So I have lately been thinking about the 5th valve on CC tubas which made me think of my own ole 5/4 rotary M-W CC. I had not really played this horn much in the past 2.5 years since I last used it in a small regional semi-pro/glorified civic orchestra; I had just doodled on it a couple times over this period. Then I remembered that M-W had advertised how you could use the 5th valve on their CC tubas (esp. the M-W 32) as a BBb and how it could be a transition for the BBb player switching to CC or vise versa. I had always wondered HOW you could do this to keep the 5th valve depressed.
Wellllll.....I pushed the main tuning and 5th valve slides all the way in and pulled all the other slides a bit. Then I cut some foam tube and stuck it between the 5th valve and brace to keep it depressed/pressed down. I stuck a mouthpiece in the horn and fooled around a bit. Surprisingly, it plays decently as a BBb tuba

. The horn settled a tad low in pitch, so I decided to cut the 5th valve slide an inch. I wont go into details because it took me a long time to get it fitting right and looking good -- lots of 'rounding out', cutting, sanding, cleaning, etc. But after I cut the male and female slides, I got the horn to play "right on" with a little wiggle room, intonation-wise. The pitch tendencies as a "fake BBb" are very different than as a CC - ALL open notes are smack dab in tune as a BBb

. There are no flat 3rd or 5th partials - F below the staff and D in staff are well in tune. As a CC, the G/Gb bottom line is flat. However, there are a few "funky" notes as a BBb - the Eb's are quite sharp

and I have to use some strange fingerings for them if sustained/longer than a typical 8th note (1,2,4 for Eb below staff, 1,2,4 or 4 lipped up for Eb in staff), but overall the pitch is fairly even. I guess those are similar to Alexander fingerings
The response as a BBb is not as good as a CC though, but the sound is pretty much the same. It's pretty weird using the 5th valve as a main tuning slide on top with the regular main tuning slide on bottom. I think after playing it a while getting used to it and settling on a mouthpiece, it will be even better
Is anyone here doing this or have tried this with any of their horns?