First thing is to see if it is the tube that is bad or the amp. You can just switch the good one and the bad one in that channel. Be careful to watch it as it warms up. If it glows very brightly, shut the thing off fast! This would probably indicate a bias problem and should go in for service. It can be a simple adjustment or something ugly-you never know.
If the bad tube still doesn't light in the good socket, you have a bad tube. In this case I would personally replace all 4. You can usually just dump the 3 remaining on ebay. If you've never bought new tubes for it, this might actually be a good thing.
If the bad tube still doesn't light in the good socket, you have a bad tube. In this case I would personally replace all 4. You can usually just dump the 3 remaining on ebay. If you've never bought new tubes for it, this might actually be a good thing.