[Fisherdude]

Well, tube substitution is certainly one thing you can try.

Keep in mind that tubes don't always fail outright. In most cases they get weaker and weaker until they're so far away from their original specifications that the circuit that they're in doesn't work properly any more, and you can then tell that something's not right.

If you're going to test tubes by substitution, you really need a set of known good, tested good, tubes to use as your reference.

If you substitute one untested tube for the questionable tube in your radio, there are three possible results, it'll get better, it'll get worse, or you won't be able to tell the difference.

If it gets better, that doesn't necessarily mean the the substituted tube is "good", it might just be that it's just not as bad as the original one. At least you'll be able to pinpoint the original tube as questionable.