VTL MB-450 Series II Signature monoblock power amplifier
By Michael Fremer   •   January, 2008 Stop me if you've heard this one: Back in the early 1990s, just after the fall of the Soviet Union, I debated professor of music engineering and magazine columnist Ken Pohlmann on a talk show on the CBS radio network. The subject was analog sound vs digital sound, but I guess when Pohlmann felt I was getting the upper hand, he felt he needed to play the tube card. Derisively, he said, "I bet you're one of those tube guys, too, aren't you?" Before I could open my mouth, he continued: "You know, the Soviet Union's military gear, including the MIG fighters, ran on tube electronics, and look what happened to them!"

"True, Ken," I countered, "but their planes sounded so much better than ours!"

Back then I was a tube guy, driving a pair of Eminent Technology VI loudspeakers with a pair of VTL MB300 tubed monoblocks. What a great-sounding system that was! If you'd have told me then that in 2008 my reference monoblocks would be the megapower, solid-state Musical Fidelity kWs, I'd have told you that was as likely as me chucking out my LPs for CDs.

But that's just what's happened (though I haven't chucked my vinyl, of course), probably because, after encountering the Audio Physic Virgo IIs—loudspeakers that soundstaged and imaged with greater specificity than anything I'd ever heard—I was tugged further in that direction by solid-state amplifiers capable of delivering high-current bottom-end extension and control, as well as the overall solidity, focus, and high-frequency extension that the tubes couldn't do. I gained a lot in the switch to solid state but lost plenty, particularly in the midrange, and in tubes' overall sense of musical continuity. Tube aficionados needn't be reminded of that.

In the years since then, designers of solid-state amplifiers have produced warmer, harmonically more expressive, texturally more supple, almost tube-like products while retaining all of solid-state's attractive qualities. Meanwhile, the tube guys haven't exactly been complacent. They've made their products more user-friendly and reliable, helped in part by a renaissance in tube manufacturing. Auto-bias circuits and other microchip-based tube babysitters have resulted in tube amps that are reasonably free of maintenance and trouble, with sound that's more consistent over time.

But does the sound of the new tube amps match that of the old? Granted, some listeners continue to crave a soft, lush, romantic "tone control"–type sound (as John Atkinson once described a well-regarded tube integrated amp), and there's nothing wrong with that if it makes you happy. But what about those of us who want at least the subjective impression of deep bass extension, iron-fisted control, reasonably flat response into complex loads, and the mesmerizing harmonic development and musical flow offered by tubes?

According to VTL's Luke Manley, his company's latest designs provide all of that, including the direct descendant of the VTL 300s I owned when the Berlin Wall fell: the new MB-450 Series II Signature ($13,500/pair).

New Looks, "Smart Tube" Technology
In the VTL MB-450 Series II Signature, the rack-mount handles and dark industrial look of the original MB300 and MB-450 (footnote 1) are replaced by a thick, elegantly handsome, curvaceous faceplate of brushed aluminum that frames a dark glass panel, through which glow the input and driver tubes. Three small buttons and accompanying LEDs select Mode, Power, and Mute. And that's it for the front panel.

On the rear panel are the main On/Off switch, a set of speaker binding posts, RCA and XLR inputs, a switch to select balanced or single-ended operation (I used the single-ended), a 20-amp IEC jack, five fuse holders, and a trigger connector for remote power-up via another source, such as a preamplifier. Much like the original VTL 300, from which the MB-300 and MB-450 are derived, the tube complement comprises eight 6550C output tubes, and one each 12AT7 and 12BH7 tube for the input and driver stages.

The MB-450 Series II incorporates many of the technological advances found in VTL's flagship monoblock, the 800W Siegfried Reference. These include "smart tube," logic-controlled auto-biasing circuitry, which continually checks and optimizes each tube's performance while providing a series of fault-sensing diagnostics for each tube via a series of LEDs. VTL maintains that the auto-bias circuit doesn't affect the sound quality because it's not part of the signal path, and monitors and biases the tubes only when music isn't being played.

The basic audio circuit, not so far removed from the original MB300, features a fully balanced input stage with separate input and driver power supplies, a "long-tailed pair" differential phase-splitter, and, of course, a push-pull output stage. Also included are a new, precision-regulated power supply and upgraded internal parts.

According to VTL, the screen power supply's "high-tension" voltage-regulation system, consisting of a 30-amp MOSFET connected between two massive banks of capacitors, working in conjunction with the main supply, to maintain proper voltages within <1% deviation at full output. That, VTL claims, helps give the amp a "fast, dynamic" sound.

With their auto-bias system's comprehensive tube-monitoring abilities, VTL strives for ultra-reliability in conditions of high current draw: the circuit immediately shuts down the amp in case of a potentially "fatal" tube fault. A separate fuse for each high-tension supply and automatic monitoring of fuse conditions add more levels of protection. Of course, protection doesn't mean complete reliability—tubes fail, and have been known to take out amplifiers when they do—but such redundancy of protection is comforting.

Installation and Use
The rack handles may be gone, but not the amp's close-to 100-lb weight, due mostly to the massive power and output transformers. The location of these at the rear of the chassis creates an uneven, somewhat unwieldy weight distribution, and makes unpacking and final placement a job best done by two people. Nonetheless, I managed to pack one up myself and ship it to JA to be measured.

The connections are straightforward. Once that's done, you flip the rear-mounted power switch to turn on the logic supply, which goes through a flashing LED mode that identifies each output tube. If all is well with the tubes, the Mute LED on the front panel glows green, signaling that the amp is in Standby mode and ready for full power-up.

---

Footnote 1: The VTL MB-300 and MB-450 Signature monoblocks were reviewed by J. Gordon Holt, Russ Novak, Michael Fremer, and Chip Stern in October 1988, July 1996, February 1999, and October 2002, respectively, while Paul Bolin wrote about the MB-450 Signature in April 2004. (All the reviews are available free on-line.)—Ed.