aboulang@BBN.COM (Albert Boulanger) (09/04/90)
This is posted for a freind. Querys, etc. should go him, not me.
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Kevin R. Kennedy
d/b/a Kennedy Audio Laboratories
273 Barker Ave. #5
Lowell, Ma. 01850
Tel: (508) 441-1010
"THE GREAT 12AX7 SHOOT OUT ?!"
I think some of you audiophiles may be interested in the admittedly
subjective results of some listening tests I recently performed on a
variety of 12AX7's and their clones.
Two amplifiers were chosen for the evaluation, both of which use 12AX7
type dual triodes in their input circuitry. Both are loosely based on the
Dynaco ST-70 and one features triode circuitry thruout. The amplifiers
chosen were the PAD ST-70 with ultra-linear outputs and the Kennedy Audio
ST-70T, a short description of each is provided in the following paragraphs.
The Purist Audio Design (PAD) is a elegantly simple design utilizing a
single 12AX7 per channel as a cathode coupled (differential) phase inverter
with no negative feedback - loop or current in the voltage amplifier
stages. This amplifier is well suited to this application since all of the
colorations and distortion products are audible due to the absence of
loop feedback, output linearity is good due to use of a limited amount of
current feedback in the cathode circuits of the output tubes. Tube
generated hum and noise are very audible in this amplifier, assisting in
determining the overall quality of the tube. Overall performance of this
amplifier is excellent into my AR-58S loudspeakers.
The Kennedy Audio Amplifier ST-70T (KA) utilizes a single 12AX7 voltage
amplifier shared between channels driving a pair of cathode coupled
12AU7A's which in turn drive the respective pairs of output tubes. Open
loop gain is high enough to allow for up to a 20 dB feedback margin
depending on selected output mode. Local current feedback is employed in
all stages to enhance linearity and to allow for precise control of overall
loop gain and bandwidth. Overall design philosophy means to optomize
circuit performance in the face of varying tube parameters. This amplifier
will drive difficult loads with ease and is relatively unaffected by
variations in load impedance. Overall it has a slightly different sonic
character than the PAD, and overall speed and linearity are better. Slew
induced distortions caused by inferior tubes are quite audible when
present.
The testing was performed under relatively uncontrolled conditions,
with a very limited sample size and with only two amplifier designs, even
so the information should prove useful in a lot of cases. Source material
was mostly CD with a smattering of analog source material. (CD's - Dave
Grusin's Baker Boys album, Charlie Brown, Harmonia Mundi's Tarantula, as
well as the Stereophile test CD among others, LP's were Lars Erstrand and
Brothers Four on Opus 3, Respighi's Pines/Fountains of Rome on Chesky.) The
CD player is a heavily modified 14 bit Philips, the turntable an AR with a
modified Mayware arm, the cartridge a modified Shure V-15, the pre-amp. is
one of my designs utilizing passive equalization, and no over all loop
feedback. Speaker cables are Apature Signature 500, and interconnects are
various home brew items... (I know that this is far from the ultimate -
but wait until I'm making a little more money....)
Please bear in mind that the rankings are really only scientifically
meaningful in the context of the specific electronics used, but that gross
deviations from best to worst will probably be audible in most tube
equipment presently used. Usual disclaimer: I don't have any connection
with any tube vendors, or manufacturers, and furnish tubes only as a
courtesy to my customers. The following is a ranking of tubes based on my
sonic preferences in the two above amplifiers, from best to worst:
1.) Tungsram ECC83/12AX7A (Hungary)
Substantially superior in the KA and PAD amplifiers, very quiet,
clean and detailed. Superiority particularly pronounced in the KA
amplifier where its low distortion and high resolution were very
obvious to all listeners. Detail present that was lost with all other
tubes, excellent sound stage, lots of "air" between instruments,
excellent harmonic structure, very neutral and cleaner sounding
subjectively than all others.
2.) Philips ECG 12AX7WA/7025 and Sylvania 12AX7A (USA)
Slight hum and hiss audible in PAD, dead silent in KA, very detailed,
clean, transparent, quick. Very good harmonic structure, and "air"
between instruments, excellent sound stage. Fairly neutral sounding in
KA. Good bass extension in both amplifiers. Considerable loss of
resolution as compared to the Tungsram tube above.
