Product Review - A Comparison of
6550 and KT-88 Power Tubes - June, 2002
Among my first equipment reviews for this website was an
evaluation of the Bruce Moore Dual 70 tubed power amp. At that time, I praised
everything about the amp save for its excessive sweetness. Its timbre was not
true, I protested; most acoustic instruments do not sound this sweet.
For this reason, I initially decided against purchasing it.
Why did I change my mind and, a few months later, end up with the Bruce Moore
Dual 70 amp in my reference system? Simply put, because once I re-auditioned
it with tubes other than Svetlana 6550s that had powered it during my initial
evaluation, I found it far more to my liking.
Welcome to the tube game, the sonic equivalent of Clairol. Want more bass,
more warmth, more openness, greater depth, increased color, truer timbre, a
warmer midrange, less brightness, more focus, and/or a tighter bottom end
(don't we all)? Change your tubes. Often this will involve hunting down Old
Stock (OS) tubes, which become more costly and rare with each passing month.
But when your amp or preamp can take on different sonic characteristics simply
by changing a few tubes, who with the time and money to experiment can resist
This review recounts my experience with a host of 6550 and KT-88 tubes, all of
which I tried in the Bruce Moore amp. The different sonic characteristics of
these tubes are, for the most part, so distinct that, even in equipment with
an entirely different tonal signature, you can expect to experience similar
differences between tubes. You may end up preferring different tubes in your
amp than I prefer in mine - some amps need sweetening up, others toning down -
but I am certain that you will experience the relative differences between
tubes that I hear.
6550 and KT-88 are power pentodes used in a variety of applications. They
are closely related. Often, they are wired in "Triode" mode, with the screen
grid connected directly to the plate. The amount of screen coupling can be
varied, to produce what is called "Ultralinear" performance. There is a switch
on some amplifiers to allow you to choose Triode mode or Ultralinear mode,
depending on the need, since Ultralinear mode delivers more power.
JJ Tesla KT-88
This readily available tube came to me courtesy of Bob Bergner, main rep for
the Bruce Moore line of electronics (http://www.rbaudio.com).
Bob touted the JJ Teslas as eminently neutral. This is true. But there's far
more to add on that front.
I began using these tubes shortly before Mike Farnsworth visited the fair city
of Oakland to position the Talon Khorus X chez Serinus. The JJ Teslas remained
in my amp a few days later, when many of the Bay Area Audiophile Society
members who visited to audition my system commented that the Talon Khorus X
sounded flat and dry.
Some months later, when I told Mike that I could not get all that I wished out
of his speakers, he suggested that maybe my equipment wasn't good enough to
all that the Talon X can offer. Since my knowledge of my equipment suggested
otherwise, I decided I needed to make, at the minimum, a number of cable and
powercord changes. Some of my discoveries on this front are revealed in my
review of the Nordost Valhalla interconnects (coming soon); the rest will
appear in a month or two in a major powercord survey.
I made many changes to my system prior to Mike's return to upgrade the Talon X
crossover. By the time of his second visit, I had replaced the JJ Teslas with
the Electro-Harmonix 6550s I obtained from Bob Cohen at The Cable Company (http://www.fatwyre.com).
Mike was amazed at how much better my system sounded. I attribute these sonic
improvements primarily to the addition of Top Gun powercords and my switch
from JJ Tesla KT-88s to Electro-Harmonix 6550s.
Now, months later, with the Talon Khorus X sounding quite wonderful, I
returned to the JJ Tesla KT-88s.
For my listening experiments, I chose four discs:
1. The first movement, "The Pines of the Villa Borghese," from the fabulous,
award-winning Respighi: Belkis, Queen of Sheba, etc. disc performed by Eiji
Oue and the Minnesota Orchestra (Reference Recordings 95-CD).
2. My favorite "Blues No More" track from Terry Evans' Puttin' It Down (JVC
3. "Byambasuren Sharav's "Legend of Herlen" from Yo-Yo Ma and the Silk Road
Ensemble: Silk Road Journeys (Sony 89782).
4. The CD transfer of one of Mercury Living Presence's great monaural
recordings, Hindemith/Schoenberg/Kodaly/Bartok, with Antal Dorati and Rafael
Kubelik (depending upon the track) conducting the Chicago Symphony Orchestra
(Mercury 289 434 397-2). This CD may be bright and aggressive, but it boasts
lots of air and a wide dynamic range.
The Reference Recordings disc is quite vibrant and alive, with sound glorious
enough to win it a Grammy Nomination for Engineering and the Independent Label
organization's award for Best Recording of the Year. The disc is so good that,
unless a system is much too bright, it's hard for it to sound bad.
Good it did sound, and decidedly neutral in color. But, with the JJ Teslas, I
could not help noticing that, as much as the triangle rang in space, the space
seemed flatter than I was accustomed to hearing.
Moving on to Terry Evans confirmed my initial impressions. The cymbals sounded
quite delicate and lovely, Terry's voice very mellow. But the overall
impression I received was that the sound was somewhat dull and lifeless. Yes,
the timbre seemed real and correct, but the sound was not sufficiently
engaging. Drum snaps in particular seemed a bit flat and dull, as though the
transients had been smoothed over.
