MP gap supposedly doesn't effect intonation equally?

[Robert P]

I saw a video where it's claimed that an increase or decrease in the mouthpiece annulus/gap doesn't effect intonation equally over the entire instrument the way changing the tuning slide does.

I'm skeptical of this both because it seems counter-intuitive and due to recent experience. I have a Chinese C trumpet that sounds great but I had to adjust the gap because the tuning slide had to be pulled in all the way to be up to pitch. When I checked the gap with a digital caliper it was well over half an inch (~ 17mm) - whoa. The horn is otherwise well made but this obviously wasn't right. Desoldered the receiver, reamed out the leadpipe end (not the mouthpiece end), drilled the internal leadpipe stop back farther and got it to where the gap is now around 2.5 mm with a Bach 1C.

The horn was by my reckoning in tune with itself before with the ridiculously large gap and it still is. Granted I didn't check it with a tuner through the whole range before but I don't perceive it as having changed in character other than now I can have the tuning slide pulled out a normal amount to have it at correct pitch.

If the mouthpiece is farther away from the leadpipe the horn is longer, ergo the pitch is lower. Why would this be impact different notes any differently than making the horn longer by changing the tuning slide?
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[Trumpetingbynurture]

Hi Robert,

It's true. The reason why is that you're dealing with two differently tapered pieces of tubing, and changing the gap is basically altering the overall tapers of both.
The overall intonation is also altered by the additional length added, but think of changing the gap as effectively changing the overall length and taper of the mouthpiece itself.
If you made a mouthpiece where the shank and backbore was longer, and stepped to a larger diameter toward the end, then changed how long that extra section was, you can probably imagine how that would effect intonation quite drastically, just as changing the taper of a backbore can quite significantly alter intonation too.

That other interesting thing with regards to changing gap, which I seldom see mentioned, is that doing so changes how much the mouthpiece shank is in contact with the trumpet. The more the mouthpiece is in contact with the trumpet, the more the mouthpiece is in control (slots feel looser). The less contact, the more the horn is in control (slots feel stiffer).

You could drive yourself nuts with it all.

Hope that is somewhat of an answer?

[JayKosta]

[Robert P]

[Trumpetingbynurture]

[Robert P]

[scottfsmith]

[JayKosta]

[lipshurt]

If you have some time, digest the patent of the “gap” by Earle Kent. It’s a bit hard to get the verbiage, cuz it’s a patent, but it’s good information.

Basically, the gist is:
The gap should be square at both ends, no bevels.
1.108 is the gap that gives the best intonation of the overtone series.

More or less gap changes the overtone series as shown in the graphs, and it’s not linear. Some go up and adjacent overtones can go the other way for instance.

Backbore shape also affects the overtone series intonation but the gap size and shape (square) works the same against the placement of the overtones.

I have found that 1.108 is a good gap size as longs as the edges are square. On a Bach the inner edge of the leadpipe is not not square. Kind of square but not square. (pipe is flared out to be tight in the receiver before soldering)

Horns like the new generation conn vintage one are square, as are many others. Not sure about the newer Yamahas, but the older ones were not square. Calichio square, benge not, etc

So the shape of the edge of the leadpipe, and the shape of the edge of the mouthpiece factors into the gap.

GR on their website as a gap formula that works real good (not surprising) and takes into account the diameters of both the leadpipe entrance, and the mouthpiece exit wall thickness. I ahave always assumed that the formula is for square edges, not beveled.

Some mouthpieces are beveled, like a schilke a4a, and one of the stainless steel mouthpiece makers bevels the exit wall. A beveled exit wall lakes the exact gap length less critical but also changes how the note front feels and the slotting, maybe for better, maybe not. If the leadpipe is Beveled, it helps if the mouthpiece is beveled close the the same angle.

All of that goes with the caveat that “this is generally true” and exceptions won’t be that hard to find.
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[mm55]

[Robert P]