Posted by Klaus on February 09, 2003 at 18:24:32:
In Reply to: Re: "double" tubas posted by B.G. on February 09, 2003 at 16:18:09:
The horn and the tuba are not comparable in this respect.
Even if we all know the wild expansion of the horn bell, that instrument for most of its length has a cylindrical bore. The principal difference between the F and Bb sides of a double horn is, that the F side has a much longer proportion of its bore being cylindrical.
A fact that actually makes some horn players prefer the F side over the Bb side, which can tend to be a bit too much like a euphonium in sound.
Of course makers like Paxman (of London) and Holton have addressed that problem. Holton has a Merker-Matic model (after Ethel Merker), where the Bb side has a smaller bore, than has the F side. These instruments are reported to have equalised the sensed differences in resistance to some degree. However I can not imagine tuba players accept their instrument having so long portions of their instruments being cylindrical, as it after all is found in all versions of double horns.
The best compromise of making instruments of extended ranges having egality of sound is found in compensating instruments. With two story rotary valves as found in the Paxman double Bb-F Wagner tuba. With long pistons as found in euphoniums and tubas following of being derivatives of the Blaikly patent.
The original and most common version of these, 3+1, inherently has a length of the main bugle running between the 3rd and 4th pistons. This length is used by most makers, if not all of them, to expand the bore, so that the bore of the 4th valve and of the compensation loops can be wider, than the bore of the 3 first valves.
That fact has two implications. First of all a desirable conical expansion is achieved. And the often complained, but not always existing, stuffiness of the compensated instruments is counteracted.
This distance of expansion does not come naturally to the compers with 4 front action valves. Instruments like your own Besson 983 take the effort to interject a seemingly superfluous loop between the 3rd and 4th valves, where that expansion can happen over a shorter distance than with the original 3+1 comp concept. The Paxman Wagner tuba has same expansion loop (to become documented sometime during my not so small project of making pics of as many brass designs as possible).
The only illustrations available, that I can point to are these
http://photos.groups.yahoo.com/group/yorkmasterpublicphotosx/vwp?.dir=/Besson+1935+prototype+front+action+euph&.dnm=Front+total.jpg&.src=gr&.view=t&.hires=t
(the expansion loop can be seen sitting to the right of the 4th piston)
http://photos.groups.yahoo.com/group/yorkmasterpublicphotosx/vwp?.dir=/Besson+1935+prototype+front+action+euph&.dnm=Backside+total.jpg&.src=gr&.view=t&.hires=t
(the expansion loop can be seen curling back towards the bottom bow)
http://photos.groups.yahoo.com/group/yorkmasterpublicphotosx/vwp?.dir=/Besson+1935+prototype+front+action+euph&.dnm=Valve+section+close-up.jpg&.src=gr&.view=t&.hires=t
(the text says it all)
It might shine through, that I am for the comp system. On conditions!
When the instrument has a comparatively wide bell like with the Besson 981 Eb tuba, that allows for wide slots, then intonation in the comp register can be steered also on the two troublesome chromatic steps right over teh open pedal note.
When the bell however is less widely flared like with the YEP641 euph, then a main tuning slide trigger is called for.
In the case of the Besson 983 Eb tuba, which has a narrow leadpipe and a not so wide bell, facts creating narrow slots, Mr. Sheridan has been quoted to say (possibly by you), that the only desirable after-market modification is the venting of the pistons. Because the best alternative, on the 983, to a slide trigger is to pull the valve slides.
Long again, but then I not even have touched the not so happy Alexander compensating double tuba, which has the larger bored compensating loops placed before the main valves of the F side. Retrograde step-bore is a bad idea in conical low brasses:
http://groups.yahoo.com/group/YorkMasterPublicPhotosVII/files/AlexanderLowBrass.pdf
or
http://photos.groups.yahoo.com/group/yorkmasterpublicphotosvii/vwp?.dir=/Alexander+low+brass+from+1985+catalogue&.dnm=AlexanderTubaB.jpg&.src=gr&.view=t&.hires=t
Klaus
(who apologises for the sickness caused in the Kansas City area)