Tube Preamp Reviews

The 6H30's plate resistance is four times lower than the 6922's (800 ohms), and its transconductance is higher (18mA/V), to deliver just a touch lower gain than the ubiquitous 6DJ8/6922. The SuperTube can't be used as a direct replacement for the 6922, but BAT developed a proprietary circuit to "unleash its true potential." Amazingly, this tall, sturdy tube delivers up to 6A of "pulse current" per section, and can withstand 1050V of plate voltage. Because of its low impedance characteristic, the 6H30 allows a much lower output resistance in the gain stage without resorting to "crutch devices" like cathode followers.

The gain stage operates at a "truly spectacular" bias current of 150mA per channel. BAT points out that the output stages of many power amplifiers are run at far less. "Operating at this high current allows us to achieve a tremendous load drive without using cathode followers or buffers. The VK-50SE can source up to 40mA of signal current into a load—a feat normally reserved for power amplifiers and in sharp contrast to many preamplifiers capable of perhaps only 2mA or so."

Using the four sections of the eight 6H30s connected in parallel, one has, in effect, a tube topology yielding an amplification factor of 15, a combined plate resistance of 200 ohms, and a transconductance factor of 72mA. So the VK-50SE's differential plate-loaded gain stage delivers an output resistance of 200 ohms per signal phase and a load current delivery of 40mA. As usual at BAT, this is done without buffers and/or cathode followers.

Upgradeability has always been an important feature of BAT products. The VK-50 can be upgraded to SE status for $2500, and the VK-5i to 5SE spec for $1950. This includes a set of 6H30s, a Six-Pak of output capacitors, new toroidal transformers, and "numerous other modifications to the power supply and gain stage." The Six-Pak mod alone can be had for the VK-50 or VK-40 solid-state model for $500.

SuperTube SuperSound
Whatever you do, don't make the mistake of judging the VK-50SE unless it's been well broken-in. This seems to be a contentious subject for some, though I can't imagine why. Everything benefits in all ways from being left on (preferably playing music) for the first 24 hours, after which a slower curve of improvement usually follows.

Fresh out of the box, the VK-50SE sounded positively nasty; the grain put my teeth on edge. Anything else about it, good or bad, couldn't be heard behind the scrim of grime. It improved significantly after 24 hours, and after 72-100 hours the VK-50SE tightened up, shed its grain, and bloomed into its true performance envelope. I can't remember the last time a preamp changed as much during break-in.

I started my listening with something refined. Although I usually find interpretations of classical music in the jazz idiom pretty lame, I dearly love the Jacques Loussier trio. Try a simply delicious Telarc release, Boléro (CD-83466), skip the trite cover of Ravel's warhorse, and play the disc from track 2 through the end. Loussier's own composition, "Nymphéas," is gorgeously recorded music, unique and accessible in the way Bill Evans is available to even those who aren't jazz fanatics (if there be such creatures). The soundstage was quiet, yet filled with shimmering air charged with the acoustics of the piano, acoustic bass, and drumkit.

Even on this laid-back material, the VK-50SE was a charger, getting the cut'n'thrust, the timing of the music, just right. Listening to the percussive effects of the drums on this disc so beautifully rendered—the snap and pace, the timing and startling dynamics, along with the lovely array of tonal colors—made for an involving listening session. The VK-50SE's linearity seemed above reproach. From the deepest bass right up through the top end, all was beautifully rendered. When the frequency response is linear, I think it better involves the listener, as it removes another reminder that we're listening to a mere re-creation of a musical event, not the real thing.

The sound of the piano's action, the damped cymbal strikes, the acoustic bass that drives "Presto" along, the acoustic quality of that bass—its power and control and differentiation, the sound of fingers slapping and stroking the strings—were completely engaging. I found myself returning time and again in my notes to descriptors like "involving" and "engaging." The VK-50SE generally got everything so right that I couldn't help but fall into the magic of the music.

Listen for the bell during the opening of "Cantabile" for the wonder of extension, shimmer, and truthfulness of reproduction, the cymbal work riding effortlessly above the accompanying piano and bass. A sheer delight for the senses, this recording is sure to set many an audiophile's ears alight. At the same time, there was real authority in the VK-50SE's sound, the piano highlighting the linearity and broadband power delivery. The air was delicious, the integrity of the performers on the soundstage was beyond reproach.

Changing up, I spun the soundtrack of an exciting recent film, Run Lola Run (TVT 8220-2). It was recorded in Santa Barbara and Berlin, the opposed cultural bookends of the audio apocalypse. However, the CD comes to us via TVT Soundtrax at 23 East 4th Street in Manhattan's East Village! (www.tvtrecords.com) I couldn't believe how fine Run Lola Run sounded, so I 20/20'd the liner notes and discovered it was mastered at Bernie Grundman Studios in Hollywood, USA. I recommend this CD to those who enjoy world/trip-hop/techno music—and catch the film too, which is imaginative and way out of the box.

A huge soundstage was laid out clear as a bell, rife with intricate scoring and out-of-phase surprises wrapping around the listening chair in a most entertaining way. Don't tell me two-channel isn't virtual-reality immersive! The driving rhythms lifted me bodily out of the Ribbon Chair and flailed my sorry butt around in spastic response to filmmaker T. Tykwer's driving, engaging, evocative music. There is prodigious bass embedded in this CD.

When listening to Run Lola Run and other Big Bass recordings, it became evident that the VK-50SE was limited only by the bass response of whatever amp it was coupled to. The Forsell offered a bloomier sound, happy for the grip and control. The Linn Klimax Solos loved the VK-50SE, which took them in hand and boogied hard! The Accuphase M2000s also appreciated the control and linearity pouring out of the VK-50SE's balanced outputs.

Track 6, "Running Three," highlighted the VK-50SE's precise, expert way of organizing and presenting dense, complex scorings while dishing out sweetness and light in the midrange and above. In fact, "Casino" has the kind of midrange I wish all pop recordings would strive for. I noted a continuousness of tonal color—I think that's the best way to put it—that was simply astounding. Around 3:55 into the track there's bass work that'll tell you all you want to know about how well your system does the very bottom. It should literally shudder your body, be felt as much as heard—as it did while I listened to the rest of this track at the goosed-up, lease-busting levels at which the good K-10 likes to play.

All that, accompanied by eerie vocals that should develop way back behind the speakers, very focused and well lit, as was the entire upper range. That light—the special inner illumination that tubes do so well, and that these particular tubes get so right—was evident throughout the entire frequency range. There were no shadows, as it were; the light got down into the many nooks and crannies in the music where dwell the details that make it all live. It was awesome; I was breathing hard by track's end. Through an instrument like the VK-50SE, one perceives all.