Product Review - A Comparison of 6550 and KT-88 Power Tubes - June, 2002

Jason Serinus



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 the temptation?

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 . 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 ( 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 XR-0014-2).

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.

Electro-Harmonix 6550

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 color.

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].

Svetlana KT-88

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 Moore.

Tesla KT-88/KR-Audio Electronics KT-88

When I attended CES 2002, Joe Fratus of ART Audio ( 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 (

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.

Electro-Harmonix KT-88

When I reported the findings of this tube comparison to Bob Bergner of RB Audio, 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 engaging.

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 -

Review System:

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 6922-equivalent tubes;

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.

© Copyright 2002 Secrets of Home Theater & High Fidelity
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