Q: My guitar amp’s tubes are glowing blue instead of sort of a yellow glow. I want to know are they bad?
A: The tube is fine as that blue glow is just a side effect of a power tube — a fluorescent glow in the blue spectrum. It actually indicates that the vacuum inside the tube is very good, which is what allows this phenomenon to occur. It will not have any effect on the performance or tone of the amp.
HOWEVER: If you notice a red glow on the metal plates of the tubes, turn off the amp immediately. This indicates that the tube is overheating and is red hot. It’s likely that the tube bias in the amp is set incorrectly, but there may be some other problem causing the tube to overheat. Have the amp checked by a technician before you use it again.