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What length tubing has to be cut from a trombone-length instrument that is 20 cents flat with the tuning slide all the way in? Asking for a friend.

pull the slide until it's 20 cents flatter than it is right now, and you'll have your answer.

Mary Ann wrote: ↑Sun Jan 26, 2025 1:40 pm What length tubing has to be cut from a trombone-length instrument that is 20 cents flat with the tuning slide all the way in? Asking for a friend.

An easy non-mathematical way to get close is to mark the playing slide with a mark or a thin strip of tape were it enters the cork barrel.

Ignoring the failure to function at A=440, just make a note of where some given pitch tunes (sure: 20c flat) in 1st position.
After having made that observation, move the playing slide out until the same "pitch" is now 20c FLATTER. Measure the distance, multiply by two, and that's the amount of too-much length (to achieve A=440...ie. B-flat=466) there is...

...and the same thing can be done with the instrument's TUNING slide, obviously.

A couple of considerations are these:
- To be able to play ABOVE A=440 (as so many ensembles do) will require MORE shortening than that (perhaps an ADDITIONAL total of an inch - 1/2 inch per "side" - would be advisable...??)
- To FLATTEN an instrument by 20c requires ADDING more tubing than to SHARPEN an instrument by 20c...but not that much difference with an instrument that's only 9-feet long (in its shortest config).

I'd probably do the observations using the TUNING slide (as it's easier than with the playing slide) and - particularly if the two "legs" of the tuning slide bow come out fairly straight for a good bit) un-solder the tuning slide bow, shorten the ends by ["determined amount plus another 1/2 inch in order to be able to play ABOVE A=440] and solder it back in place. (It might - probably-would - be desirable to expand the large end of the tuning slide bow just a bit, and shrink the small end - just a bit - prior to reinstalling the tuning slide bow.)

unlikely confusion:
Above, I referred to the total amount to shorten...Obviously (if shortening a tuning slide bow) it would be done in equal HALVES (left-side/right-side) of that amount (the same as the amount of PULLED distance observed when observing the pulled-out length required for yet another 20c of "flatness").

bloke "Read @LeMark's answer...much more concise - lacking all of the typical blokeblather...THOUGH (again) consider shortening the instrument MORE THAN the minimally A=440-achieving amount.

Thanks but it isn't a trombone. It is trombone length and does not have a slide. I could theoretically figure that out with my P-bone but it has been noted by others that my chops will play the pitch I want even if the slide is quite far from the "correct" position for that pitch. Drives them nuts sometimes.

Mary Ann wrote: ↑Sun Jan 26, 2025 5:14 pm Thanks but it isn't a trombone. It is trombone length and does not have a slide. I could theoretically figure that out with my P-bone but it has been noted by others that my chops will play the pitch I want even if the slide is quite far from the "correct" position for that pitch. Drives them nuts sometimes.

Do this with a two-minute-borrowed trombone and reliable player, and you'll still have your answer.

Mary Ann wrote: ↑Sun Jan 26, 2025 5:14 pm Thanks but it isn't a trombone. It is trombone length and does not have a slide. I could theoretically figure that out with my P-bone but it has been noted by others that my chops will play the pitch I want even if the slide is quite far from the "correct" position for that pitch. Drives them nuts sometimes.

I am jealous of good horn players for this trick.

Well, if there is 100 cents in a half step in tempered tuning, then it seems that if you measure a tuned second valve slide port-to-port, you then know that 1/5 of that length is 20 cents.

The mathematics of it is a little bit more complicated (involves the 12th root of 2), but having a trombone player pull out his main tuning slide until he put he's playing 20 cents flat, and take away a smidge from that, is less complicated.