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I have a friend who has a Conn 12J. He uses a Bach 24AW mouthpiece. The horn plays very sharp. He has his tuning slide almost all the way out and is still sharp. I tried my Symphony mouthpiece with it and it is significantly better, but still has the slide mostly pulled out. The horn has not been serviced in a long time. Other than having him change to a Conn 120 S mouthpiece, are there any suggestions on how to get this horn back in tune?

I noticed the same thing on my son's 12J. Even with a stainless steel Kellyberg, the slide was most of the way out. I do feel that a full size Helleberg style mouthpiece works best for these horns to get the most out of them, having such a small bore on a BBb tuba. Be careful with pulling the main slide too much, because on the three valve tuba all the valve slides are made long. You might find that pulling out enough to make Bb and F perfect makes many other notes flat. I considered making slide extensions but decided that would make many other notes just too flat.
If your friend is in high school or older, I am not the biggest fan of these tubas for anything other than very small ensembles. Outside of the bell flare these are very small tubas. In my opinion these are great horns for younger players but when pushed in a big ensemble they can get very ugly. My son's particular 12J has a lot of extra brass on the bell which helps to raise the usable threshold on it. (done by a previous owner, not me). He is already wanting a bigger horn after 2 years on the 12J but I am making him wait another year or two until he is taller.
Putting a .687 king valve set on these instruments would probably help a lot, I may try that and possibly even a cut to CC when he outgrowns his 12J.

I should have included that my friend is about 64 years old, plays in a city band and is a very good technical tuba player. I know, he should know better and work on playing in tune. I'm trying to help him with that.

I have an Amati ABB323 that's quite sharp. I use a tuba/sousaphone neck bit with a Conn Helleberg mouthpiece which helps my intonation. I had to get used to a slightly different playing position with the curved bit, but it wasn't drastic & I don't have to pull the tuning slide out as far.