This time I did a pair of C12s
I designed new power supplies with complete overkill in mind. A choke in the B+ and regulated heaters. An extremely slow time constant on the power supply means B+ ramps up slowly over the course of about 4-5 minutes.
I spent a lot of time on airplanes this year, and most of the design work for the power supplies was done in the air.
After simulating the supplies in LTspice (which I had to learn!), I laid out some PCBs and had them made up for me by JLCPCB in China. A small amount of prototyping/tweaking and they work really well.
These supplies need to provide 120V of ridiculously-clean DC, 1.1V of even-more-ridiculously-clean bias voltage, and 6V DC for heaters (also needs to be very quiet, but not quite as critical as the other two voltages).
A company in Germany makes a pretty nice enclosure that's sized right for a tube mic PSU, so I used those.
Guts:

On the bottom are two test points and an accessible trimmer (by the lower-left foot) to allow fine-tuning of the 120V B+ voltage without opening the enclosure

NOS smooth (unfaceted) blue glass jewel

The cable is 7-conductor shielded with an obsolete and hard-to-find (read: expensive, even used) "Großtuchel" connector on the mic end. For the other end, I used a currently-available and only slightly expensive Binder connector (similar to what you'd see on a U67).

Bodies are from FLEA in Slovakia, and they're fantastic. Historically-accurate to almost every detail... the only thing that doesn't look like the original to me is the nickel bead-blasted finish (which is truthfully more reminiscent of Neumann, and is actually much nicer than what you'd find on an original AKG C12). The downside is that they were pretty expensive... but really fantastic in every respect, including the historically-accurate capsule mount (most cheaper "clone" bodies don't duplicate this detail).
Capsules are from T. Campbell in Denmark, who's making the consensus-best CK12 capsules right now.


The vast majority of old C12s have a very nice, small transformer called the T14/1. A few early examples have a larger transformer (barely fitting the mic housing) by Henry Radio, with the model numbers V2148 or V3044. Even fewer (mostly Siemens-badged variants that are badged SM204) have an even-rarer variant of this variant called the V2545.
My favorite C12s always seem to have these earlier, larger Henry transformers, and the two very best I ever heard had the V2545.
This is probably a coincidence, but I put the word out a year or two ago that I'd love if someone could re-create that early V2545. There are many, many great re-creations of the T14/1, but not the larger Henry Radio iron.
An internet friend in Serbia who is a mic transformer genius (Sergio Moby) had a V2545 sent to him for rewinding (it had an open primary coil). He took the chance to painstakingly unwind and document it, as well as undertaking some metallurgic forensics to obtain not just the proper lamination dimensions and alloy, but also the exact correct annealing for magnetic properties. I got the very first ones he ever made, and they went into these mics.

He even duplicated the paper bobbins and the cloth tape. He really went to every effort to make these an exact replacement part for vintage microphones.
Resistors are NOS Philips RN65 military metal films, except the extreme high-resistance values (250M and 30M, two each) are Ohmite Mini-Mox.
The output coupling cap is a Sprague Vitamin Q (not exactly like the metallized-paper-in-tar Bosch/Siemens originals, but not worlds apart). The other capacitors are NOS Philips polystyrene film-and-foil.
I underwent some rather extreme research... poring over datasheets of old parts to get the exact variants of each part that I wanted. I now know more about mid-century capacitor part numbers than any 2022-era human rightly should. But it was a decent way to pass the time on airplanes...

Tubes are GE five-star 6072 types, triple-mica variety. These are the very best version of the very best 12AY7 type ever made, and they're certainly rare and expensive. But if there was ever a place to go for a tube of that quality, it's in a C12.
Different makes and models of tube can make an absolutely shocking level of difference in this application. If you think you can hear the difference between tube manufacturers in V1 of a guitar amp, you'd be floored by just how drastic it can be in a microphone head amp.

Pretty happy with how they came out

FLEA also made me some nice boxes, and were kind enough to delete their logo for me.
Speaking of logos, I'm in talks with a friend about doing a small bit of engraving on the body tubes just so it's a bit easier to tell at a glance which side is the front. It'd also be nice to serialize them so I can easily keep each mic with the specific power supply that was trimmed out for it. Right now I've got a little P-touch label stuck to the back, but that's not super elegant.

They sound pretty stunning; I'm very pleased. You never really know when doing your own builds whether it's going to meet your expectations. These definitely do. They do "the thing.

A friend has a few original C12s, and I went over yesterday and shot them out against his best one. As drum overheads they're pretty much indistinguishable... I'd easily use one of mine as a pair with his vintage one. Mine had a touch more low-end authority (which is consistent with what I always felt about the old big-transformer C12s).
On acoustic guitar and voice, there were a few more distinctions audible. Mine have ever-so-slightly less of a midrange scoop. This is another hallmark of the "big-transformer" C12 signature to me... almost like it has about 5% U47 quality about it. My builds actually sounded a touch better than the original on my voice, but this says more about what my voice likes than about the microphones.
Anyway, happy to answer any questions about the build process
