I've built a few that are 'loosely' based on the C37 capsule. There are a lot of metal parts in the original, all with rather fine threads. I don't have the correct tooling for my CNC to make those fine internal threads. One nice thing about this capsule design, however, is that the tension can be set in-situ. The diaphragm only needs to be glued to its mounting ring with even, loose, tension. Mine follows the same acoustic principles, and a very similar hole pattern, but I used a glued-in diaphragm and diaphragm spacer arrangement like an M7 or M8. I don't really have much intention of making an exact clone of the C37 - for one, I don't have a vintage C37 to compare it to - I only have access to a pair that I redid the diaphragms on. To be quite honest, I don't think I'll be making a lot more mic capsules. It's getting a bit tiring and I'm trying to learn to play the bassoon again.
I don't really like redoing diaphragms because although the mic sounds good, I can't reference an original unless I have one that hasn't been wrecked. That is usually not the case. Also, although I can tell 'good' from 'mediocre', I have a hard time consistently telling apart 'good' from 'excellent' - there are many people in the business (including the host of this forum) with more skill at that than I have - this is a hobby for me, and not my living.
In this case, a previous owner decided to 'clean' the diaphragms off. He did that to a Nylon CK12 as well. Fortunately, the C37 is easier from a repair perspective because the tension adjustment is not permanent - in fact, the capsules can be matched fairly easily with slight adjustments of the tensioning adjustment. Surprisingly enough, looser tension does not necessarily mean more bass output on this capsule design.
The output of the C37 capsule is limited primarily by the acoustic design. The output is roughly 10dB lower than the M7 capsule (for the same sound pressure level). There are only sixteen blind holes in the C37 and around sixty on an M7.
This also means that you can't just take any old 6AU6 tube and shove it into a C37. I had a microphonic C37 on my bench, and we just had to live with it. All of my replacement 6AU6 tubes had too much noise. One was almost acceptable. I had about twenty of those tubes. Maybe one in a hundred would be ok? Or maybe another manufacturer? Even the Altec M11 'Coke Bottle' puts out much more signal, it'll tolerate a bit of hiss from the tube and not be so noticeable.
The mics can be retensioned but you need to know the internal structure. All of the little spanner wrench holes in the back need to be used in the proper manner. They can be serviced!!! The outer set is a lock ring. The inner set holds the backplate. You need to loosen the lock ring first, then you can adjust the backplate in or out to adjust the tension, then tighten the lock ring. You will need to have the correct wrenches for the job and you can't buy them at your local hardware store.
-Dale Ulan
10,000 Cows Recording Studio