My impressions were further confirmed by listening to the Silk Road Project. I
remember being blown away by the drum thwacks that occur several minutes into
this track. Here, they were merely loud drum thwacks - nothing special. Again,
I did not find myself deeply drawn into the listening experience.
When Bob Cohen of the Cable Company sold me these tubes, he extolled them
as far more open on top and bottom than other KT-88/6550 tubes he had used.
To test out this hypothesis, I replaced the two JJ Teslas in my amp's left
channel with my usual E-H 6550s. This may seem like sheer madness, but, thanks
to manual bias adjustment, it presents no threat to either the tubes or the
integrity of the Bruce Moore amp.
First, I listened to the initial three discs in my review pile. It was quite
an interesting experience. Consistently, the left channel, powered by the E-H
6550s, seemed more alive, more resonant, set farther back in the soundstage,
and more filled with air.
There was only one way to confirm my impressions: play the Mercury
Living Presence mono recording, first through the left channel, then the
right, and compare the sound.
I chose "Vorgefühle (Premonitions)," the first of Arnold Schoenberg's Five
Pieces for Orchestra, Op. 16, which offers a wide contrast of instrumental
The experiment confirmed what I had heard on the stereo tracks. The E-H 6550s
offer far more freedom on top, far more air, and far more three-dimensionality
than the JJ Teslas. Most striking was my discovery that the trombones, set so
far back in the soundstage as to become a vague presence on the right channel,
took on real life on the left channel. For the Bruce Moore amp, the Electro-Harmonix
6550 proved vastly superior to the JJ Tesla KT-88.
[Note: My E-H 6550s and KT-88s have simple black and white, small print
lettering, as opposed to the large yellow lettering pictured].
I now returned to my old pair of Russian Svetlanas. For a while, Svetlanas
were unavailable in the US until a new importer returned them to our shores.
Though the labeling has since changed, the tube's innards remain the same.
The Svetlana KT-88s originally powered my Bruce Moore amp, having replaced the
Svetlana 6650s at the time I purchased the amp several years ago. Eventually I
replaced them because, as I continued to upgrade my system, I discovered them
too sweet. Now, with umpteen system upgrades, I feel far more capable of
distinguishing their characteristics.
Simply put, if I thought the JJ Teslas were dull, they sounded like heaven
compared to these tubes. I started listening to the Mercury Living Presence
disc. Monochromatically sweet, flat, truncated at both ends, everything
sounding alike, a distinct lack of air on high - like the difference in taste
between saccharine and properly collected honey. Ugh. I had the same
experience listening to the Terry Evans track: the cymbals lost their
ping, and Terry sounded like he just graduated from the Academy of Lost Souls
and was preparing for a new career as a Born Again Balladeer.
These tubes reportedly work quite well in amps whose sound needs sweetening
up. They also supposedly last for ages. While I would not question anyone who
found them appropriate for their amp, I would not choose them for the Bruce
Tesla KT-88/KR-Audio Electronics KT-88
When I attended CES 2002, Joe Fratus of ART Audio (http://www.artaudio.com)
urged me to audition what he considers the best sounding KT-88 on the market,
the KR-Audio KT-88 available from Dr. Riccardo Kron of Prague, Czechoslovakia.
A subsequent visit to the KR-Audio room revealed the doc in negotiation with a
potential US importer, Ken Chait of ATSI Advantage Tube Services (http://vacuumtubeaudio.com).
When import negotiations fell through, I learned that the KR-Audio Electronics
KT-88 and Tesla KT-88 (different from the JJ Tesla) are made in the same
factory, and may possibly be the same tube with different cosmetics and
labeling. I therefore obtained a quad of Teslas from ATSI for evaluation.
Alas, these attractive, copper-bottom Tesla tubes are incompatible with my
Bruce Moore amp. I was able to listen for only a few hours before they blew my
main and left channel fuses. Once the fuses were replaced, and everything in
the amp checked out fine, I turned it back on, only to hear the right channel
fuse blow within a minute or two. The tubes were returned and found to be in
perfect working order.
To test the presumed similarity between KR-Audio Electronics and Tesla tubes,
Joe Fratus, currently the sole U.S. source for the KR-Audio KT-88s he uses in
some of his amps, sent me a quad of KR-Audio KT-88s. These aluminum-bottom
tubes arrived packaged in beautiful, large foam insulated boxes, each holding
a pair matched to a set of specs, with values handwritten on a comprehensive
list. Regardless, they made it through exactly three minutes of warm up before
blowing the Bruce Moore amp's left channel fuse.
I don't know if the Tesla and KR-Audio KT-88s are identical. All I can say for
certain is that they behaved similarly in my amp. Of all the tubes I
auditioned, these are the only ones that proved incompatible.
