Thanks for the replies. I was some pretty serious fireworks throughout the tube. I did try another tube which is working fine, so it does appear to just be a bad tube. Had never seen anything like it before though, so I thought it best to ask the experts!
Tube Fireworks
Tonight I turned on my amplifier, starting hearing popping and hissing from one speaker, and turned around to find one tube sparkling like fireworks! Obviously the tube is bad and I won't use the amp again until I can replace the tube, but does anyone know why such a thing would happen? Thanks!