Inside your expensive and prized vintage Neumann mic there might NOT be a tube!
Just because a mic is old doesn't necessarily mean that it is a tube mic. We know all about tubes - they have warmth and presence, qualities that transistors simply do not have. Yet during the 1960s, tube mics were phased out of production and gradually transistorized mics came in. And they just don't sound the same!
So if you want the genuine tube sound, then a genuine Neumann mic, such as the U47, will be the one for you. (That's the tube U47; they made a transistor one later, which actually is not a bad mic at all, and will cost you a lot less.)
Your vintage tube U47 will be a joy. A joy to handle, a joy to use and a joy to listen to the sound it produces.
But are you sure there is a tube inside?
The Neumann U47 uses the venerable VF-14 tube. The VF-14 was developed for portable communications equipment way back during World War II.
Needless to say, it ain't made anymore.
So what can you do if the tube in your U47 becomes noisy, or simply gives up the ghost, as all tubes will sooner or later?
Well you could buy another. There are still stocks around, but expect to pay $1500 or more, just for one tube.
Or, you could have the mic converted to transistor operation. Yes, it can be done, but I doubt if you would want that.
There is a middle path, and that is to convert the mic to a Nuvistor. The Nuvistor is a miniaturized vacuum tube, built in a metal can rather than a glass envelope.
It was originally introduced by RCA back in 1959 as a rival to the transistor that was just coming into popularity at the time. However, although the Nuvistor had advantages over the early transistors, none of those advantages remain, except for the fact that it is indeed a vacuum tube and can hopefully be expected to produce the characteristic tube warmth.
So if your VF-14 fails, that's your best option at a reasonable price. You can buy a converter from Music Valve Electronics, and it doesn't involve modifying your mic.
However, if you have a U47 that's working fine, you might care to take a look inside. Perhaps someone has swapped the tube for a Nuvistor already. Or - perhaps a transistor amplifier!
Now wouldn't that be a surprise?