Yes if you're lucky just a blown fuse and possibly blown resistor but as stated above things can get much worse.
Can using a bad tube hurt an amplifier?
If you're using a tube for a while, and everything's fine, and then voltage changes over time gradually, and then it damages an amplifier? are amps Built with controls to prevent that from happening?
How can you trust using a tube if it potentially could harm an amplifier?
Assuming You're using the correct tube in the slot provided.