The Adams Adjustable-Gap Receiver
I suspect most euphonium players have never worried about the gap between the end of the mouthpiece shank and the beginning of the instrument's leadpipe. To understand the terminology, the receiver is the two or three inches of tubing attached to the end of the leadpipe. The mouthpiece inserts into the receiver. The gap is hidden underneath the receiver. The drawing below shows this:
Different manufactures may use different sizes and degree of taper for either the euphonium receiver or the mouthpiece shank. This means that in some setups the gap may be narrower or wider. As a practical matter, it is necessary to design so that there will be a gap. Otherwise a particular mouthpiece might hit the leadpipe before it is tight within the receiver. That could cause the mouthpiece to fall out of the receiver or to "rock" inside the receiver.
On the Adams Custom euphonium, which I play, there is an innovative "adjustable-gap" receiver. The concept is simple. The receiver is threaded on to the leadpipe instead of being soldered. Turning the receiver one way or the other will adjust the gap. The photo here shows the receiver adjusted all the way in and then a considerable distance out. There is a collar around the receiver that can be loosened for adjustment and tightened when the optimum setting is found. It seems logical to assume that the narrowest possible gap is ideal, but in practice that is not the case. The amount of gap affects response, tone, and intonation.
One might still think it is not worth worrying about because we usually pick out our horns playing the mouthpiece we prefer. So if that combination feels right, why worry about the gap? For one thing, if we can adjust the gap further we may find a position we like better. And for another thing, if we buy a new mouthpiece it may fit differently and increase or decrease the gap. If you test a mouthpiece that has a significantly different gap in your horn, you may reject the mouthpiece because of the gap, as opposed to the qualities of the mouthpiece itself. Below is a chart showing six mouthpieces I tested in my Adams euphonium. It is presented in inches (top) and millimeters (bottom). The Overall Length is the distance from end to end of the mouthpiece. The Shank Insertion is the amount of the shank that fits inside the receiver. The Overhang is the amount of the mouthpiece that extends outside the receiver. Then in Italics beneath each set of numbers is the difference between the largest and smallest measurement in each column.
As you can see from this chart, the insertion (which controls the gap) varies more than 1/3 of an inch (about 7.5mm) between my own Denis Wick 4AL to a Schilke 52E2. But even between two mouthpiece that one would think would be similar, the Wick 4AL and the Wick Ultra SM4U, the difference in insertion (gap) is about 1/4 inch (more than 6mm).This series of measurements was educational for me for two reasons. First, from a general interest perspective, I didn't know the gap varied so much. And second, I have heard and experienced for myself the difference it makes on my Adams euphonium when I modify the gap adjustment; it then stands to reason that to fully test/compare the SM4U against my 4AL, for example, I should at do some of my testing with both set to the same gap.
As a practical example, at ITEC 2010 I was visiting all the displays and trying every euphonium. On my rounds of the second display building, I was playing the Adams Custom from the display, trying unsuccessfully to play Claude Smith's Rondo for Trumpet. The piece has some powerful jumps over an octave up to high Bb and B-natural concert. I could not make the jumps with accuracy. I mentioned to Miel Adams that I was having trouble finding the horn's center. He took it from me and noticed that the receiver was turned in pretty far (toward a narrow gap), so he loosened the lock and turned it a while to open the gap. Then he gave it back to me. As you can imagine, I was skeptical that it would make much difference. But I felt much more secure as I tried the same passage. That was very easy to detect in such an on-the-edge passage. It made a difference in tone, too, but that was harder to judge (Better? Worse? Just different?). But the experience made me a believer. Once I got the loaner that I am currently playing, I experimented and found that I was most comfortable with a position about halfway between the two shown in the photo.
The adjustable-gap receiver was a feature that I thought was interesting when I first heard of it, but I have since learned that it is indeed quite useful.