Solid State Power Amp Reviews

The 175, though not designed by the senior Manley, is based on the basic circuit guidelines outlined in his book on tube design, The Vacuum Tube Logic Book. The power supply features a solid-state rectifier bridge and a pair of 3800µF caps—meat'n'potatoes stuff. The power tubes are more unusual: 807s—inexpensive, relatively low-power radio transmitter devices (no longer in use for that purpose) that feature ultra-wideband response said to reach from above 125MHz down to DC. Using the 807s (similar to KT66s, according to Manley's book) as power tubes is tricky, because their wide bandwidth makes them prone to oscillation, but using 807s would be the "push-pull answer to the sound of a single-ended amp." The tube is a five-pin, classic Coke-bottle-shaped device complete with anode cap. The design uses a modest 6dB of negative feedback and is switchable between triode and tetrode modes. Claimed frequency response is 20Hz-20kHz, and a low 1.5% THD at full-rated 175W output into 4 ohms.

Manley told me that average tube life is a relatively high 2500 hours, in part due to the design's low 30mA current draw. Combine that with the 807's low cost—currently about $20 each—and you have, theoretically, a low-maintenance, high-performance amp that can be completely re-tubed for about $150, including the 12AT7 and 6350 input tubes (dual triode drive and phase splitter, respectively). Given the long tube life, re-tubing should be necessary only once every four or five years, with average use.

The key to getting high-quality performance and sound from any tube amplifier is its output transformer. The Signature 175 uses VTL's newly developed, highest-quality, and most expensive "Signature" transformer, which has been precisely matched to the plate impedance of the output tube for greater efficiency. Manley told me that the tight, carefully layered windings of each section result in "smooth sound" and "very clean squarewave response."

Build quality was fine, though the 175 lacks the point-to-point wiring found on some other amps, and doesn't make use of the most expensive, highest-tolerance electronic components. Manley was forthcoming on this: He believes that tube designs are somewhat more forgiving of wider-tolerance parts, and that his use of high-quality if lower-tolerance parts on a printed circuit board has not degraded his amp's performance. Besides, he told me, he was building a high-quality product to what he thought was a reasonable price point: $4990/pair.

Before leaving a pair of MB 175s with me, Luke cautioned that they were not designed for big, dynamic speakers with 12" woofers. "It's excellent with smaller speakers and with horns," he told me. Since I was going to audition the amplifiers with my reference Audio Physic Virgos, which feature a pair of 8" woofers per side and whose impedance drops to around 4 ohms over much of their frequency range, Manley left satisfied that I'd get a good measure of what the Signature MB 175 could do. He was correct.

Simple setup
Like most other VTL amplifiers, the 175 features a pair of large front-panel-mounted handles. Good thing, as the compact 50-lb units are ungainly as hell to move around. With both heavy transformers mounted to the rear of the chassis, setting the 175s down without crushing the rear-mounted speaker binding posts is a skill best learned quickly. But as you hopefully only have to do it once, it's not a big problem.

I rested the 175s on a pair of Target amp stands, sandwiching four A.R.T. Q-Dampers between each amp and the wooden Target shelf. Later I switched to three of Lloyd Walker's lead/brass Valid Points (best name, don't you think?) under each amp, each Point sitting on its (supplied) heavy lead/resin disc.

Hookup was straightforward, as was monitoring the bias to each tube—"on spec" at the outset, and remaining so for the entire review process. I tried a variety of AC cords during the 175s' lengthy stay in my system, including samples of WireWorld, TARA Labs, Transparent, A.R.T., and finally the latest Yamamura Millennium 6000 and Quantum power cables, plugged into the Yamamura Ciabatta—a passive power-line conditioner.

I'd sold my VTL 300s for two reasons: Because the power-hungry Eminent Technology VI speakers didn't fit into my new, smaller listening room, I no longer needed the mega-wattage; and the 300s had a mechanical transformer buzz that was obtrusive in the new space. But upon powering up the 175s for the first time, I was greeted by a pleasant surprise: nothing. Both 175s were mechanically and electrically silent. Luke Manley assured me that all current VTL amps run equally quietly, and that the 300's buzz was due to the less than stellar QC of a former transformer vendor. I ran the amps with their covers off and a Shakti Stone on each power transformer.