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Are you absolutely sure about the correctness of your observation?

Holding down permanently the 5th&6th valves on an F tuba would turn it into a Db tuba with 4 valve loops all being too short to make any musical sense.

Klaus

Tuning on a 6 valve tuba is 5th valve = long whole tone, 6 the valve = long 1/2 step. Together, they tune a sharp 2 & 3 like the old Mirfaone 5th valve set up. They are never used alone, but rather in combinatiuon with other valves. I can't think of a situation where one would hold them both down, and play the other 4 valves, UNLESS the playing is in the 'Tritone of Death' between low b and pedal F.

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I was informed about this post by a student of mine.

On a rotary F tuba you cannot use the 5th and 6th valve to transpose the instrument,because usually are not provided with compensating system.
Some one use those valves only for low range and for some alternative fingerings.

What I do is to consider the 5th and 6th valves at the same level of the other 4, the reasons are many:

- intonation, if a note in a specific chord has a better intonation with an "alternative fingering" this become a traditional conbination in that situation, I don't want to be scared if I feel better (in some situations) to play an F with 4th and 5th instead without valves

- technique, there are much more possibilities to find the right fingering in a fast passage if you include in your choice 5th and 6th, ex. in VW Tuba Concerto I mvt, I play the trill (C-Dflat) playing the C with 5+1+2 and I trill with the second valve, the quality of the trill is much better (specially on a rotary tuba) instead using 0+23...you can try