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.
You may want to replace tube 7. You're lucky to know which one it is. I'd get a new set plus one extra tube that matches. Antique Electronic Supply in AZ can probably match the numbers you have now. 480-820-5411 also check your fuses and find out if any are blown. Do the tubes bias individually or all at once? I'd also call Audio Research to make sure the bias is set to their specs. Hang in there, it will be worth the effort for tube sound. Good luck.
Yes it's blown.

My guess is that tube 7 is bad. If you live near some place where you can have all the tubes tested, I would do that just to make sure there are no other problematic tubes.

I would also read the part about tube bias in the manual. I have never owned or biased a VS 110. But I have owned a number of Audio Research amps and currently own a Ref 110. Tubes in Audio Research amps are normally biased in pairs. Again I would check the manual but normally the bias is 65mvDC. If all the other tubes are fine I would get a replacement tube that matches the other tube in the pair. If the tubes are biased in pairs which I am guessing they are. You don't need matched quads, just matched pairs. Though it doesn't hurt to get matched quads.

When you bias the tubes the amps needs to be warm with no music playing. 15 or 20 minutes is good. Normally if I am going to check my bias I do it after a listening session. On new tubes it is best to check the bias a few times. As new tubes break in the bias can drift a bit.
Make sure that the Amp is connected to your Speakers when setting the Bias and as stated above give the Tubes a warm-up first.
Thanks for your responses. I appreciate the reference to Antique Electronic Supply in AZ since I live in Scottsdale. The Audio Research manual does say 65 mV DC- but the seller said he was told 55mV from his tube supplier for these particular tubes. I might change it to 65. The tubes are matched pairs- so I guess I'll get a new pair.
In the mean time- I assume I shouldn't use it, but someone told me I could listen at low volumes with a single blown tube.

I run SED winged "C" tubes in my Ref 110. That is the 6550 tube that Audio Research recommends for all their amps. These tubes don't need to be run at that low a bias. And a bias that low will adversely affect the sound quality of the amp. I run the tubes in my Ref 110 at 63mvDC.
FWIW, I don't know about the VS110 but a lot of ARC amps will take out a resistor in the bias circuit when a tube shorts out (as yours did). That resistor acts as a fuse so to speak. Don't be surprised if you find when you replace the tube that it won't bias. If that occurs you best have a techie (or ARC) replace the resistor.
Arizona Hi-Fi is an ARC dealer. You might want to take the tube (and the amp) down to them to check the tube(s) and verify that the bias resistor is okay. If you're lucky, all that will be needed is a new tube. Also, having the dealer check ALL the tubes will verify that they are new tubes.
Don't laugh, I have replaced tubes in my used tube amp purchases with fresh tubes purchased by me. The tubes that came with the amps didn't see duty until I checked their condition. Some of them were on their last legs.
I had the vs 110 and the manual states to use 65m v but that was with the older 6550 not the 6550c that the new ones had for info .
I checked the resistor with a multimeter- fortunately it's OK- just waiting on a pair of tubes. Thanks for the Arizona Hi-Fi tip!