This happened to my Hughes and Kettner pretty soon after I bought it. It was the first all tube amp that I've owned. The tubes needed changing. Well, I changed the tubes, but the amp still wasn't working right and two of the power tubes were glowing extra hot. Apparentally, if you don't use tubes with the same exact power rating (and some cases, the brand) to replace the old ones, then you're gonna have to bias your amp to adjust to the new tubes.

To bias your tubes and your amp, it is my understanding that you first need the right equipment to do so. My suggestion is that you take it to a shop where they do amp repair. It's fairly cheap, and you avoid the risk of messing anything up by trying to do it yourself. The amp guy explained to me and said "If you don't bias your amp when you change tubes, then your amp won't be putting out enough power through the tubes, or it will be running too much power through them. Either way it isn't good for your amp to be played through, let alone running at all." Hopefully, that's all that needs done. Good luck.