One of the difficulties with the great old Martin detachable-bell tubas is finding a suitable detachable upright bell. The original 20" Martin uprights are relatively rare and not all that good in my opinion (thin metal and not much resonance). The Kanstul repros can be ok if you get a good one. They have a slightly larger diameter (approx. 21") than the original Martin bells and are shorter. The metal is also a bit light.
The collar on the most common size of Martin BBb tubas is 7 and 7/8", limiting the selection of bells that can be cut and adapted successfully. Most will be too short if they're cut where they need to be for the collar. After some searching, Martin Wilk suggested a Fafner bell and so I asked him to cut one for me. The Fafner bell is just under 19" which tightens up the sound nicely, but more importantly it comes in at just the right height when cut for the collar. The pitch is just right with the Fafner plus the sound won't break up at higher dynamic levels, (a problem with both the original Martin bells and Kanstul repros).
Admittedly, Fafner bells are not cheap plus you will have to locate or fabricate a tenon to fit a Martin,but the results I have seen are just great. The top-action 3-valvers are not going for too much money and if you are willing to sacrifice a recording bell you can harvest the tenon for an upright bell. As far as putting an entire uncut Fafner bell on a Martin as a one-piece bell goes, I don't know if that would work. I've never looked into it since the detachable bell doesn't bother me.
Here are two photos showing the bell with both a side-action and top-action Martin. The proportions just look and feel right to me. Incidentally, it was amazing for me to watch Martin cut and adapt this bell for me. He's truly a craftsman. I left the bell unlacquered which accounts for some of the smudges and spots you might see.






