The Telefunken VF14 Tube used in the original U 47s has become incredibly scarce. To purchase a NOS (New Old Stock - a 70 year old tube kept in a deep freeze) VF14 can cost between $1500-$2000… if you’re lucky, and that price is only going to go up as the years march on.
So, you’re faced with some options in building your “new” “U 47” microphone. You can plop down $2000 and get your true-to-life original VF14, or you can find a suitable substitute like the EF14, UF14, or… dare I say it… even the EF86.
The VF14 was made by Telefunken in the era of the tube based AC/DC radio receiver. Sadly Georg Neumann chose this tube for his fated microphone just as that market was beginning to dry up. Telefunken shortly thereafter chose to discontinue the production of the tube due to cost issues. Since the VF14 wasn’t made for use in microphones, it wasn’t quite suited for the spec, so a high percentage of the tubes were ill-matched for use in the U 47. Those suited for use in microphones were marked with an “M.” So if you’re buying a VF14, look for the VF14M.
Disclaimer: If you’re restoring a true U 47, get a VF14. Just do it. The amount of value your mic will lose with another tube + the alternate wiring you’ll need to compensate will equal far more than just plopping down the $2000. It’s egregious, but what can you do?
Now, you must understand that ANYthing other than an original VF14 will make your U 47 a bastard child in the eyes of the purists. And absolutely… it will not be a “true” U 47. So, purists… read no further! Turn back now! Abandon all hope, ye with sticks up your butt!
I’m not restoring a true U47. I’m building a mic from scratch. I have no inherent value to lose. So I’m going to use a different tube, and—if you’re building your mic from scratch as well—I think you should join me. My top choice is an EF14, which is identical to the VF14 in every regard, except for two. Number one: it requires 6V instead of 60V for the heater. Number two: it costs about $1800 less than a VF14. Why am I sure that the sun will still rise tomorrow if I use an EF14 and not a VF14? Well, Wunder Microphones, which makes fantastic U 47 replicas, specifically states that they use EF14s in their remakes and that the use of a VF14 isn’t worth the increase in cost. I’m sold. How about you?
Now, if you’ve been studying up like I have, your next question will be: “Russ, the original VF14 heater was notably underpowered for its application in the microphone. If the heater voltage is different in the EF14, do we still have to underpower it?” The answer is yes! power the heater of your EF14 with only 4.95V instead of the 6V indicated in the spec sheet. All else should take care of itself.
The other benefit of the EF14 is that it is shaped exactly like a VF14. In fact, if it weren’t for the “EF 14” marked on the top, you couldn’t tell the difference. This means, that if at some point down the road you go all Daddy Warbucks and purchase a VF14, you can slide it right into the EF14s place, make a couple mods in your circuit, and you’re good to go.