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I had never been alone with a Russian-manufactured 6C33C tube. At least not at night, in the dark. The first night Balanced Audio Technology’s VK-56SE tubed amplifier was in my system, I sat on the floor studying the unusual shape and dark orange glow of its four 6C33C-B output tubes. I noticed their brightly lit, cathedral-like innards. My Russian neighbor told me they were used as regulator tubes in MiG jets during the Cold War. I could believe it—their exposed cathodes were the exact color of the Soviet flag. From more than a foot away, I could feel the heat from their high-amperage filaments.
The next morning, I pulled a cold 6C33C-B from its socket and examined it in sunlight. Its glass envelope seemed thick. It felt like a tube that wouldn’t break if I dropped it. I was fascinated by its vast crown of chrome-colored getter-flash, punctuated by three support-rod nipples. I was impressed by the thickness of the bronze side rods and the laminated graphite-titanium plate structure (footnote 1).
Sitting at my desk in the winter sun, I remembered: Once upon a time I was skeptical of using industrial-strength series-regulator tubes as audio-frequency amplifiers. Why? Because in my DIY experiments, 6AS7/6080 dual-triode tubes had sounded clean, but decidedly not subtle or rich of tone.
Historically, smart engineers designed vacuum tubes to operate in specific ways under specific conditions. Rarely did tubes excel at jobs they were not created to perform. Typically, audio designers repurpose tubes for one of two reasons: they don’t want to spring for an expensive audio-frequency triode like a 2A3, 211, or 300B—or they want more power than the few watts those Hall-of-Fame triodes can deliver. But in the case of the 6C33C-Bs in the VK-56SE, I suspect BAT had another, more relevant reason.
Balanced Audio Technology’s chief of design and engineering, Victor Khomenko (the “VK” in VK-56SE), was born in Leningrad (now St. Petersburg) and grew up two blocks from the Svetlana tube factory where 6C33C-B tubes were manufactured. According to Stereophile‘s Robert Deutsch, who interviewed Khomenko in 1995, he “attended the prestigious Leningrad Polytechnic Institute and received an M.S. in physics and electronics, specializing in electronic emissions. He spent his early working life in the Russian electronics industry, then emigrated to the US in 1979—with $400, a family, no home, and no job.”
I imagine Victor Khomenko dreaming about these hot-filament tubes at an early age. He probably began designing this amp way back then. He knew the 6C33C-B was a sturdy, indirectly heated triode tube capable of dumping substantial current into low-impedance speaker loads. He knew it would work well with low-turns-ratio, wide-bandwidth output transformers. Khomenko knew the 6C33C-B would repurpose well, and deliver a unique and gratifying sound in audio-frequency applications of his own design.
Description
The VK-56SE is the deluxe version of Balanced Audio Technology’s VK-56 stereo amplifier. Both are three-stage, fully balanced, push-pull, class-AB amplifiers designed around the 6C33C-B triode tube, and include full-time, active, automatic tube biasing. Both are capable of putting out 55Wpc into 8 or 4 ohms at 3% THD, and of doubling that output when bridged for mono. Each comes in a heavy, painted-steel case measuring 17″W by 8″H by 16″D and weighing 52 lb. The VK-56SE’s 3/8″-thick, machined-aluminum faceplate matches the sweeping arch of the aluminum handle at the amplifier’s rear. The standard VK-56 costs $4995, the VK-56SE $8495.
Because this would be my first in-home experience with a BAT product, I choose the VK-56SE—I wanted a full dose of Victor Khomenko’s unique design aesthetic without going full dose on the price.
I asked Khomenko how the SE differs from the standard VK-56.
“In the SE version, the first gain stage is changed from our standard 6SN7 twin-triode to a 6H30 super-tube. Then [the first stage’s] passive current source, formerly a resistor, is replaced by an active tube-based one. The polypropylene decoupling capacitors are replaced with oil parts. The power supply is substantially beefed up—there are two additional power reserve boards added to the VK-56SE as well. Finally, the electronic protection circuit, similar to that found in our REX II Power amplifier, is added, replacing the standard VK-56 rail fuses.
“The 56SE is a three-stage design, fully differential from input to output. It has no phase inverter, but if a single-ended signal is connected to its input, then the first gain stage generates both phases.
“The VK-56SE uses only 3dB of global feedback. The power supplies use a very simple, straightforward architecture. There are no voltage regulators, but there are very high quality capacitors, including oil capacitors in the SE version. The last capacitor your circuit sees is usually the most important in terms of its quality, and we are using oil capacitors in that application. We use very high quality custom toroidal power transformers with low turns ratios and wide bandwidth.”
On its back side, the VK-56SE has only balanced (XLR) input jacks; single-ended RCA-to-balanced-XLR adapters can be ordered at a nominal price. Output speaker connections offer more choices: Low (2–4 ohms), Med (4–6 ohms), High (6–8 ohms). I experimented with all three, but 90% of my listening was done with my speakers connected to the Low terminals.
I include all of these arcane engineering details so you can understand how little this pure triode push-pull amplifier has in common with generic EL34/KT88/KT120 pentode amplifiers.
Listening with Harbeth M30.2s
The BAT VK-56SE arrived the day after the deadline for the previous month’s reviews. Usually, those first days after submitting copy, I space out, read books, do laundry, and visit friends. I also install the gear I’m to review for the next issue. This little post-deadline “vacation” gives me a week of absent-minded, uncritical listening, and a general feel for a new component.
On my second day with the VK-56SE, I was folding laundry and listening, purely for pleasure, to Ladilikan, by Trio Da Kali and the Kronos Quartet (24-bit/96kHz FLAC, World Circuit 93/Qobuz). Suddenly, I had to stop pairing socks, turn around, face the speakers, and admire the sound. Damn! I thought. This amp sounds really good.
Trio Da Kali is a traditional African group whose main instruments include the xylophone-like balafon and a bass ngoni—a skinny, guitar-like instrument. The Kronos Quartet, founded in 1973 by violinist-composer David Harrington, specializes in parlaying surprising musical collaborations into challenging new musical experiences. Ladilikan is a captivating, audiophile-quality recording whose best feature is how its idiosyncratic mixture of African and European instruments frames the powerful voice of Hawa “Kassé Mady” Diabaté, who sings in Bambara, a language spoken in Mali. I thought the VK-56SE sounded better than the Rogue Stereo 100 amplifier because the pure-triode BAT exposed more complex harmonics in the sound of the balafon, and revealed more power in Diabaté’s voice. The Rogue reproduced Ladilikan with greater detail and sharper focus, but the VK-56SE made the more sensuous, more tangible sound.
In the quiet of night, with the VK-56SE driving the M30.2 Harbeths, I listened again to Ladilikan and noticed how earthy—though not warm-tubey earthy—it sounded. I also noticed more non-earthy digital artifacts, but their effect was minimal. The sound was still liquid and clear, effervescent and richly toned.
I’m pretty certain that the effervescence I heard was generated not by the BAT but by the PS Audio DirectStream DAC. Later, with the HoloAudio Spring “Kitsuné Tuned Edition” Level 3 and Chord Qutest DACs, the VK-56SE sounded less bright and not bubbly.
Footnote 1: From the tube data sheet: “The 6C33B-C consists of two triodes (A and B) in the same hard glass envelope. Cathodes, grids and plates are internally connected in parallel. The filaments can be wired in series for 12V at 3.3A or parallel for 6.3V at 6.6A.”