tube amp help

I recently purchased my first tube amp- a used Audio Research VS 110 with new matched SED winged C 6550Cs put in by the seller. They were biased at 55mV per his recommendations. I've been listening for 1 week- it sounded great until today. When listening there was a brief pop sound and a quick spark from tube 7. I turned it off, waited a couple hours...turned it back on, they all glowed orange, I checked the bias. tube 7 had 0 mV, all others 53-55. Is the tube blown? Do I need just need to reset the bias? If I need to replace the tube do I need to replace a pair?? Any help would be appreciated - I am a tube amp newbie.