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While the C12 and the C12a share a capsule, the CK12 (of which there are many versions including the current AKG part made into a Nylon housing with an entirely different backplate than the original part), and they definitely were made by the same company, that's where any similarity ends.
The C12a is a very compact mic with a stylish stand mount that is integral (though detachable) to the mic. This mount had a modified Amplenol connector which will not stand the test of time and will break eventually. Fortunately there is enough room to drill & tap the part, rendering radical modification unnecesarry. There is an aftermarket part which allows you to change the base of the mic and use a simpler 414 clip (or similar) with the mic, but it requires changing the connector from threaded locking to XLR6 (boo. Alway go threaded locking when possible, if you're a pro). This aftermarket part makes an already service unfriendly mic moreso.
The C12a circuit is a very simple cathode follower mated to a low turns ratio transformer. Cathode followers (like this) have a gain of slightly less than one. For more information : http://www.valvewizard.co.uk/accf.html It utilizes a tube even smaller than a subminature, the Nuvistor, which was the future of the tube indiustry at the time of it's "collapse" (begining of slow death). It does feature quality internal components including a nice wet slug tantalum output cap which should last forever. The fact that this mic is a cathode follower makes objective interpretation of charachterizing the tube type (as frequently seen with other mic tubes EF vs VF 14, 6072 GE vs GE five star, AC701 vs everything else) unlikely. It is the combination of crude cathode follower and low turns ratio xfmr which places this mic in the "murky" audio descriptor. Yes, it has all the frequencies, but if you speak into it (while wearing headphones) I'd be surprised if you said it wasn't "murky". This murkyness does not lend itself to most vocal stylists, BUT, because of that very cathode follower/low turns ratio xfmr = low pecieved gain it does have excellent high SPL capabilities, while retaining the fullness of sound associated with tube mics. THAT is why they have value. Close up on high SPL sources, they are a great choice and make many other mics in the same spot seem like compromises. It is most often found with the small silver or black N12 (and variants N12b etc) psu which is a unduely compact and under rated for a C12, but functional enough for a C12a. It puts out 120v B+ and 5 or 6.3v (selectable on some units) for heaters, utilizing a good transistor capacitance multiplier circuit.
The C12, on the other hand, is an iconic vocal and drum overhead mic (even though the original 1950's pamphlets I've seen have a pix of an orchestra behind them). It is for the most part a classic plate loaded circuit, though is unique in the world as it's a combination of fixed and unbypassed self bias. This plate loading means the tube is utilized to provide gain (for more info: http://www.valvewizard.co.uk/gainstage.html) . The tube utilized is the GE 6072 in a single triode configuration with nothing but high quality components inside, include the paper in oil output cap and fragile T14/1 transformer. The first 200 C12's had a larger Hiller or Henry xfmrs, which had different (lower) ratios than the later and more commonly known T14/1. The lower ratios of the Henry's and Hillers result in more apparent gain and different frequency responses. Not to be confusing, but, the same ratio xfmr from three different manufacturers place in three otherwise identical microphones would yield us three different sounding microphones. It (as well as the C12a) uses a different scheme to connect the capsule to the tube than most classic Neumann, there is a 1000pF capacitor before the grid of the tube. I am not familiar with anyone arguing the merits or differences of these two systems. So in general the C12 is known as a very "open" if not bright mic, which this time is not just the combination of tube/xfmr/circuit but also now the single layer mesh super open headgrille. The C12 is best powered by the large brown N12 (not to be confused with the small silver or black units) which utilizes a companion S12 pattern select box. It also supplies 120v B+ and adjustable 6.3v from a Capacitance multiplier.