What does it sound like?

Kehut, nice response.

Most likely. Sometimes they surge bright red and then die. Some amps will shut down and blow a fuse when the tube goes. Most tubes give little warning of their time to go, unless they are very weak and then exhibit lower output volume in that channel. If you have acess to a tester, have them checked, or replace the tube in question immediately. Dont turn the amps back on with a bad tube, that could do more damage. Good luck with your amp.