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A lot of the older horns were a "low pitch" and were tuned to A435, A438 instead of A440. Many of those horns have had their tuning slides cut down to accommodate. whether your tuba is A440 or not can be found in the older Conn serial archives.

SJP wrote:Did those old tubas originally come with smaller, shallower, maybe smaller bore mouthpieces?

To make a long story short (pun intended), yes.
You actually are on the right track using a bass 'bone mouthpiece, because they are actually more along the size of an original Eb mouthpiece.

I'm not aware of anyone making copies of those originals, because people like much bigger, deeper mouthpieces to use these days. They [the originals] show up on ebay from time to time. Look for "Conn Standard" and "Conn Giant" mouthpieces, both of which were made for those early 20th century American-style Eb's.

Or, try to get a BIG shank contrabass 'bone MP. As I said, they were more along the lines of the originals. Not only was it still early in the escalating brass arms race, but a lot of Eb players ended up coming from converted trumpet players (bass clef was the same as transposed treble, just add two sharps). If they couldn't cut it as tuba players, then they'd be moved down to drummers. If they couldn't cut THAT, they could always be conductors...

I have a Pan American Eb Giant Bass. If I remember right, mid '20s vintage. It runs a hair flat with the slides all in. It could be leaky, certainly has been a little banged up and worn down in almost a century of use.

I also have a Conn Giant mouthpiece. It's a good one, I like it a lot and particularly with that tuba, but I have never noticed any difference in pitch. It's a lot like the Conn 3 I have, which has the distinctive flared outside profile you'd recognize from the Conn 2, but inside it's the same shallow funnel and large throat. Closest current production I can think of is Schilke 62, but seriously, the Giant is a better mouthpiece for my money. It's a shame that among the hundreds of tuba mouthpieces made today, there can't be one that's anything like it.

But will it make a tuba play in tune? I fired up my tuner. I tried the trombone mouthpiece. (I was actually working on this recently with my little Eb, where a trombone mouthpiece fits.) I tried the Conn Giant, a Schilke 62, Schilke 66, Conn 7B, Marcinkiewicz H1. That last is the biggest thing I have - width, depth and throat, all truly large. Honestly I'm not totally sure any one of these mouthpieces is inherently sharper or flatter. They can all be flat in the lower range if I don't support the note, but with more support I can zero the tuner easily enough. The trombone mouthpiece (Josef Klier 1AK) was very easy to support; the Marcinkiewicz was not as automatic but it's quite responsive. Cup size might have made a little difference - favoring the trombone mouthpiece and the Conn Giant - but it could just as well have been the shank size, which varied a little. The Schilkes were harder to get dead in tune, and they didn't go in as far as the Giant and the Marcinkiewicz.

It is common for pre-1920 brass instruments to not be pitched at A=440. More often than not, the older American instruments are flat. And, on the old horns with the main slide in the leadpipe there is not that much pull length that can be sacrificed, so it is a problem to bring these instruments into tune. Just this week, I removed the 4th branch from a 1919 Conn monster Eb, trimmed an inch from each side, re-fitted the branch to the ferrules, soldered it back together, and the pitch is not absolutely perfect (is it ever?), but everything is in the ballpark now. With the changes, just about every pitch is within 5-7 cents with a modern mouthpiece, without pulling slides.