I have a 25-J Conn tuba. When I tune this instrument I am always slightly flat, but when I do manage to get my B flats in tune it is with all slides pushed in. While keeping the slide positions the same, my F will be extremely flat, and my C will be very sharp. I do not have this problem on any other tuba, therefore it is not an embouchure problem. I would like to know if there is anything I can do to fix this and if so how I should go about that?

What year was this 25J made?
In order to play in tune with others I have had to have the main tuning slide on my older Conn's shortened.

It is a 1959.

Yep, I had 5/8ths cut off my main tuning slide of my 1959 horn to be able to play A440.

Ok!! Thank you very much!