DON'T PANIC, That's a bad power tube. Mess with the Tube Mix and you'll find the bad side, then swop them for new. If it was me I would buy a new set of both power tubes, Egnaters do work the power tubes hard and you can bet the other side will go bad soon enough. I like JJ tubes for the power tubes, they seem to last longer and are cheep, all the tone is in the pre amp tubes if you like messing with different tubes but power tubes just seem to be power tubes to me. The half power thing in Egnater's is kinda cool, most other amps would just stop working. I had one of my Marshall's go up in smoke at a gig once when a tube let go.