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.
3 comments:
Given the 12AU7 vacuum tube's / ECC82 vacuum tube's popularity - why is it that most audio designers with ears during the first half of the 1990s tend to favor to use the "older" 6SN7 vacuum tube over the 12AU7?
Maybe the 6SN7 vacuum tube can operate with much lower amounts of negative feedback compared to the 12AU7 / ECC82 vacuum tube - or maybe the 6SN7 just subjectively has a much better sound quality.
The older tubes have larger plates and operate at higher voltages. Some feel this results in lower noise and better sound quality. I'm not sure this is the case. Like transistors, tubes that measure alike across a large swath of measurements tend to sound about the same. It's the circuit and associated passive components that makes the real difference. And even then resistors and capacitors aren't going to matter nearly as much as the output transformer and attached speakers. The difference from one tube to the next in a circuit are small assuming the circuit isn't running the tube too hard. IMO
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