Hello LR Rec,
the answer is yes and no,
to clarify:
If you are hunting for the best possible sound in your U47 the EF14 solution cost nearly the same than a good working VF14.
I did several EF14 conversion for so called modified U47, were some people gutted the mic and put some funky electronic inside. The cost restoring the mic to original splendor on the top of what people already paid would have been insane.
The EF14 is nearly identical from a technical point of view and only made by Telefunken and RFT, both tubes share the same part aside the filament, but there are some major differences in how those parts were put together.
The VF14 was a special request tube and due to the high filament voltage, the same cathode shape, same material thickness was used, but the VF14 has different base material and a special insolating coating inside the cathode sleeve.
Also the mica waver that hold the electron system in place, had a special coating to prevent creep currents plate to grid, plus a few minor other details that would to be to technical to get into.
All those production measurements were employed to insure a tube that could deal with super high grid leaks as it is required in a condenser mic circuit. Last but not least the VF14 was equipped with a special super high impedance/resistance tube base, here again to reduce leak currents.
Anyway, there is still a possibility to use a EF14, but you have to special select them, and only one out of ten made by Telefunken, and one out of 20 of the RFT made will be suitable for a high impedance amplifier, including the cost for changing over the circuit, changing the power sup, etc. the cost is nearly the same than a good working VF14 replacement
Even those two tubes are nearly identical, and even with the reduced filament simulation, the final answer remains, "DOES THIS MODIFICATION SOUND THE SAME?"
Knowing that even missing the target by 0.1mm is still missing, the answer has to be NO.
The EF14 works in the circuit with same technical specs, the sound is as close as can be...
So the individual mic user has to decide is the EF14 solution is good enough for him/her...
Hope that helps,
Oliver Archut
www.tab-funkenwerk.com
