Miraphone 1291 5th Valve Venting
Posted: Fri Sep 11, 2009 11:14 am
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This doesn't directly answer your question, but...
I have noticed that when the manufacturer notes that a particular model has vented valves, it is usally on piston tubas, and it usually applies to only the 4 pistons. I have never encountered a vented 5th valve on a 4+1 tuba.
A more detailed explanation of valve venting might have been "lost in translation" when Miraphone was converting information to English or it could have been a simple error of omission right from the start. The Miraphone folks are generally pretty responsive if you contact them through the website, so if you really want a "from the horses mouth" answer, email them and ask about vented 5th valves.
Unless you are frequently pushing/pulling the 5th slide, I personally don't think I would find it necessary to undergo this procedure.
Even if you are, consider this...
I am temporarily playing a very fine 5 rotor Hirsbrunner HB-2 CC with the 5th valve slide setup for frequent, easy manipulation from an absolute top level professional orchestral tubist. I have noted that despite the setup, none of the valves on this tuba are vented, including the 5th.
I believe there was a rather extensive discussion of valve venting that went on within the last month or so. You might want to check that out for even more insight.
Good luck!