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Hi all, I have started teaching again after many years and have a couple of students with some issues I am trying to pin down. Both are older men 60ish. One is playing an old Yamaha Eb 4 valve and has some notes that refuse to speak cleanly around the F at the bottom of the stave. It's not the horn and I have worked with him to get the valve slides tuned. Tried a few different mouthpieces with no change. Better breath support helps.

The other plays tenor trombone, he has been playing for a long time and finding his tuning deteriorating and a buzzing edge creeping into his playing.

Any ideas or suggestions as to what is actually going on here and information resources?