Ken Chait has sold over 100 quads of Tesla KT-88s, and reports that I'm the
only person who has experienced an incompatibility problem. Joe Fratus and a
lot of other audiophiles love the KR-Audio KT-88. There's a 99.5% chance that
either "brand" of these tubes, identical or not, will work fine in your amp.
In fact, one or both may prove the best KT-88 you audition.
Valve Art KT-88
I received a quad of these tubes from ATSI Advantage Tube Services. They're
quite good, and work well in many amps. In fact, when I initially placed two
of them in the left channel, keeping the Electro-Harmonix 6550s in the right
channel, I couldn't tell which side I liked more. It was only when I made the
complete switch back and forth between quads that I ascertained that, in my
amp, the Valve Art KT-88 offers a leaner, less colorful sound than the E-H
6550. This was especially evident on the Terry Evans track; the superior
soundstaging and air between instruments that distinguish that recording make
it ideal for such comparisons. My hunch is that for amps with too fat a sound,
these will prove a good match.
When I reported the findings of this tube comparison to Bob Bergner of RB
Audio (http://www.rbaudio.com), he
repeated an oft-heard assertion that a manufacturer's KT-88s always sound
better than their 6550 equivalents. Knowing how jazzed I was about the E-H
6550, Bob immediately contacted various distributors, and learned that
Electro-Harmonix had just begun producing KT-88s.
As of mid-June, 2002, many tube distributors that carry Electro-Harmonix have
their KT-88 on backorder; I obtained my quad directly from Bob at RB Audio.
This tube looks identical to the E-H 6550 except for a change in lettering.
Simply put, in my amp, I find the E-H KT-88 superior to the E-H 6550. It
offers more color, more depth, more refined highs, and a tighter bottom end.
The sound is richer, fuller, and more exciting.
On a new recording of Mozart Piano Concerto No. 24 in C minor played by
Gilmore Award-winning pianist Piotr Anderszewski directing Sinfonia Varsovia
(Virgin Veritas 7243 5 45 5042 3), the strings, hardly the most
gorgeous-sounding on the planet, come across as less strident than with the
E-H 6550; the timpani have greater impact; the soundstage is more
three-dimensional; color is more pronounced; and, most importantly, the piano
itself is more clearly focused, with greater ring on top. This may not be a
five star recording, but it is far more satisfying when heard with the KT-88s.
On my favorite Terry Evans "Blues No More" standby, everything sounds better
with these tubes: the cymbal brush strokes are more focused and precise,
Terry's voice is rounder and richer in color and midrange, the bass is far
more focused and impactful, and the soundstage is more three-dimensional and
I auditioned the Evans track in numerous rooms at CES 2002, hearing it through
amps and preamps far more expensive than my own. I cannot forget how good it
sounded in two rooms equipped with the dcs Purcell, different models of Verity
speakers, Nordost Valhalla cabling, and amps more powerful and costly than my
own. Now that my Bruce Moore amp is equipped with Electro-Harmonix KT-88 and
Siemens NOS CCa tubes, and I too have Valhalla cabling, I am hearing sound
that approaches the level of richness that I heard on those significantly more
expensive systems at CES.
The Electro-Harmonic KT-88 is the clear winner of this comparison. It now
powers my amp. Regardless of whether it proves optimal for your equipment, it
deserves an audition.
- Jason Serinus -
Talon Khorus X speakers
Bruce Moore Dual 70 tube poweramp equipped with various 6550 and KT-88 tubes,
Siemens CCa tubes, and (at times) cryogenically treated Russian
Bruce Moore Companion III tube preamp with Siemens CCa and Chinese AU7 tubes
(rewired with Nirvana hook-up wire)
Theta Gen. 5A single-ended DAC;Perpetual Technologies P-1A with Monolithic
Power Supply;Audio Alchemy DDS-Pro transport
Sony DVP-NV500V SACD/DVD-V with detachable powercord
PS Audio P600 Power Plant power synthesizer with MultiWave;
PS Audio Ultimate Outlet;
PS Audio Power Ports in wall
Nordost Valhalla interconnects, digital interconnects, and speakercable;
Shunyata Python powercable on the transport;
Nordost powercable on the preamp;
Custom Power Company Top Gun High Current powercables on the amp andP-1A
Elrod EPS-1 cord on the Theta
PS Audio Lab Cables or Top Gun High Current cord on P600
Michael Green Deluxe Ultrarack, Basic Racks, and room treatment; Black Diamond
Racing cones and MG audiopoints; inner tube, maple cutting boards, bags of
sand also under transport; sand and maple also under preamp, amp, and P600;
homemade bass traps; Shakti stone atop Theta and Shakti On-Lines on some
powercords; Bedini Dual Beam Ultraclarifier, Audioprism Stoplight and
Blacklight, Gryphon Exorcist; Sheffield/XLO degmagnetiser and break-in disc.
Analog (hardly the strong suit of the system, rarely used):
Dual 1219, Sumiko Blue Point and a Classe 6 phono preampwith the optional
umbilical cord. Paired with Tara Decade and Nirvana SL-1 interconnects, and a
Shunyata Sidewinder powercord.