Somewhere on this site there's a good article on how to tune slides on a euphonium. It's the same for tuba. Find that article.
If by the "upper slide" and adjusting it, you mean to refer to the 1st valve slide that sticks up, then ...
It is common for tuba players to use this slide to adjust the pitch dynamically as they play, on certain notes. I did this on my Cerveny BBb and consequently almost never used the 4th valve for intonation issues. HOWEVER, ...
You can do this ONLY if that slide is lapped to the point where you can (a) easily move it by hand as you're playing (almost like a trombone hand slide, but not quite so slick), and (b) not lapped so much that it won't stay in position (so NOT like a trombone hand slide to that degree) if you want it to. This is a bit tricky to achieve, and if you want to go in that direction, I recommend you take it to a (good!) repair tech and have it done. If you do that then ...
You should have the first valve vented. Without venting the valve, then when you pull or push the slide as you're playing, you'll create either a vacuum or a positive pressure that resists moving the slide. In addition, you'll get a little "pop" if you use the valve when there is such a vacuum or pressure. You really want to avoid both of these things. If the additional expense isn't too much, have all the valves vented. There's no reason not to, and it makes adjusting slides a bit easier. Venting piston valves is very simple and involves virtually no labor -- so shouldn't be too costly.
Lapping the first valve slide (on a non-compensating horn) is well worth doing and learning to use -- IF it's done correctly.
Last edited by ghmerrill; 07-14-2018 at 10:00 AM.
Gary Merrill
Wessex EEb Bass tuba (DW 3XL or 2XL)
Mack Brass Compensating Euph (DE N106, Euph J, J9 euph)
Amati Oval Euph (DE 104, Euph J, J6 euph)
1924 Buescher 3-valve Eb tuba (with std US receiver), Kelly 25
Schiller American Heritage 7B clone bass trombone (DE LB K/K10/112/14 Lexan, Brass Ark MV50R)
1947 Olds "Standard" trombone (Olds #3)