when not to be a purist...
There are some mic companies I won't name who have taken wild liberties with the mics they say are 'inspired' by this or that... like using a K67 capsule, 12AY7 tube, tiny mic body, and still calling it a 47 as some folks I've seen do... (guys, at least use a K47)... I guess I'd certainly be a purist compared about half a dozen other companies making affordable reproductions, no doubt; but I never thought of myself as that. Quite a few makers I know really are purists, and I admire the heck out of them for it. But here's a few things that might interest you to know, where I'm not really a purist and quite content to not be...
One thing that makes a good preface to this conversation is the realization that the acoustic measurement tools available to most people today were simply not available half a century ago or more. A lot of things were guesswork and relied upon by ear. I still think the ear is the most important tool for evaluation; but its also good to sometimes get a verification on what you think you're hearing. For instance, if you ever see a really close U47 replica, its capsule is generally raised within the headbasket. This is a traditional thing, to have a high capsule mount, to where the chrome or nickel bar from the top of the headbasket literally crosses in front of the top portion of the capsule. I've never heard of a convincing reason why this is done; but it is tradition, and purists carry it forward. I don't. I've run sweep analysis of a K47 capsule in a U47 headbasket at different raised positions. That crossbeam in the line of sight of the capsule produces an interference pattern that slightly raises the bands that most consider within the sibilance region for most voices (i.e. making 'SSSS's slightly worse). Granted, a K47 already has a slight bump here and doesn't need any more of it. I am not superstitious enough to believe that Georg Neumann meant to do this and I shouldn't dare question it. I respect the hell out of these guys; but I also know the limitations of the tools they had to work with and I see things like that for just what they probably are: an engineering oversight. So, its not something I reproduce, the heck with tradition. I've only ever had one person notice this; and when I explained why, they fully agreed.
Here's another one... Now, I have made historic 47 units before with the proper Binder type connector and nickel-plated aircraft aluminum tube body. They sound fine; but feel a bit lightweight in my hands, to me. I do know that this is historically correct; the first units were light aluminum bodies although the headbasket was brass and everything was nickel plated. All brass body replicas came in later; and this is what I actually prefer to use. Much heavier, and I feel that there's a mild effect on the sound, although the headbasket area and body tube don't acoustically interact much on most mic bodies because there is a dividing plate. I imagine aluminum was less expensive to come by at the time than brass, and the same is true today. What I do know, however, is that the lighter a metal is, the higher its resonant frequency will be compared to heavier metals of the same dimensions and size. Brass probably would resonate longer than aluminum when struck in a basic accelerometer test due to its mass and stored inertia; but the lighter aluminum body would likely be more subject to picking up resonance from its acoustic environment. Nitpicky as hell? Why yes... but the first time I made a U47 with the truly historic metals, I heard a difference and I personally favored the brass bodies I had been using, even if they are less historically correct. The differences were minuscule, don't really affect the overall quality of the product; but I felt that I had heard them.
One last one... One reason I haven't been in a super hurry to use bodies with the much more costly open-frame traditional insulated headbasket switch for pattern selection is because I have a hard time believing it would be of higher fidelity than the remote-controlled (on the PSU) audiophile relays I use now. The way I do it now, while not historically correct, is with a very expensive vacuum sealed, silver-tipped reed relay. The relays are potted and mu-metal shielded, and even mounted to the board with a touch of acoustic dampening adhesive to make them immune to vibration. It's hard to imagine the open-frame switch being as clean as that; even though its possible that they could be almost as clean, at least when new. I do know that those switches are exposed to the elements; the copper is interacting with oxygen in the air, attracting particles, and physically striking the surface of the adjacent contact when switched. Kept clean, they're fine; but you don't want to see what these switches look like on 60 year old models sometimes... they can look like something Indiana Jones discovered. Just a propensity to attract dust and to oxidize. With a sealed audiophile relay, you are going to get perfect switching, basically forever. For the life of the unit. It's free of air and epoxy sealed, dampened and shielded to boot; it is just never going to corrode or change in its behavior. At least not for the life of the unit. Something tells me that if cost and R&D were no object, they'd have done this originally. The electronic relay was invented in 1835 and the evacuated reed relay was in use by the 1930's; so it surely could have been possible. I just think that there was a mandate to keep costs down on the U47 and to keep the design as elegantly simple as possible; so they just manufactured a high quality switch in-house. So, I've just never been in a hurry to do it. Nothing but respect for those who do. It's hard to get right, and takes some 3D modeling skills. I get requests sometimes; but it just doesn't quite excite me the way it does some, for the technical reasons above.
Anyway, those were the things on my mind today. :)