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I've just received a 1930 Buescher 690 sousaphone and noticed something odd in the plumbing. The leadpipe goes directly to the tuning slide which is tapered. However, the taper goes from large to small (.775 to .765) in the way the outer slides are set up. Everything looks to be original and unmodified plus the horn responds ok-I've just never seen a bore going from large to small before. So for all of those in the know out there, would there ever be a reason to design a horn this way?

The instrument itself is wonderfully preposterous: a 30 inch bell with a 9 inch collar and a bore averaging .770 or so. A nice light horn for quintet work...

I'm thinking manufacturing tolerance as opposed to any actual thought about increasing or decreasing bore size.

No, this is definitely not just a manufacturing inconsistency. The difference is great enough that it is impossible to reverse the tuning slide (plus you can actually see the rather quick taper in the crook itself. But thanks for the suggestion.

As per my PM I have my doubts about it being changed but I will take a closer look tomorrow. The bell throat is 9" at the collar and when it gets to the joint at the bell flange it measures just short of 12"

Paul, Contrary to what we have all been taught to belive It often works to reverse the taper. I built a Bnatural tuba some years back for Lew Waldeck. I installed an rotary assending valve before the valve set and reversed the taper It worked great The horn played open and free. No intonation problems. I believe its the straight tubing thats the killer. Including extra valves. I know you have a few 3 valve horns. I have used a 3 valve on 80% of the work Ive done. 3 valves always play better than 4, 4 than 5 etc Its all the straight tubing. I am suprised that the horn you have has reverse tapers. but , not really. The old American makers Built horns they weren't influnced by todays myths and wives tales.
Id love to see the horn someday. Maybe we can get toghter and play trios on bell front 3 valve Martins with Heter!!!!
Ray Noguera

I have read that the Meinl-Weston 45SLP F tuba has a fifth valve with a bore that is smaller than the 4th.

I double checked that with the MW website and apparently, it's true:
# 4th valve 20mm / .787˝
# 5th rotary valve (right hand thumb operated)
# bore: 19,5mm / .768˝

I have also read that the valves on the Miraphone firebird are similarly odd like that, the 5th is before the first valve, but is larger than the first valve.

I think this is where I read that:
viewtopic.php?f=2&t=33949&p=299482&hili ... ne#p299482" target="_blank

Ray,
Given that information it now makes more sense to me why Buescher did this. The combination of bits, neck, tuning slide and tubing to the valve block add up to well over 2 feet of tubing. Even with a gradual taper, if the tubing had continued to expand to the valve block the resulting valve-bore would be pretty huge (and it's big already). So it appears that in order to avoid cylindrical tubing they reversed the taper after expanding it beyond the actual valve bore, (perhaps on the theory that any taper in any direction is better than no taper at all!). The sudden "de-taper" occurs in the very short crook on the main slide (picture the main slide crook on an old King sousa-a very tight turn). I wonder if this quick change somehow compensated for the necessary cylindrical tubing of the tuning slide. Since the main slide is located before the valve block, the bugle after the block can expand smoothly to the bell.

Whatever the reason is for this design, IT WORKS. Pitch and response are excellent (as you described in the Waldeck horn) and the instrument is surprisingly easy to play. BTW, what ever happened to Lew Waldeck's BBb Conn with the two bells. What a nice instrument!

Thanks for responding and yes, you Mark and I should have a tuba-sousa-beer get-together over here soon!