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Alright so I am originally an euphonium player. For high school marching band this year I am going to play sousaphone. I have been using a bach 24aw because its what my band director had. The other day I was fooling around and tried playing sousa with a bass trombone mouthpiece. It was suprisingly easy to play with it and I still am able to get really low notes. The moutpiece doesn't have any markings on it but I think it is similar to a yamaha 60. Is there any reason why I shouldn't use the bass trombone mouthpiece? Besides the fact that the other sousa players will make fun of me for it.

They'll make fun of your sound, is the problem.

You might be able to find something that suits you better than the 24AW, though. Faxx for example makes high quality copies for $50 plus postage, a 7B might be worth a try. Or a Kelly polycarbonate 18 for $34. 24AW though not a huge mouthpiece has an unusually large bore, maybe not the ideal thing for someone moving over from a smaller horn.

Good news - there aren't many unbreakable rules when it comes to playing sousaphone.
Mouthpiece selection is one with lots of room for change.

I can think of 2 things to check:
1 - How is the tuning with the Bass Trombone mouthpiece? As long as it doesn't get weird as you play the standard Bb and F major scales, you're probably ok. Play the scales at volume level f and do 2 octaves for a good picture of what's going on with the tuning. Playing soft can cover a few minor quirks since you'll naturally lip the horn into pitch. It's harder to do that while playing loud.
2 - How does the horn sound in comparison to your section mates? Can you blend your sound with them? Does the director notice a difference (from the front sideline) when you're all lined up across the back hashmarks of the football field?
As long as both of those check out, you're probably good to go. Nobody can see your mouthpiece from across the field anyway.

Of course, if either of those is an issue, you should check back here for some recommended options on a Tuba mouthpiece.
[Try searching first - the topic of good mouthpieces for marching tuba/sousaphone has come up a few times.]

I was a Euphonium player also (and still am) I tried the small mouthpiece route thinking it would be easier, but I actually found that on Tuba Bigger is better. The Euphonium and Tuba are different instruments, and to get the required sound from each you need different mouthpiece sizes. Although I've now settled on the Conn Hellberg myself because I like the tone I get with the sharper inner Rim, but I also had good luck with the Blessing 18. Do yourself a favor and see if you can borrow a bigger mouthpiece size to try for a week, and see what you think. The Kelly plastic mouthpieces are also an option to see what size works for you.

+1 on the Kelly. I have tried dozens of mouthpieces on souzys, and the Kelly 18 is about the best for projection, tone, and price point. Put a ring of golfer's lead tape around the outside of the throat to damp vibration so it is stable at dynamic extremes. Plus, it comes in school colors!

Don't worry about the transition in the diameter of mouthpiece. I switched from trumpet to souzy as a freshman in high school, and after about a week of getting used to it, I was good to go, and have been ever since.

I agree about the Kelly mpcs. They are good and cheap. Try a Kelly 18, or if you want a smaller one, the 25. A bass t-bone mpc will not make a good characteristic sound on a sousa or tuba - it will sound much too "farty" and not have enough richness/depth in the sound compared to a real tuba/sousa mpc.

Hey guys thanks for the input. But thanks especially to tubarepair. He recommended a shilke 62 and even sent me one, free of charge. He's a super nice guy and just an all around great person.

I have always felt that a Bach 18 was a very good Sousaphone moughpiece. I don't like blatty Sousaphone playing, and I have found that the Bach was easy to control, great volume when needed and good range. Fairly universal mouthpiece.

Steve