Tube Power Amp Reviews

It's been a while since I've had a classic amplifier in my system, and McIntosh Laboratory's MC275 is as classic as they come. Introduced in 1961 as the "powerhouse" of that era's newfangled stereo tube amps (two 75W amplifiers in one chassis!), the MC275 retained its position as the amplifier to own—challenged only, perhaps, by Marantz and a few others—until 1970, when it fell prey to the widespread wisdom that transistors were king and tubes were dead, and the model was discontinued. The MC275 briefly returned in 1993, in a limited "Commemorative" edition to honor the late Gordon Gow, longtime president and chief designer of McIntosh Labs. To everyone's surprise, that edition sold well, and McIntosh, gingerly at first, crept back into the tube business.

The present version of the MC275 is its fifth incarnation, though it looks strikingly (and, for retro-cool's sake, deliberately) similar. Looks, of course, can be deceiving or revealing; in this case, they're a bit of both.

Description and Design
Like the MC275 of yore, the current model is rated at 75Wpc (though McIntosh Labs' tech man, Chuck Hinton, says it can actually pump out 90Wpc), and it weighs the same 67 lbs. Much of that heft comes from the two output transformers, which are, with the exception of one quality, identical to the original's, even to the point of being wound on the same equipment at McIntosh's production facility in Binghamton, New York. The wire in the old transformers was made of lacquered copper, which tended to harden and become brittle. The new wiring is still copper, but it's insulated with a far more durable synthetic material.

The MC275's transformers utilize the same "unity-coupled circuit" that McIntosh invented in 1947. The two primary strands—one connected through the power supply to the plate and cathode of one of the output tubes, the other connected in the same manner to the other tube—are tightly wound together, turn for turn, for complete magnetic coupling. This creates almost instantaneous local feedback, which is said to reduce distortion. In the original and all subsequent versions of the MC275, including this one, a third winding is connected to the plates of the cathode follower driver; this was said to extend the ultra-low distortion over a much greater bandwidth, especially into the high frequencies.

Also like the original, the new MC275 uses four KT88 power-output tubes, three 12AX7A input and phase-inverter tubes, and four 12AT7 voltage-amplifier and driver tubes.

All else, though, is different from the original, to varying degrees. The chassis, once chrome-plated steel bent to shape (and thus prone to cracks and rust), is now highly polished stainless steel. The tube cage, formerly of fine mesh with solid ends, now has larger perforations and slotted ends, to allow more even ventilation. (When listening, I removed the cage, in part because I liked watching the tubes glow, and in part because, in my experience, cages of this sort usually vibrate. First, however, I received assurances from McIntosh that the cage was entirely for safety purposes, and that removing it wouldn't harm the sound.) The plating on the tube pins, once nickel, is now gold, and the ceramic output-tube sockets have heat chimneys that let cool air flow more freely from under the chassis, thus prolonging the life of the tubes—a McIntosh innovation that began with the MC275's fourth incarnation.

The new model also uses 1% precision resistors and polypropylene capacitors, for lower noise; the specified signal/noise ratio is 100dB, or 10dB higher than the original's. Balanced inputs have been added; a rear-panel switch lets you choose that option, and if you do, the first 12AX7 tube is bypassed for a more direct connection, though this results in lower gain. The original MC275 had several circuit boards, with all component parts wired point to point. The new version has just one military-grade board, on which the components and tube sockets are directly mounted. Hinton maintains that this results in less induced noise and crosstalk. A rear-panel switch allows the amp to be converted into a monoblock, with the two channels driven in parallel to cope more efficiently with low-impedance speakers.

Finally, the new model has a detachable power cord and an On/Off switch (though, as it's placed under the terminal panel, it's a bit awkward to get at). More welcome still, the old-fashioned terminal strips for speaker cables have been replaced by gold-plated, five-way binding posts. This is the only significant change since the version that Sam Tellig reviewed—very positively—in the July 2004 Stereophile.