[miniman82]

If the tester isn't finding the short, run the tube with the chassis and note the voltages on all the elements (K, G1, G2, ect). You might find that the grid voltage is significantly higher than it should be, or the cathode is very low. In either case, this would drive the tube out of conduction leading to the low brightness situation. I suspect an open has developed in one of the elements of the gun, the voltage measurements will confirm which one. If you find an open, some testers have a 'weld' function which can sometimes repair a fault like this.