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Hello Tubenetters!
I've lurked on these boards for years, but never really figured I had much to add, so didn't say anything. I think you guys can probably figure this out, though.
I bought a sousaphone on eBay a couple weeks ago. It had been listed as possibly being a Conn 38K from 1924. I was really excited to get it, as the horn that I normally play is (I have always believed) a 38K, and I love it. The problem is that my normal horn belongs to the band I play with - it's not mine, so I wanted one of my own. When I finally got my grubby mitts on the new horn, though, it was clear that this is not the same model. The new horn is noticeably larger and heavier. I don't have a bathroom scale to weigh it, though.

I think the new horn is a 42K with a front-facing bell instead of the rain-catcher bell. I'm not exactly sure where one measures the bore of a tuba, and I'm certain I don't have the correct tools to do so anyway. My only real clues are the facts that it's a 3-valve Bb, and the serial number, which Conn Loyalist estimates at 1924. Conn Loyalist also puts this as the first year that the 42K was manufactured. The bell collar is about 7 1/8 in. The bell itself is marked with a '41', but from what I've read (probably here) and elsewhere, that marking isn't necessarily indicative of model number. The old horn has a serial number that puts it in the 1915 range, though Conn Loyalist also says that the 38K wasn't manufactured until 1919.

So, my thoughts are that either the new horn is a 42K because it's larger than the old horn, or maybe the new horn is a 38K, but the old horn is something else. What do you guys think?

I'm not totally familiar with all of the Conn model numbers but It appears that your bell collar is actually 7 1/4". Measuring from inside to inside gets easier if you start at the 1" mark instead of the end of the tape and then subtract the inch. A 7 1/4" tenon was (is) used to the 20/22K and I think the 37,38, 39, 40, & 41K sousas sousaphones as well as some Holton sousas.

What is the inside diameter of the second valve slide?

I think the part that I was getting hung up on is the lack of (readily available, anyway) information about Bb sousaphones as old as the white one. I double-checked the serial number, which Conn-Selmer puts between 1911-1912. Conn Loyalist doesn't show any sousaphones nearly that old (helicons, maybe, but not sousas). Granted, he says himself that the information he has is incomplete and/or incorrect.

iiiPopes, as best I can measure, the interior measurement of the 2nd valve slide is 23/32". I'm guessing my measurement's probably not quite precise enough, though - but I do see on Horn-U-Copia that a 42K would have a .773 bore, which would be over 3/4".

At any rate - I'm really happy to have this horn. It's not a Jumbo, but it's still a beast! I played it on a gig last night, and it puts out sound. I am going to have to strengthen my back, though!

Thanks for the help, guys!

Sure - the old horn has serial number 123579. The 38K has serial number 210024. Now you've got me curious... What are you looking for?

.... but schtupid?

Hmm. That makes the bore @ .719, which is the "#10" bore on the Conn Loyalist Chart, and could be the 38K. We've all thought that the 38K had the "10 1/2" bore, or .734, as does just about everything else since, but if your serial numbers pan out, meaning that they all match, that the number stamped in the top of each valve underneath the felts and corks matches the external serial number, that may actually be the case.

Please measure again with a ruler or caliper that can do decimals or 64ths, not just 32nds. And measure at least three different "round the clock" points to get an average.

But it is definitely not a 42K, which, yes, has the .772 bore like the later Conn 2XJ tubas.