I am currently in college and play a Conn 18h. I love the horn and have played it for 2 years. Unfortunately the sleeves of the slide are losing their plating and it is becoming noticeable when playing. I know this issue is a hassle to fix and it’s often better to buy a new horn instead of getting it fixed. I was thinking of trying a new, larger horn and I love the dark sound of the coprion bell so would a conn 10h with a coprion bell be a good choice? Would love to hear from anyone who has played a 10h or preferably both an 18h and 10h. Any other horn suggestions are also welcome as regardless of if it’s the 10h or something else I will be testing it before making any purchases so having more to compare to would be nice! Thank you!

Are you sure you can’t just have a tech rotate the inner slide? Usually chrome wear isn’t so bad that it’s totally unsalvageable. Inner slide replacements aren’t that bad either.

That said, 18H is an unusual choice. Very small bore and the copper doesn’t blend well in my experience. If that really your cup of tea, you can get a replacement slide. Edwards and Getzen small bore slides should fit no problem as will most small bore Conn slides.

It's kind of unusual to see a college player talking about a vintage horn. There are a couple of steps you can take before replacing the instrument. First, as MattK says is to rotate the slides, and I would recommend a slide straightening job at the same time. Next would be to replace the tubes with new ones. If you like it and want to keep playing the instrument, this kind of maintenance is not unreasonable. The 18h is not exactly a collector item, so repairing with non-original parts shouldn't set off alarms. Another thing you could do would be to just replace the slide. The small Conns were popular, so you might be able to swap a slide with something like a 24h or 6h. There is a thread on here somewhere that talks about the compatibility between different model slides. Regardless if you get a used slide, you might wind up right back where you are now. If it were mine, I'd just get the tubes replaced, or buy a 6h if you want to upsize. 6h are relatively cheap and plentiful, but might also need some repair.

The 18h was a student coprion model. You can't just go down to the corner music store and replace these. They stopped making the bells long ago. Conn 10h are hard to find, and will likely have some slide wear as well. 12h are more common, and are the professional equivalent of the 18h. There are some other copper bells floating around. Kanstul used to make some, but these are also hard to find. There are some European models, but I'm not too familiar with them. Even so, a modern copper bell is not the same as a coprion bell, which was electro deposited onto a mandrel in a vat of chemicals. Modern copper bells are just built from sheets of copper.