The 12AX7 Vacuum Tube: The Miracle Postwar Vacuum Tube?

After almost single-handedly ushering in the post World War II hi-fi revolution in America and the rest of the world, can the 12AX7 preamplifier tube lay claim to be as the “miracle postwar vacuum tube”?

By: Ringo Bones 

Currently used in myriads of products intended to sound musically consonant to the musically-trained human ear, the 12AX7 small signal thermionic vacuum preamplifier tube seems able to resist and be dethroned by the tide of solid state progress of reliable silicon transistors and ultra-miniature integrated circuit operational amplifier revolution of the mid 1960s onwards. But does anyone still remember the origin of this humble and ubiquitous small signal preamplifier tube that almost single-handedly usher in the “miracle” of postwar hi-fi revolution? 

The 12AX7 small-signal vacuum tube – also known as the ECC83 in the European Union – is a miniature indirectly-heated dual triode vacuum tube notable for its high voltage gain. It was developed around 1946 by a team of RCA engineers in Harrison, New Jersey under the developmental number A – 4522. The tube was released for public sale under the 12AX7 identification code on September 15, 1947. The 12AX7 was originally intended as a replacement for the 6SL7 and 6SN7 family of small signal dual triode amplification tubes, but the 6SL7 and 6SN7 soldiered on because they are much better sounding when used in zero negative feedback single ended triode audio amplification applications. The 12AX7 tube is so popular with high fidelity vacuum tube based amplifier enthusiasts during the Golden Age of Stereo onwards in its ongoing use in both vintage and authentic replica vacuum tube based high fidelity audio equipment makes it one of the few small signal vacuum tubes in continuous production since it was introduced.  

In truth, the 12AX7 tube could be regarded as the most widely used modern low level, small signal vacuum tube known to mankind. A double triode with 12.6-volt / 6.3-volt series / parallel heater, B9A base and high amplification factor make it convenient to use in a wide variety of audio amplification applications. Gains of 60 to 70 in one of its stages can quite easily be achieved. This allows one section of the 12AX7’s triode (or ECC83’s triode) stage can quite easily be achieved. This allows one triode section of a 12AX7 tube to replace a pentode in a lot of circuits, leaving the other triode section free for some other use. 

The very high gain of this tube also makes it ideal for cathode coupled phase splitters, ensuring accurate balance. The Mullard 5-20 used one for exactly this reason. The 12AX7 tube is quite linear, but because of its high impedance, it doesn’t accept high input / output signal levels. A good 12AX7 / ECC83 has a warm, smooth sound, really tubey sounding – the Sovtek made ones, like the ones stamped with the Electro-Harmonix brand, really excel. 

Noise may be a problem in moving coil pre-amplification stages because of the 12AX7 tubes’ rather low transconductance or gm. Though it haven’t stopped many leading electric guitar amplifier manufacturers during the late 1980s and early 1990s for making 12AX7 tube based electric guitar preamplifiers that use 7 compliment of 12AX7 tube gain stages able to do gains of 90dB or 1,000,000 despite of the resulting noise. The 6DJ8 or ECC88 tube or the ultra reliable military spec Sovtek 6922 small signal preamplifier tube is way more suited for such applications. 

Even though the 12AX7 tube / ECC83 tube is primarily a preamplifier tube, an audio engineering genius by the name of Tim de Paravicini of Esoteric Audio Research managed to use the 12AX7 preamp tube as the output stage of the power amplifier he released back in 1998 – the Esoteric Audio Research EAR Yoshino V20 which uses 10 pairs of 12AX7 tubes in the output stage. The preamp tubes were connected in Enhanced Triode Mode, a circuit configuration originally applied to triode-connected pentode output power tubes, whereby the grid is maintained positive with respect to the cathode, so that the grid normally flows. Under these conditions, a typical thermionic vacuum tube now behaves effectively as a current-controlled device – rather like a solid-state transistor – than a more normal voltage-controlled mode of vacuum tubes. Though Tim de Paravicini is very much a genius when it comes to using unusual tubes in audio or using familiar audio tubes in unusual applications, such audio engineering iconoclasm allowed him to use the Enhanced Triode Mode principle to use the PL509 tube – a tube normally used as a saw tooth wave generator for the vertical deflection amplifiers of old TV sets - into a very good sounding PL509 tube based audio amplifier during the early 1990s.