RCA 6681/12AX7A (USA)
Evaluated only in the ST-70T, this tube is an enigma, not particularly
well known, if its performance in the KA amplifier is an indication,
it is worth another look. It easily outperformed the Sylvania 12AX7A.
Very clean, excellent harmonic structure, inner detail, sound stage
and depth, freedom from overload effects under all conditions, quick,
open, lucid mid-range, extended well controlled bass, accurate treble
under duress. I would rate this tube as better than the Sylvania, but
limited experience prevents me stating this to be the case under any
other conditions...
3.) GE/Sylvania 5751 (USA)
Slightly quieter than the Philips in the PAD amplifier, dead silent
in the KA amplifier, very detailed, slightly forward balance in both
amplifiers. Quick and clean, detailed, lively, otherwise very similar
to Philips ECG above. Frequently outperforms 12AX7's in pre-amplifier
circuitry.
4.) Brimar 12AX7WA/CV492 (Britain)
Slightly noisier than any of the above tubes in the PAD amplifier,
very quiet in the KA, good detail, fairly neutral timbral balance in
both amplifiers, clean, good bass extension. Good sound stage depth,
very pleasant without being euphonic, more neutral than 5751. Less
noise would have put it in the number 3 position... Excellent overall.
5.) Mullard CV492/12AX7MC (Britain)
Relatively quiet, some hum evident in PAD, dead silent in KA.
Clean, neutral timbral balance, good inner detail and sound stage.
Good bass extension. Agressive, yet controlled presentation with
excellent harmonic detail and attack characteristics, dynamic with
no sense of strain in the KA amplifier. Very similar to the Brimar
and highly recommended.
6.) AR 12AX7B (India)
Very quiet, slight hum in PAD, not tested in KA, very solid well
controlled bass response, slight thickening of texture in the
mid-range and some brashness in the treble, good timbral balance,
slight loss of detail, overall very good.
7.) India ECC83 Allegrosound (India)
Structurally identical to the AR, and "Allegrosound Frosted", very
quiet, slight hum in PAD, silent in the KA. Clean, well balanced
sound, some thickening of texture in mid-range, slight brashness in
the treble. Good harmonic detail, somewhat lacking in depth, with some
high frequency distress on crescendo's in the KA amplifier. These
Indian tubes are very similar overall, but the AR is selected for low
noise. Good.
8.) Allegrosound 12AX7A Frosted glass envelope (Indian manufacture)
Significantly noisier than any of the above, substantial hum in PAD,
quiet in the KA. Very solid bass response, well controlled, slight
"chestiness" in mid-range, slightly brittle treble, good detail in
the PAD. Not so great in the KA amplifier, additional distress on high
frequency crescendo's and loss of harmonic detail are quite evident.
Not too bad, it seems to be a better match in inherently slew limited
applications.
9.) EI 12AX7A/ECC83 Penta, National, Procomm, etc. (Yugoslavia)
Fairly noisy and moderately high level of hum evident in the PAD,
fairly quiet in the KA. Substantial loss of detail in both
amplifiers, thick almost syrupy mid-range and soft, sweet treble.
Bass extension is ok, but loss of control is evident to some extent
in both amplifiers. This tube is good in the PAS-3 however.....
Quite inexpensive, but barely acceptible in some applications and
good in others. This is the same tube that some vendors sell for
$25.00 each and up - not worth the expense in my opinion, they sound
no better than the $2.95 version of the tube.... Not recommended,
except to those on a serious budget, or PAS-3 owners.
10.) Shuguang 12AX7A @#**?!?@#!!!! (Chinese)
Very quiet, some hum audible in PAD - one of the quietest tubes of
the lot.... Too bad it doesn't sound better. Rough, edgy almost
shrill sound quality in the treble regions, very dry, antiseptic
mid-range, lack of harmonic detail, shallow sound stage, bottom end
lacking in punch in both amplifiers. Fairly poor sound, but very
consistent and reliable. These don't sound very good in most pre-
amplifiers either... Not recommended. (Strangely enough some early
samples from around 1983 seem to sound better!?)