swear by this, others recommend not doing it at all. a test can be performed to see if the low output you are having can be remedied with a rejuvenation or not (same equipment). It's pretty easy, takes about 5 mins. may make your tube look better. ALSO may render your tube unusable or fail prematurely. so try this at your own risk.