Tube Power Amp Reviews

The tubed VTL MB300 monoblocks mated so well with the Acarian Alón V loudspeaker that I wanted to reacquaint Stereophile readers with them, and comment on developments since J. Gordon Holt first reviewed the amplifier in October 1988 (Vol.11 No.10).

Though the sweet, liquid, holographic quality of triode tube operation has often been written about, it remains difficult to describe unless you've lived with it in your own system. It's...seductive. Detail is presented in a casual, graceful manner that sounds natural and real. The music simply exists in space. You don't tire of the sound as it continues to surprise and charm. In a quiet, darkened listening room there are few experiences quite so magical. I believe that the Alón Vs provided the venue for the full character of these amplifiers to emerge.

The problem historically has been the lowered power of triode operation and its marginal ability to handle dynamic peaks with insensitive speakers. That has not been a valid concern for several years now, as several companies are producing high-powered triode amps.

I remember auditioning the VTL 225 with the Mirage M-1 speaker six years ago. It sounded smooth but distant, and bass definition was a problem. The big Mirage is very current-hungry and needs a muscular solid-state amp to gain full control. With the newer M-1si in my living room and the VTL MB300s on hand, I took the opportunity to experiment. I was astonished by how well these amps disciplined and directed the speaker. A well-controlled speaker actually does the opposite of what the word suggests: Dynamics and bass response have that casual, natural, almost offhand feel that indicates sufficient power is available—even in triode mode.

The neat thing about the VTL is that it can be switched between triode (250Wpc) and tetrode (nearly 500Wpc) operation with a flick of two switches on the front panel. The earlier model ran in tetrode mode only at 350Wpc. You don't lose a lot in tetrode mode—a bit of stage width, a bit of liquidity and sweetness—but you gain much added power for those thunderous Saint-Saëns Organ Symphony crescendos. However, I rarely needed that power with the Mirages, and never needed it with the Alón Vs.

Luke Manley tells me there have been substantial changes to the MB300 since it was first introduced. The current version's overall circuit topology is similar, but a 6350 has replaced the 12BH7 phase-splitter tube. The 6350 has a higher amplification factor and transconductance, higher current, and lower impedance. Manley feels it smooths the sound compared to the original MB300. One circuit board with tighter, simpler circuitry replaces two boards on the original model; this is said to result in a lowered noisefloor.

The main reason for the power increase is the all-new Signature output transformer. There's an art and a science to winding transformers, and tube devotees will note the changes here. Manley states that they've increased the number of sections in the transformer, which improves couplings between windings and increases efficiency by lowering insertion loss (whew!). The primary impedance of the transformer is more closely matched to the plate impedance of the output tubes for more efficient current transfer to the speaker.

More windings and better winding-to-winding couplings are claimed to give a wider frequency response, especially at the top end. Manley reports that the amp measures flat from 20Hz to 25kHz at full power. The larger transformers also allow more output current—the 1988 version of this amplifier could deliver 6.6 amps, while this version will do 11 amps—which might account for the vast improvement in performance with my Mirages. VTL uses 9-gauge wiring in their Signature transformers to give a 20-amp capability.

There will have been some cosmetic changes to the MB300 by the time you read this. Tube biasing will become an internal procedure—it's currently accomplished with a switch and meter on the front panel and a screwdriver adjustment at each output tube—and an additional power supply will be added for the driver stage. Total power output will remain the same. The model number will become the MB450 Signature and the price will increase to $6990/pair.

Audiophiles who have read about but not yet heard a high-powered triode amplifier in their own homes need to do so. Get friendly with a tube head; make him loan you his amp for the weekend. If your speakers are capable, the improvements in sound quality should not be subtle.—Russell Novak