Does your Alexander have a Dillon Adjustable Gap Receiver (AGR) installed? That is what immediately comes to mind after reading your description, but without photos or more details it is hard to say for sure.
Knowing what you have for a receiver is important because of the variety of shank sizes that are available. If your current 7B actually fits the horn now, I'd guess that your receiver is setup for American shank mouthpices, as EVERY Conn Helleberg or 7B I've ever encountered is this way. That seems like an awfully small shank and receiver size for an Alexander to me, but if that's what you've got, that's what you've got.
Shank size info (originally posted by Matt Walters of Dillon Music):
TUBA SHANK SIZES (primer)
In the world of tuba mouthpieces, it seems there are five (5) general shank sizes and that, can lead to confusion. The Standard American and European Shank sizes, followed by the Large Shank, are the most common sizes that fit modern production tubas. From smallest to largest, we offer the following information to shed a little light on the subject.
EUROPEAN SHANK: This is an increasingly popular shank size of about .530" diameter at the small end of the shank. It is found in mouthpieces like the JK Exclusive, Perantucci, Laskey "E" shank, and of course our wonderful Sheridan Series of mouthpieces. Most every European and American tuba built these days, will accept this shank size.
STANDARD AMERICAN: The reference point for this size of about .520" at the small end of the shank. A good example would be the Bach, and Conn Helleberg tuba mouthpieces. This is the most common size here in the United States. In the Dennis Wick line, you need to make sure it has the "L" in the model number to get this standard size. Our regular size shank Dillon Mouthpieces are also of this size. Besides American built horns, many European horns like Miraphone can use this size.
LARGE SHANK: This is a larger size that measures about .550" diameter at the small end of the shank. It works well on the larger European tubas. Especially horns with large diameter leadpipes like the "York" copies and most Alexander model 163 CC tubas. This is the size of our Dillon Music "L" shank mouthpiece and the new Wick 2XL.
SMALL EUROPEAN: Having a diameter of .490" at small end of shank, this size is offered by Dennis Wick in the sizes 1 through 5. Just make sure you order the one without the "L" in the model number. This size is most commonly used on old Besson tubas and old American Eb tubas. I hear this referred to as the Eb tuba size. Some old German, etc. 3/4 size BBb tubas have also shown up with that receiver size. If you are having trouble figuring out what small size shank your old tuba needs, it is about the same size as a large shank trombone mouthpiece. Borrow one from a trombonist and see for yourself. You can order a Denis Wick mouthpiece, or to have a wider variety of mouthpieces to choose from, consider replacing the receiver for about the price of a mouthpiece.
KAISER SHANK: With a measurement of about .585" in diameter at the small end of the shank, this truly is a "Kaiser" size. It fits the largest Alexander model 164, a few ‘one off’ model 163's, and some original York receivers we've come across. Anyone needing this size shank will need to get a custom built mouthpiece, or have an adapter made.
I rarely say that any particular mouthpiece is not a good fit for a given instrument, but will make an exception in this case. A Conn 7B is just not the right type of mouthpiece for an instrument like an Alexander. I have tried MANY options in my Alexander 163 just to see what happens and always end up here:
First choice: Doug Elliott setup (R cup, #7 or #8---pretty big sizes--- for backbore/shank).
Second choice: Dillon M1CL (not made anymore, ask around on the for sale forum)
Third choice: Schilke Geib Euro or Laskey Geib series
I also have the original mouthpiece that came with the horn and have tried out several other factory Alexander tuba mouthpieces. They are all very wide, quite shallow, and have small throats, and wide rims on them. I've found that the factory mouthpice does make the instrument seem more efficient and easier to steer, but that I prefer the sound of something deeper and not nearly as wide (some Alex mouthpices are 36mm or more wide). The factory piece is a bit like a really wide version of a PT-64, for reference.
Alexander tubas, in their unaltered state, are rather mouthpiece sensitive. The biggest thing I've discovered in my own experimentation is that fit in the receiver makes a significant difference on a Alexander. Having the right shank sizes for my receiver (from Doug Elliott) honestly does make a very noticeable difference in the way the horn plays and how managable pitch is (and mine's an excellent one).
I had a nameless wonder (looked like a Perantucci -very round outside w/ a couple rings cut in) which was a joy to play. Now I'm starting to wear through the plating on the 7B (which was my backup) too. The sound I get with the 7B feels somewhat choked/stuffy by comparison. If I had the nameless one replated (gold), it would cost as much or more than a new mouthpiece (and would still have no name).
