Stop tuning. Don't tune anymore. Just say no to tuning.
I never tune. And I almost never move my tuning slide. I might as well not have one. When the oboe plays an A for the brass, I don't even pick up my horn unless it's just to screw around. What's the point of it?
My thesis here is this: A bad trombonist plays out of tune on an in-tune trombone, and a good trombonist plays in-tune on an out of tune trombone.
Trying to tune-up with the band or orchestra is a fool's errand. We play a giant tuning slide, after all. Each note will be in or out of tune depending on what you do with that particular note, and tuning to an A or a Bb before starting is absolutely useless. It's worse than useless, because it deceives you into being lazy and thinking your horn is in-tune.
The very best advice I ever got was from Roy Main, my teacher at the time, who once suggested in a throw-away comment that I push my tuning slide all the way in and leave it there, and simply play in tune.
The light just came on. Doing that was a huge revelation for me in both listening and playing, and it forever changed how I play.
Bass trombonist Bob Sanders writes the following anecdote on his website:
My point is that if you're tuning, you're missing the boat. Each note has its own place on the slide and on your chops, and it changes depending on the harmonics of the moment and where everyone else is playing. Good players don't rely on their tuning slide, but listen to every note and make small adjustments so quickly that you don't even notice that the note may have been out of tune when it started (although it probably wasn't in most cases). I haven't moved my tuning slide in months, and I play in-tune without difficulty.For a decade or so, I played a Conn 62H with no tuning slides whatsoever – main or valves. I had Larry Minick tune it so 1st position was about ½ half inch out. It responded great all over the horn – and had a super light-weight hand-slide. So . . . one night at Donte’s, Bill Watrous noticed this and told the audience, “I thought I was arrogant. I don’t pull my tuning slide, but he doesn’t even have one!” [After all, the trombonist has a couple feet of tuning slide in their right hand.]
Here's my suggestion to you: Stop playing with your tuning slide. From now on just leave it wherever it happens to be right now, and just listen and play each individual note in tune.
You're welcome.
Okay, commence firing!