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I’m envious of cellists’ ability to extend and retract a metal rod that takes the weight of their instruments.

Has anyone tried to make a similar device for a tuba?

You mean the Stewart Stand?

Not really, but close. The Stewart Stand provides a foot to go on the chair between the legs. My BAT's bottom is already below chair seat height when I'm playing.

I'm thinking of something that could drop to the floor. In my dream-world version, it could be extended a little (say 12-15") for playing while sitting or much more for playing while standing, all the while giving me an anchor to the floor.

Same thing. Longer pin.

Is it, though?

I don't see that it's been made with a longer pin. The one on the Stewart seems to go at quite an angle (not straight down from the bottom bow or even nearly so). It would need to be telescoping and very strong, like a photographer's monopod or tripod leg. The hard part, it seems, is mounting it on the horn -- or mounting the pin/leg on whatever is mounted to the horn.

No. Make a copy of that design with an appropriate length and thickness to support a tuba. It is a simple design. It is just a euphonium endpin strapped to the bottom bow. Look at photos and make a design out of paper, take it to a saddler and have him make it for you. The hardware to hold the pin and make it removable/adjustable might take some time to source, but again, photos will give you ideas.

And why the inherent weakness of hollow, telescoping tubing and not a solid, single piece of steel rod?
And you can set the angle yourself for one of your own design.

In direct answer to your question: No. No one has made this for a tuba and commercially sold it. You could be that guy…

Maybe I just will! Thanks!

I'm thinking of hollow tubing just because it is telescoping and therefore adjustable. If it were a solid rod, I'd need a few different ones based on length.

Depends on the method of adjustment.
You could use hollow rods from an adjustable cane.
Or, solid you could use an adjustable string endpin and slipstop.

You could use hollow tubing in several lengths and secures with those little round nubs like this:

https://www.acehardware.com/departments ... gJINPD_BwE

You would then just need a short socket on your bottom bow and could have different lengths for sitting (either going to chair or to floor) and for sitting and perhaps a couple or really short ones for fine tuning the height.