aboulang@BBN.COM (Albert Boulanger) (05/29/90)
A friend of mine is producing a high end audio newsletter. Please, if
you have comments, write to him directly. This is Part 1 of Issue 1:
The First Newsletter Kennedy Audio Laboratories
This is the first of a regular series of newsletters where I hope to
express some of my viewpoints on things that I feel to be of importance in
audio. This is such a subjective field that the views I express may be
diametrically opposed to yours, but prove me wrong and everyone interested
in such things should benefit.
The first topic I should like to address is the poor sonic quality of
those ubiquitous Chinese 12AX7A's. I find them to be harsh sounding with
poor frequency balance and lack of inner detail. These tubes share some
design features with VHF triodes, such as a rectangular cathode structure,
plate contruction and an internal shield between the two sections. I
surmise that the strained sound quality of these tubes is due to poor
linearity and I will attempt to verify the hypothesis. (Please note that I
don't use over all feedback in most of my designs and that this phenomena
may be less audible in high feedback applications.)
A Chinese friend of mine tells me that the quality of these tubes is very
unpredictable because of the quota system employed to set production
guidelines. In place of these tubes I would recommend the cheaper
Yugoslavian tube made by EI which somewhat reassembles the old Telefunken.
It is not the most detailed, but it is extremely quiet and pleasant
sounding. You can often get them from Penta Labs, Triode Electronics, and
Antique Electronic Supply. My favorite 12AX7A is made by Philips ECG and is
available from Newark Electronics for about $9.00 each.
A tip for those of you who own AR SP-11 pre-amps. is to replace the
OEM tubes with Amperex 6922 when they wear out. You'll be surprised at the
difference these tubes can make. The 7308 should also be interesting, but
I haven't tried it.
I have also been investigating various brands and types of output
tubes for replacement use as well as for my Citation II power amp. which
uses KT88/6550A (yes I know they're not identical). For general use you
can't go wrong with N.O.S Sylvania's, GE's, or Amperex. Today most Philips
ECG tubes are very good. I have some vintage KT-88's knocking about the
place and have listened to them extensively and I can tell you that they
do sound different than the 6550A. They are detailed, smooth with good
ambience and depth, a little dark sounding, but overall I prefer the GE
6550A for its speed, neutrality and resolution. How many of you will write
to tell me that output tubes have no sound of their own? I think this
should be audible to some differing degree in any good power amplifier,
and I'm not talking about merely replacing any worn out tubes with good
ones either. About Chinese KT-88's, they aren't! These tubes are exactly
the same as the various versions of the 6550 the Chinese make. In less
demanding circuits these tubes do perform well and in some cases even
sound really nice. GSI uses a good version of this tube, I have some that
I used in my Citation for over six months with no problems and I admit
that I was very pleased overall with the sound/service I got from them.
You can recognize them by their "Coke Bottle" envelopes otherwise known as
the venerable ST-16 bulb. YOU really should have an RCA/SYLVANIA tube
manual, what a wealth of information to be had....
Sometimes you can significantly improve the performance of a tuner or
pre-amplifier just by replacing tubes, even when they test ok. This seems
to be particularly true of types; 6BL8, 12AT7, 6U8 and all heavily used
Telefunken's. All vintage gear should be re-tubed unless they have low
hours before making any judgement of merit.
For more information and an extended newsletter please send an SASE
to: KEVIN R. KENNEDY/ dba KENNEDY AUDIO LABORATORIES
273 BARKER AVE. #5, LOWELL, MA. O1850