[Arran]

The 6X5GT is fine as far as I know, I have several sets that use a 6X5GT and there is no evidence of any problems, some still have the original tube, the ones with the X pattern plates seem the most robust. I guess it may depend on the internal structure and how much current load is put onto the tube. Before you power the set up again I would double check your work and also see if maybe there is a short somewhere down the line after the rectifier, when they glow blue or spark that's often the cause. It could be a failed newer component, it does happen, or it could be a blob of solder they got into the wrong place. I'm not suggesting that the tube didn't fail but it isn't the only thing that could fail.
Regards
Arran