Tube Power Amp Reviews

Ron Cox, Zen master and good friend from Zuni, New Mexico, gingerly navigated the crowded streets of Amp City—the essentially all-tube amp collection sprawled on my listening-room floor between the speakers. Ron had no trouble spotting the four chrome-and-black chassis of the JA 200s. He pointed a tentative finger: "Are those the Jedi?"

Jedi, Jahdee, Jahdiss, Jadiss—I wasn't really sure. But one thing was beyond doubt: These amps indeed befit a Jedi Knight. The oversized transformers, massive chassis, and elegant profile bespeak nobility.

The JA 200 is but one step removed from the Jadis JA 500—the summit of the Jadis line. Like the JA 500, the JA 200's power supply occupies a separate chassis, connected via an umbilical cord to the amp proper. Remove the tube cage, and the eyes feast upon a sea of GE 6550A power pentodes: ten per channel. An awesome sight, but a sobering one—think of the retubing costs. As with an exotic racing car, not only is the price tag steep, but the maintenance costs are high.

Technical details
The heart of any power amp is its power supply—the reservoir of current for the output stage. Many a brilliant design has been sabotaged by an inadequate supply, or one that was slow to recover from overload. Jadis did not skimp here in terms of parts quality or reservoir rating. Solid-state bridge rectification is used exclusively, followed by an RC filter network. Filament voltages are regulated, but, as is almost always the case, plate-supply voltages are unregulated.

Using the two halves of a 12AU7A dual-triode tube, the input/phase-splitter stage comprises a Schmitt cathode-coupled circuit. This clever push-pull stage has rarely been used in audio amps, primarily because it requires two tubes and offers only moderate amplification. It was originally intended for scientific instruments (eg, oscilloscopes) for which extended low-frequency response is desirable. Coupling caps are not required, and hence frequency extension to DC with good balance is easy to achieve with this circuit. The phase splitter is DC-coupled to a driver stage that uses a 12AX7A dual-triode. Thus, the entire front-end of the JA 200 is DC-coupled.

The output stage uses five pairs of triode-connected 6550As in push-pull fashion. Each output tube is individually fused as protection against excessive bias-current draw. Other output tubes may be used. Fanfare International's Victor Goldstein, Jadis's US distributor, tells me that customers worldwide also use EL34 power pentodes and KT88 beam power tetrodes. I have also experimented successfully with Gold Aero's KT99A. Victor strongly warns, however, against the use of the Chinese KT88. Apparently, the failure mode of the Chinese tube is so rapid that the fuse offers little or no protection from circuit damage. Accordingly, use of the Chinese tube will void Jadis's warranty.

Two pairs of speaker binding posts are provided, wired in parallel to facilitate bi-wiring. The output transformer is configured at the factory for a nominal 8-ohm load. It may also, however, be configured internally for 1, 4, and 16 ohm nominal loads.

The amp is intended to be powered up in two steps. Setting the On/Off switch to On applies less than full B+ voltage to the output stage. After a five-minute wait, the user should move the Operate/Standby switch from Standby to Operate. Optimal sound quality is reached after about an hour of operation, as internal temperatures reach their ideal "cooking" level.

Preliminaries
I discovered that the JA 200 is quite sensitive to AC-line–borne hash and RF noise. It is essential to provide, at a minimum, some form of line-noise filtering. A pair of Berkleonics CEA-25 2kVA isolation transformers (one per channel) brought about a dramatic cleansing of harmonic textures. Even though such transformers are only effective in filtering common-mode noise and do nothing for normal-mode noise, the improvement was akin to a good hosing-down of the soundstage. Layers of grit and grain were washed away. Harmonic textures sounded purer and sweeter, as if more music and less garbage were getting through.

It's no surprise, therefore, that Victor Goldstein has found AC power cords to make a difference with the JA 200s. Sonic differences among cords are due to the degree to which they provide some small measure of RF filtering. Properly, the problem ought to be tackled at the wall with the right tools. Once this is done, there is no reason to invest in exotic AC power cords. In my opinion, such money is simply flushed away. Maybe it's not snake oil, but the recent craze in power cords seems to me to be little more than a blatant attempt to capitalize on the Audiophilia nervosa epidemic. Save your pennies and buy a genuine line-conditioner. [It is fair to point out that Victor Goldstein does not recommend AC line conditioners with the JA 200.—Ed.]

Listening impressions
Having listened to scores of amps over the years, it gets easier and easier for me to form a sonic baseline that defines the performance level of the ordinary or average amplifier—and a piece of cake to pick out the extraordinary. The JA 200 fits into the latter category. In two fundamental respects, it sets a new standard by which to gauge the state of the art.

With the 1993 Albuquerque Hot Air Balloon Fiesta scheduled to take place just before this issue hits the newsstands, I thought it would be appropriate to fashion an analogy based upon this colorful event.

