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You're hitting your head on every stroke, try choking up on the shaft.

Oh, sorry I thought you played golf.

What I think my learned colleagues are saying is the best advanced techingue to do this and most things on the tuba is practice. There are ways of imporving your practice time, but it comes down to the way to be better is to spend time on the horn. To clear up this note you might try sluring chromaticly to it slowly focusing on good tone and intonation. I would do that everyday for about 5 minutes of your practice session. The note will clear up.