Here's a video I saw on YouTube that is interesting by mistertonemaster. He took the time to capture a glowing output tube on tape. These come in the shop sometimes. If you experience this in your amp, turn it off immediately and take it in for repair. Simply replacing a tube may fix the problem but you could have an open bias resistor.
What this means is the bias resistor acts as a gas pedal when the input signal hits the output tube. It's usually biased as a negative voltage. If it opens from heat, wear and tear, pulls away from the circuit board, etc. This presents a 0 voltage to the input of the output tube. This effect causes the output tube to run wide open, putting a strain on your amp as well as the tube.