[Rimshot]

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What could cause a eflat alto to go flat. That is the mouth piece has to be pushed in all the way and one has to lip up. This causes chops fatigue and flatness of the high notes.
Rimmy.

 

[Saxplayer67]

Reed gone soft, embouchure needing more work, time for a new mouthpiece as your chops develop more, keycups too close? I'd look at reeds and embouchure first.

 

[tomalite]

Play long tones into your tuner til the neighbours are biffing rocks again. Seiko sat 500 chromatic tuner is the pick of the dozen or so I have tried and discarded. A new or harder read perhaps
A trap for newbies is a poorly maintained (sealed) horn, Dead giveaway is white knuckles and cramps. Your touch should be feather light.
a dirty mouthpiece, neck or horn.
Little trick I use when I'm travelling, "glad wrap" "saran wrap" cling film what ever you call it over your bell, finger a low Bb and suck.
If the horn is sealed the cling film will pop, if not you got a leak. Squeaks, flat notes or wild intonation, reluctance to form notes that do my head in til I find the reason.
If you need help find a sax tech that actually plays a saxophone.

 

[jbtsax]

The make and model of sax as well as the brand of mouthpiece would help to come up with an educated guess as to why the sax is flat.

John