EF12(Spez) replacement tube

The Phædrus Audio EF12(Spez) is a drop-in replacement for the EF12 as the active component in capacitor-microphone impedance-converters.

The EF12(Spez) is a Phædrus Audio Electronic tubeTM: a modern emulation of every aspect of the original thermionic device. The technology is described here.

The Telefunken EF12 tubes are increasingly seen as favourites for substitution for the VF14 device in mod's, clones and copies of the famous U47/U48 microphones. Sadly, even these low-voltage heater devices are now becoming rare; at least, those with sufficient performance for use as a microphone impedance converter.

The Phædrus Audio EF12(Spez) is designed as a drop-in replacement for the EF12 (or the EF12 Spez) tube in U47/48 type circuits. Because the EF12 has a slightly different pinout to the similar EF14, we now produce a fully "plug and play" pin-compatible EF12 Phædrus Audio Electronic tubeTM.

What's Spezial about the EF12 Spezial

The EF12 Spezial Ausführung (special version) had a slightly different pinout to the standard EF12 in that the metal casing connection was broken away from the pin which carries the cathode, the suppressor grid and the casing connection in the case of the standard tube (see pinout diagram).

This was done (at the time) to improve performance at RF/IF frequencies, but there are certain advantages at audio frequencies too. Ideally pin 4 (M) should be taken to a local ground. (The Electronic tubeTM will work without this extra connection, but better performance is possible if it this direct ground connection is installed.)

As with the newer EF14M Electronic tubeTM, heater polarity does not need to be observed to offer maximum compatibility with a wide variety of microphones.¹

Click for the datasheet for the EF12(Spez).

The Phædrus Audio EF12(Spez) is available in a standard and Red Dot version. Electrically, the EF12 (Spez) is similar to the EF14M Phædrus Audio Electronic tubeTM


1. EF14M Electronic tubesTM, prior to serial number #1131, heater polarity must be respected: thus, terminal h+ must be positive with respect to terminal h-. From #1131 and above, this restriction is no longer present. The restriction is not present in all EF12 Electronic tubesTM.


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