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I have a significant dent in the third valve slide, just above where the slide (REALLY LONG) ends.

https://photos.app.goo.gl/bzxx6Gsj3VSSXdQd6" target="_blank

Would this significantly affect the sound of any pitch played with the third valve, or am I just imagining it?

Also, this horn is super old, so yeah, compression is probably junk as well, but I'm borrowing it from a university out here, and I'd like to return it in better condition than when I got it. I banged out the bell today and now it sits pretty.

It should be just fine. I've heard that dents aren't too much of a serious problem unless there are a lot of them, or they intrude about 40% into the bore.

Okay, that's good enough news. Maybe it's just the shoddy valves. I'll take pictures of them tomorrow. It's not the best tuba (I like my Conn 20j and Cerveny Helicon more), but it's what I use for the ensemble out here.