The time is just before mass ascension. A stream of hot air is inflating a collapsed balloon. As you strain to decipher the image painted on the balloon's crumpled exterior, imagine that the image represents the soundstage. At this stage in the balloon's inflation, the image is collapsed, its true breadth and depth difficult to discern. It takes quite a bit of mental gymnastics to try to imagine its true extent. So it is with most power amps: soundstage dimensions are merely hinted at, not fully revealed. Sometimes the depth perspective isn't fully fleshed out; at others, the impression of width collapses, to near-mono proportions, to the center of the soundstage. This compression of spatial information is at the core of my aversion to most solid-state gear, and continues to fuel my infatuation with tube gear as the giver of life to the spatial illusion.

The balloon, now fully inflated, strains at its tether, the full glory of the image revealed at last. So it was with the Jadises: They consistently made the dimensions of the original recording space abundantly clear, and strengthened the impression of standing on the threshold of the original space. The JA 200s sculpted the extent of the soundstage so that the original space appeared to pop out, expanding to fill the front of my listening room.

Another remarkable attribute was the degree to which the Jadises illuminated the soundstage's inner recesses. This transparency made it easy to gaze far into the soundstage—all the way to the back of the hall. Most amps muddle or fuzz the soundstage, making it difficult to resolve spatial layering. It's like trying to glimpse an object through a field of tall grass. Not so with the Jadises. Not only were instrumental outlines sculpted with great precision, but they were illuminated as if by a searchlight. The JA 200s blazed a trail right up to each spatial outline. It became easy to pinpoint outlines within the soundstage with respect to their relative locations in the hall. The spatial perspective was so believable that I felt tempted to pack a lunch and go exploring in this newfound field of sonic dreams. The total effect served to more effectively engage my attention, to draw me further into the illusion of "live." This gestalt of being there is a crucial attribute that separates the merely good amp from the truly great one.

Retrieval of low-level detail bordered on the best that solid-state has to offer, without the latter's propensity to rub the listener's nose in it. At no time did the JA 200 sound analytical. The lower treble lacked tube glare and the harshness of odd-order harmonic distortion. Treble transients didn't sound etched, but were allowed to decay smoothly into a velvety-black background.

The notion of inherent upper-octave aggression was totally lost on the Jadis. Yet, when the music demanded attack, it was perfectly capable of portraying the proper degree of brass bite. The treble sounded fast and in control, without the deplorable tendency that so many tube amps have to soften the leading edges of musical transients. That's not to say that it sounded exactly like good solid-state. Its voicing was more liquid and a shade softer, with the sort of natural flavor one might experience in a middle-row seat at a concert hall—but not at all like the blunt and toothless presentation of, say, vintage Dynaco tube gear.

At last, here was an amp fully capable of harnessing the dynamic range dished out by the Jadis JP 80MC preamp. Even with average-sensitivity speakers, loud, complex passages were allowed to expand and bloom without congestion or stress. This is an area that many audiophiles miss out on, married as they are to minimonitors, wimpy electrostatics, or other gutless planar speakers. With a speaker that can boogie (like the MartinLogan Quest Z), the JA 200 is capable of revving your heart rate. There are but a handful of tube amps out there with the Jadis's clean power reserves.

A musical joy for me is a large chorus in full voice, and here the JA 200s really delivered. The spatial finesse exemplified by smaller tube amps was here in spades. But just as impressive was the muscle and ease with which the Jadises lofted a full orchestra and chorus to dynamic peaks. Mountain climbing was never this much fun.

All of my listening was conducted with the factory-set, 8 ohm transformer output tap settings. It's possible that bass performance might have tightened up with, at least, the ML Quest by deferring to the 4-ohm connection. In general, however, I felt that bass extension and definition were quite remarkable for a tube amp. Tube amps lack the current drive and bass damping of solid-state output stages. A transformer-coupled output stage depends on a benign impedance magnitude for optimal power coupling to the load. Often, however, the impedance magnitude below 100Hz is far from flat, and can't be described as 4, 8, 16, or whatever ohms. Peaks, and especially dips, in the impedance curve cause trouble for tube amps. It's possible for a woofer's impedance to fall below its DC resistance value because of the shunting action of a crossover network. I would never attempt to mate a tube amp with a load whose impedance falls below 3–4 ohms in the range below 500Hz (the power range of the orchestra). Those sorts of loads are best left to a Krell or a Mark Levinson. With both the Mach 1 Acoustics and the Nestorovic speakers, however, the Jadis acquitted itself very well in terms of bass extension, punch, and definition.

Many years ago, H.A. Hartley said that, while treble extension beyond 12kHz or so didn't much matter to him, the lower the bass extension of his system, the more realistic and enjoyable it sounded. I basically agree with his viewpoint. Given a choice, I would opt for flat response to 30Hz rather than treble extension to 20kHz. It's exciting, therefore, to find a tube amp that can at least compete with the transistorized big boys. It's clear that the Jadis was designed to excel in the bass. From its multitude of output tubes, colossal output and power transformers, and front-end design, the JA 200 possesses the resources to succeed in the lower octaves.

Trouble in paradise
Despite my admiration for the Jadises, I wasn't totally in love: they weren't doing enough for me in the midrange.

