it is possible for different tubes, esp. tubes from different MFGs or production lots to have subtle differences. if the difference can be compared like a 15watt bulb vs a 300 watt bulb, check voltage, and if possible current draw. if the ghetter has turned milky in the "brighter" tube, the tube may have lost its Vacuum = bad tube. Otherwise need better description of problem.
What will cause my EL-34 filaments to glow differently.
I have a modified Dyna St 70 with a Curcio driver board. My right channel tube filaments glow one brighter than the other. I have checked the filament voltage and it is the same on both sides, left and right. There is nothing in the circuit that I can see that would cause this imbalance. I have moved the tubes around and the condition remains with all combinations.