Even though the 12AU7 / ECC82 vacuum tube is very popular in hi-fi audio circles, is an audio designer justified in choosing another type in the name of sound quality?
By: Ringo Bones
Some audio amplifier designers often chose to use the 6SN7 tube in place of the 12AU7 / ECC82 in almost every application in the name of sound quality does it still make the 12AU7 / ECC82 the most popular tube in high fidelity audio circles? Maybe, but it seems this somewhat controversial practice of replacing a more recently designed vacuum tube with something from a previous generation of vacuum tubes a sound audio engineering practice?
The 12AU7 is a miniature 9-pin (B9A base) medium gain dual triode vacuum tube. It belongs to a large family of dual triode vacuum tubes which share the same pinout (EIA9A). The 12AU7 vacuum tube was first marketed back in October 18, 1946. Known in Europe under its Mullard-Philips vacuum tube designation ECC82, in fact, the 12AT7, 12AU7 and 12AX7 or the ECC81, ECC82 and ECC83 are all exactly the same vacuum tube but with varying electrode spacing and grid pitch to achieve the different characteristics.
There are many equivalent vacuum tubes with different names, some identical some designed for ruggedness, long life or other characteristics, examples are the US military 5814A and the European special quality E82CC and E182CC. There is also the 6067 SQ version, B329 GEC version and the 6189W military version. The 12AU7 tube is very popularly in high fidelity vacuum tube audio and is used in low noise line level preamplifier, driver and phase splitter, phase inverter in vacuum tube push pull power amplifier circuits. There are opinions which indicate that this popularity is caused by the audiophile industry / hi-fi review polarization as opposed to its true audio performance. The 12AU7 was originally designed and intended as a low impedance driver vacuum tube. Gains of around 12 are typical for a single stage, but more can usually be squeezed out of it if needed.
Its special quality versions like the E82CC and 5814A are used in specialized non-audio applications during pre-solid state semiconductor days in early digital computer circuitry. Using the special quality versions outside of the purpose they were designed for – like hi-fi audio use during the 1990s – may not provide the intended optimal results. For example, a version of the 12AU7 vacuum tube intended and optimized for digital computer logic gate use may be designed for long life without succumbing to cathode poisoning when mostly switched to low current mode in switching applications but with little attention paid to parameters of interest intended for linear – as in audio – applications such as linearity of transfer characteristics, matching between two sections in push-pull phase splitter audio amplifier applications, microphony levels, etc.
In high fidelity audio applications, the 12AU7 / ECC82 vacuum tube can deliver quite large currents at low anode voltages, making it useful as a cathode follower when a small amount of power is needed to drive the output stage. As a simple anode follower, the 12AU7 / ECC82 often finds itself as the last stage in preamplifiers or as a driver for parallel-pair output stages. It is also sometimes used as a phase splitter / driver. If used in a cathode coupled configuration, anode resistors must be adjusted to obtain balance.
Sound quality can vary depending on which type is used and of course how it is used. Generally, a typical 12AU7 has quite a dull sound, but this could be viewed as lack of coloration. The 12AU7 – like the 12AX7 – is used by a lot of hi-fi audio equipment manufacturers like Audio Innovations and Border Patrol just to name a few.