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An Olds and Son sousaphone is one of my 'dream instruments'. It's thick walls give it a good tone and amazing durability, not to mention Nat McIntosh plays one, and he told me: "If you ever see one in good condition, I'd strongly suggest taking it."
I don't need a sousaphone now (I can still use the schools for 2 more years) but once I get to college, even if I make marching band, they frown on taking the sousaphones off campus, and I use the sousaphone quite a lot over the summer, meaning that I'll probably need one in 2 years.
Now my question isn't about buying one because I don't need one right now.
My question is simply this: if I do end up buying one in the future, will parts be available if some part of it breaks down? and if there ore no Olds parts, are there any parts of other brands that would fit it?
I'm asking this stuff because I don't want to dish out a bunch of cash, then find out I can't play the sousaphone anymore because no one can fix it.

There's really nothing to break down. All horns need new corks and felts periodically. They come in assorted stock sizes and thicknesses, and they're cheap, so that will never be a problem. Valve springs rarely, if ever, "wear out," and again, if the spring brass in one of them does give, there are stock items of different diameters and lengths that can be adjusted to fit, and they're also cheap. Valves are replated, rehoned, and rebuilt with the casings about every 50 to 100 years or so, depending on how much you play it, both indoors and out getting dust and grit into the horn, and that is only moderately expensive. If red rot settles in, the offending area can be cut out and new metal brazed in. Not cheap, but can be done, but shouldn't have to be. By now, Olds horns are now so old that if that's a problem, it's going to be evident now so you can see it beforehand and not purchase the horn.

So unless you have a major accident and severely damage a valve slide or a bow, what's there to fix? Nothing. And are you going to forego a great playing horn just because you're worried about an accident and nothing you can do to prevent it? No. So if you really want one, and find one in good condition, for a good price, go for it!

Don't know about the neck, but the bits are interchangeable with Conn bits.