I recently found an old 1960s era tube amp that my grandfather built, and I hooked it up to a speaker I had laying around to see if it worked. Lo and behold, the thing worked! Unfortunately, there was a massive hissing sound coming through from somewhere.
So, I opened it up and first thing I noticed was a pair of capacitors had exploded. I ordered the equivalent from parts-express, and am currently waiting for them to arrive. I am hoping that the new caps fix the problem.
However, I would also like to know if there is a way to test the tubes as well, just in case they are the cause.