It is vitally important that the music's inherent harmonic colors are fully fleshed out. The music's true colors demand accurate reproduction of the harmonic palette. With the JA 200s, the upper mids turned slightly gray and dull, as if a dark cloud were blocking out the sun. The sweetness and sheen of violin overtones and soprano voice were blunted, at least to the point of fuzzing over some harmonic nuances. The vitality with which these instruments sang was subdued. But it doesn't take much of a swing of the harmonic compass from sunny and sweet to overcast and gray to tick me off.

Neither was I totally enamored of the lower mids, which were on the lean side of reality. I don't mean to describe the patient as suffering from anemia. Clinically, it certainly wasn't all bones and no fat. But the reproductions of double bass, cello, piano, sax, and oboe lacked full measures of weight and the sort of lifelike lushness I've come to expect from, say, the Air Tight ATM-3 or even the Air Tight ATM-2 when both are retubed with KT99As instead of the stock Chinese KT88s. I didn't find the mids as engaging or as musically authoritative as with these less expensive amps.

There was nothing romantic about the Jadis. In fact, it sounded very businesslike through the midband—somewhat mechanical, really, which made it sound about as glamorous as a sewing machine.

Tube alternatives
I've never been a fan of the 6550A, which is really not an audio tube at all. I was thus perfectly willing to ascribe my responses to the JA 200's midrange to the 6550A's inherent character. The question now was, How would other output tubes fare in this circuit? Of course, getting hold of 20 genuine KT88s was out of the question. I should be so lucky! There remained the KT99As and the EL34s to investigate. With the kind assistance of Gold Aero's Frank Morris, I was able to obtain an adequate supply for comparison.

My first impression of the KT99A was that there was a richer, sweeter upper midrange, but also a murky character that permeated and obscured the soundstage. Fortunately, focus and transparency improved considerably after several hours of burn-in. At this point the Jadises were really cooking. Bass lines were easily resolved, though lacking the tightness they had through the 6550As. Spatial resolution of massed voices was nothing short of spectacular. The soundstage came alive, as if energized by a supernova. The harmonic tapestry through the upper mids and lower treble sounded smoother, and richer to the point of being a bit too dark or chocolate-flavored. And although the lower mids were now a bit more forward and intimate, they were still a tad deficient in authority and weight. All in all, it was a smoother, less mechanical sound than that afforded by the 6550A.

Tesla's premium EL34, designated the E34L, showed a sunnier disposition, being even sweeter and better focused than the KT99As. Ahhhh! Now I was beginning to fall in love. Dynamic shadings were even more linear and incisive. The upper registers sounded cleaner and not as thick as with the KT99As. Violin overtones and soprano upper registers now sparkled and shone joyously. The tapestry of the music sounded more luscious and more harmonically accurate. Bass lines were remarkably lucid, though lacking some of the ultimate punch and power of the 6550As. The lower mids, however, still lacked a full measure of weight.

In short, the choice of output tube is crucial to the sound of the JA 200. Both the KT99A and E34L offer a smoother, less mechanical-sounding midrange than the 6550A. The E34L proved the most harmonically convincing of the three, and was the tube that got me the most passionate about the music. (Victor mentioned a Russian EL34 sourced from New Sensor Corp., which I hope to get hold of shortly for a "Follow-Up.")

There was still one problem: None of these tubes entirely addressed the tonal-balance deficiency through the lower midrange. The sound here ought to be a bit fuller and fatter (footnote 1). Ironically, the JA 200's design is so good that it pushes the 6550A to the limit—and shows the 6550A's primary virtue to be its strong bass response. In all other respects, the 6550A was outclassed by the alternate tubes examined.

I also experimented with the front-end tubes. I found the Jadis-branded Yugo/Serbian 12AX7 and 12AU7 tubes to sound best in this application. The 12AU7 was originally sourced from Richardson (National brand name), but Victor quickly sent me a Yugo replacement that I've been happily using.

Final thoughts
During their sojourn in my listening room, the JA 200s performed very reliably. They earned my respect and even love—at least with Gold Aero's Tesla E34L tube complement. Yet these are not amps to satisfy the most demanding disciples of tonal-balance accuracy. In contrast to their spatial impression, which is reminiscent of the best of French Impressionistic painting, the JA 200s' tonal-balance expression was not entirely convincing (though it came close at times). Their somewhat meager lower mids will probably disappoint old-time tube aficionados. This was not classic tube sound, sounding more New Age than Old Age, especially with its stock output tubes. In this regard, the JA 200 strikes me as characteristically French—more poignant in tone color rather than robust, in the manner of a French-style bassoon rather than a German instrument.

With this caveat in mind, the JA 200 still qualifies as one of the most musically appealing amplifiers money can buy. It's certainly the most impressive high-power tube amp I've heard. This is no small feat, since most dinosaur-sized tube amps lack the magic of their smaller brothers.

So say "Oui!" to Jadis. Take out that second mortgage. Embrace the JA 200, and the Force will be with you.—Dick Olsher

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Footnote 1: It should be noted that the intrinsic balance of DO's MartinLogan is a bit lean in the lower midrange.—John Atkinson