If it sounds good, its probably fine. Many older tube amps play fine after 40 years without any maintenance; they are a testament to simple sturdy construction. Old tube amps can almost always be repaired easily (although not inexpensively if its a transformer). Generally they need to have their electrolytic power supply capacitors replaced, and require a re-bias on the power tubes. Old tubes are often fine and really do not REQUIRE replacing - however many prefer to install fresh ones. If they sound good, they are good. Speakers are the other main concern. These can and do wear out, tear, go open, and fry rarely.
Again if the amp sounds good when turned up, then you have a winner. If not, it can probably be repaired. Hell if you have some patience, some experience with a soldering iron, are willing to learn, then you can even repair them yourself. Replacement parts abound.
Look at local pawn shops, smaller music stores, classified advertisement newspapers and you will get the best deals. Ebay is cool, but be careful about price/value. I would try to stick to names like Fender, Gibson, Supro, Ampeg because they are known to be good quality.
From my perspective I have had worse reliability issues with solid state amps. These can be much harder to repair. Any new SS amp are loaded with IC's, and printed circuits which are a headache to say the least.
Good luck!
