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At first glance, without any special knowledge, that looks simiilar to the Custom Jumbo I had some years back. Except, mine had rotors. They didn't give it a stock number according to Steve Dillon who ID'd it for me. That horn is now in a private collection deep in the heart of Texas.
this is the Conn next to my B&S F, for no other reason than those were my two main horns at the time. Pitch on "mine" was nasty. But, the sound was/is amazing, for me second only to the Martin I now have. I learned to lip everything, because slide shuffling was hardly worth the trouble. Matt Walters did a lot of work on it, apparently did some cutting and cleaning and made graven images to the gods of intonation, but said it didn't do a lot of good. It's cool to look at, though. I do miss that horn, all the same.

I think these early big Conns changed often. It seems like they're all nearly prototypes. If you find two just alike, that is a lot. Coming up with a model designation is not easy. The Tuba Exchange has two very similar to this in the historic collection, and they call them Orchestra Grand Basses, so that's what I have called them.

I owned this horn for a year or so. I noticed the same thing you have about how the intonation seems to improve almost magically after an hour or a little more. I used to start warming up way before a performance because of this. The low F is great, as you mention, and the Eb below the staff I thought was especially pleasing. C in the staff cannot be played 1, at least couldn't before Bloke worked on the first valve (after I sold it); it was so flat it was nearly a B.

It does seem like there's a leak, but I couldn't find one that seemed to matter. I filled it with water and watched. There was a small leak in the bottom bow. I posted a question about that here and the consensus was that even a bullet hole wouldn't matter where that little drip was. But the valves were pretty porous at that time. Nothing less than Vintage valve oil would do. Bloke worked on those too and posted pictures here, so I gather that's been fixed.

For all its age and faults, I liked that horn a lot and was sorry I sold it. I've gotten another one a little older but wrapped pretty much like a 34J and in a lot better shape. So the seller's remorse is over. I still have a lot of pictures of that horn though, taken by me and by previous owners. FWIW, I think the 1915 date is correct. If it was from the 20s it would have a model number on it, wouldn't it?

EDIT: here it is with another previous owner: http://www.cartoonsforkids.com/CoolTubas/DaleConn.htm

ANOTHER EDIT: Klaus preserved these pictures from the eBay auction that led me to buy it, so this is what it looked like when it came to me.:
http://groups.yahoo.com/group/YorkMaste ... 3/pic/list
The only mods it had in my possession were the valve buttons (which I did myself, when one of the original ones crumbled to pieces, I posted about it at the time) and the thumb ring, which I had moved over onto the main tuning slide. I notice the new owner has had it moved (by Bloke?) back onto the first valve slide.

BTW, approps of another thread, I am pretty sure I've got the strap rings off this horn, if your student wants them. I'm away just now, but could check & send them in a couple of days if there is interest.

I would say that this looks like an Orchestra Grand from the teens that has been cut (badly) to bring it up to pitch.

I played one of these once that was so low, it was a tuba in AA! Early Conns are known for having a complete, gradual taper from the mothpipe to the bell. If you look at the back of this horn, the inner top bow to bottom bow is too cylindrical for a Conn. Also, that third valve configuration would have never left the factory, so I believe that all the valve slides were cut as well.

The Orch Grand I refered to earlier was one of the most beautiful instruments I'd ever seen. In mint condition and played with the most beautiful sound, and was even in tune with itself. The sad thing was, I didn't see any way to bring it up to pitch without altering the taper, which would of course, throw the intonation way off. And I wasn't about to learn fingerings for a AA tuba!

Good luck with this one. I wonder if you coud get a cheap 20J, mix and match bows to come up with something that had a much more reasonable pitch center, and still maintained that big beautiful Conn Orch Grand sound?

Frank

the elephant wrote:"NO SOUP FOR YOU!"

HAHA!

the elephant wrote:

"I feel the resistance decrease with 3rd down.... I cannot locate a leak"

I'm sure you've checked to see if the piston ports are properly aligned with the cylinder ports..... thought it worth asking though.