Long 4th valve slides

[Mark]

I suspect that getting a custom 4th-valve slide on a 4-valve 191 would end up costing more than buying a 5-valve 191.

[Art Hovey]

On my Frankentubas in BBb I always try to make a long-pull 4th valve.
In order to get a low C just above pedal Bb with all 4 valves down I need to pull the slide out about 7 to 8 inches, if it is initially set for a perfect 4th. When it is pulled out that far the 4th valve alone gives a tritone.
I rarely use that option, however; usually it's easier to get the low C using the "false pedal" tone fingered 1+2.
I do use the 4th valve slide frequently to correct the 2+4 fingering for low B and low E, which requires a pull of about 2 inches. (Sometimes it's more convenient to pull the second valve slide instead.) And sometimes it is helpful to push the 4th slide in a couple of inches to correct the 124 fingering for low Eb.
Example:
http://www.galvanizedjazz.com/tuba/frugalhorn.html

[pwhitaker]

On both my Rudy 5/4 BBb and my Miraphone 186 recording BBb - both 4 valves - I'm able to leave the 3rd slide pulled out about 3 inches and the 4th all the way out, and have the 1st slide out about 2 inches and only pull it another 2 or 3 to get the low C. Both of these horns have good intonation on the 5th partial 1st valve and the 2+3 valve notes. The low B comes in very cleanly with just the 3rd valve down. The low Eb slots well with 1+4, and the low D is OK with 3+4. This might be idiosyncratic of these horns, which have the same tube configuration - FWIW. I don't play quintets or wind ensembles any more, just Trad jazz, so this tuning might not be good enough for those situations.

[MartyNeilan]

Playing through a longer slide allegedly adds some stability to a note in the lower register and consequently more projection. For example, I sometimes use the long 4th slide on a low F and finger it 1,4 rather than 5,4. This seems to make the note more solid and aggressive!

I don't know if this is fact or opinion/psychological or truly affecting the way the tuba plays...but it works for me.

Two possible physical explanations for this:
1. Pulling a slide effectively makes for a larger bore. This can make a dramatic difference in feel when going from an F to a flat E pull on a bass trombone.
2. Your 5th valve is probably a rotor. If your 1st is a piston, there is a reasonable probability that the piston blows more open than the rotor (some exceptions being Rotax, the softball sized rotor W-N uses, etc.)
Then again, it may be all in your head after all. Take off the Vader mask and get some O2.