First: How do you know that one of the tubes "went out" ?

You can probably find a schematic for your amp >here<, but I doubt that you'll need it. All of the tubes are labeled, so you can probably just read it and get another one. If it was one of the power tubes that failed, you will need to order 2 of them, and get the bias set properly.

Which brings us back to my first question, because its unusual for a tube to just 'go out'. Some other component may have failed and taken the tube with it. In that case, just replacing the tube won't fix the problem, and you would just waste your coin on blowing up a new tube.

So, if you have a competent tech in your area, you should take your amp in for a check-up. A good vintage tube amp deserves as much.