Tube Power Amp Reviews

Robert Harley returned to the VTL 225W Deluxe in October 1991 (Vol.14 No.10):

When I reviewed the VTL 225W Deluxe Monoblocks in January 1990, I concluded that they were the most musically satisfying amplifiers I had ever heard. Despite the intervening years and products, the 225s still do it for me. They have an unparalleled midrange liquidity, harmonic rightness, and a detailed yet unfatiguing presentation. After reviewing other amplifiers—many superb in their own rights—it is always a joy to return to the special qualities that make the 225s so musically compelling.

With the continued supply of the high-quality EL34 output tubes used in the 225 monoblocks in question, VTL's David Manley sought a replacement tube for the 225 and other VTL products. In a joint venture with the Yugoslavian EI tube factory, an entirely new vacuum tube—the KT90—was designed for audio applications (footnote 1). The KT90 has replaced the EL34 in all current-production VTL products.

The question begs: How do the KT90'd 225s compare to the venerable EL34'd version? I've had both amplifiers on hand for a few months, alternating between them for weeks at a time and doing some side-by-side comparisons at matched levels.

For this "Follow-Up" I used my usual reference loudspeakers system—Hales System Two Signatures—but without its usual companion, the Muse Model 18 active subwoofer. (Removing the subwoofer and running the amplifiers full-bandwidth gives a better indication of the two amplifiers' relative bass performance.)

The analog front end was a Well-Tempered Turntable and Arm, fitted with an AudioQuest AQ7000 cartridge and stepped up with an Expressive Technologies SU-1 transformer. The digital front end varied over the months, at one time or another including the Theta DSPro Basic, Theta DSPro Prime, Audio Research DAC1, Meridian 203, and Audio Alchemy DDE converters driven by Esoteric P-2, Wadia 3200, or Theta Data transports. Preamps have been the Audio Research SP-11 Mk.II, ARC LS2, or the passive EVS Stepped Attenuator. Speaker cable was bi-wired runs of AudioQuest Dragon/Clear, and interconnects were AudioQuest Lapis and Diamond.

In addition to auditioning the amplifiers in my listening room, I did comparisons at someone's house through a pair of Magneplanar MG2.6/Rs. My first impression, shared by the Magneplanar owner, was that the KT90 version was superior to the EL34'd 225s. The KT90s had greater clarity, increased resolution of detail, and a more dynamic, driving low end. In many ways the KT90s sounded more solid-state: a little brighter in the top octave, a more up-front perspective, and greater soundstage focus. While the KT90'd 225s had less soundstage depth than the EL34s, the focus and resolution of individual outlines was clearly superior with the KT90s. There was, however, a slight loss of the midrange ease and liquidity that characterize the EL34'd 225s. The KT90s tended to be more analytical, the EL34s more romantic. Despite the tradeoffs, we both preferred the KT90 version driving the Maggies.

Just for fun, I replaced my host's Cardas interconnects and TARA Labs Space and Time speaker cable with AudioQuest Lapis interconnect and AudioQuest Sterling cable (both use silver conductors). We were shocked: The difference between cables was greater than the difference between the amplifiers. With the AudioQuest cables, the soundstage depth increased dramatically. Suddenly there were layers and layers of music we hadn't heard before. Textures became more liquid, and the overall presentation took a huge leap forward in musicality. The vocal on Johnny Frigo's My Blue Heaven (Chesky JD1) became round and liquid, with the backing instruments now separate and behind the singer rather than being a two-dimensional wall between the loudspeakers. In addition, previously unheard detail became apparent as the soundstage became more transparent and three-dimensional. I don't know if it was due to replacing the interconnects or the speaker cable, since we switched both at the same time, but the Magneplanar owner knew immediately he would have to pop for the Lapis and Sterling. It was the most dramatic difference between cables I'd ever heard.

But back to the VTLs. Driving the Haleses, the KT90's superiority was less clear-cut. The KT90's characteristics that we preferred on the Magneplanars—greater contrast, closer perspective, and slightly brighter top octave—were less welcome on the Signatures. The Haleses' more detailed and forward character (in relation to the Magneplanars') seemed better suited to the softer-sounding EL34s. The ease, smoothness, and liquidity of the EL34s made for a more musical presentation with the Signatures. The EL34s have a unique quality that I wouldn't use to describe the KT90s: lush. There's something magical about the EL34'd 225s in the mids that is unmatched by any other amplifier I've auditioned. Brass instruments—the sax and flugelhorn on my own jazz recording, for example—had a particularly round, liquid quality with the EL34s. They had a more natural, burnished brass timbre rather than a polished chrome rendering.

The KT90'd 225s, however, had many qualities that made them superior through either loudspeaker: bass control, LF extension, and more effortless dynamics. Both amplifiers have excellent bass for tubes, but the KT90'd 225s were clearly a notch better. Bass drum had more punch and authority, and bass guitar had more body and rhythmic drive. These characteristics gave the KT90s a more physical involvement with the music. The KT90s' bass control and extension made them sound more like solid-state amps in the lowermost octaves. "Robust" describes the KT90'd 225; "soft and gentle" is a more appropriate description of the EL34s.

Incidentally, the KT90s seem to draw more power than the EL34s. The inrush of current at turn-on sounds louder with the KT90s, and the sound of the tubes cooling is more apparent when the amplifier is turned off. I also experienced a failure of one of the KT90'd 225s at the end of the listening evaluations: an output tube smoked, and the amplifier wouldn't work after that even with the tube replaced.

Conclusion
Which amplifier one prefers is largely a matter of personal taste and associated components, especially loudspeakers. If you have the EL34'd 225s and your system needs a little more sparkle and punch, the upgrade to the KT90 is worth the expense. If your system leans toward the analytical, keep your EL34s.

For those of you contemplating buying VTL 225s for the first time, the KT90'd 225s are superb amplifiers and are highly recommended. They have earned a continued Class A recommendation in Stereophile's "Recommended Components."—Robert Harley

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Footnote 1: For the full story behind the KT90—and the associated controversy—see my interview with David Manley in Vol.14 No.6 and the "Manufacturer's Comments" section of the same issue.—Robert Harley