Alternate Low A extension design
regular bell (right) low A bell (left)
A few years ago, I tried this low A extension concept which worked well. Then during the pandemic, I was inspired by a picture of the Moosmann low A extension I saw on Kristian Ronnes‘ website. This approach involves creating a longer bell joint for the bassoon that has additional keywork and tone hole for the lower note.
I purchased a “cheap” Chinese bassoon that had the “gentleman” design for the bell. On this bassoon, the low B and Bb keys are on the same section of the horn making it longer than a standard bell. I bored out the Chinese bell so it would fit nicely on the top of the bass joint of my Fox. Since the low B natural is already present, the “gentlemen’s” bell adds two additional half steps below that as opposed to a standard bell which only has one half step.
Then I fabricated some additional keywork so that with the flick of a switch (similar to awhisper key lock), the Bb thumb key activates the closing of both holes on the extended bell. In this way, it plays low Bb or A.
Reach up and flip the lever to the left to engage the low A
Construction:
Initial testing demonstrated that the new bell was sharp and therefore few inches short of what’s required for a properly tuned A. I added an extension which was taken from the top part of an old Linton bassoon and woodsmithed to fit onto the longer bell. This extends the length a bit more than 2 additional inches.
Steve Wilson turning the wooden piece that extends above the white ring. This was needed for intonation purposes.
With the intonation solved, I sanded the whole thing (thanks, Bill Jasper!) to remove the bright red finish it had come with.
A dark stain and spray polyurethane refinished pieces to match standard Fox brown. 
I was able to find a case that fits perfectly so I can easily carry around the additional bell when its needed for certain pieces like Mahler and Wagner.
The result is an impressive, functional extension that looks like it came from the factory!
Low A bell on a Fox 220