Tube Power Amp Reviews

It began when I reviewed the MartinLogan Montis loudspeaker (September 2012). The amplifiers I had to drive the pair of them were the PrimaLuna ProLogue Premier integrated ($2999), the Audiopax Mk.II (no longer available; the Mk.III costs $22,000), and the Simaudio Moon Evolution W-7 ($9000). The first two are tubed, with power in the 30–40Wpc range; the solid-state Simaudio puts out 150Wpc. The Audiopax, which sounds great with my Avantgarde Uno Nanos, turned out to be not such a good match for the Montises: weak in dynamics, and too soft sounding. The PrimaLuna and the Simaudio were better overall, each with its strengths and weaknesses, though neither was ideal. I really liked the ProLogue Premier's tonal characteristics, and wondered what a higher-powered tube amp would sound like with the MartinLogans.

I had two options: borrow an amp with the requisite characteristics that had already been reviewed in Stereophile, or use this as an opportunity to review an amp that had not been reviewed but seemed to fit the bill. McIntosh Laboratory's MC275 50th Anniversary Limited Edition (aka MC275LE, $6500)3 was kind of in between: previous versions had been reviewed by Sam Tellig (July 2004) and Fred Kaplan (October 2010), but McIntosh has since made some significant changes in the MC275LE, and John Atkinson suggested that it deserved a review. ST's and FK's reviews were highly positive, and the MC275 was already listed in Class A of "Recommended Components." The MC275LE's claimed output is 75Wpc, but JA's 2010 measurements indicated that this specification is quite conservative, the amp putting out about 90W before clipping. This could be exactly the extra power the Montises needed—and I was curious about the sound of a classic amplifier that had obviously passed the test of time.

What's New?
There are a number of differences between Version V of the MC275, which FK reviewed, and the new Version VI: some cosmetic, others functional, still others technical. V.VI has a gold-plated rather than a stainless-steel chassis, the binding posts are much more substantial, and the faceplate identifies it as the 50th Anniversary Edition. The power switch, which ST and FK criticized for being hard to access, has been moved to a more convenient position on the left side panel, near the front. A switch on the right side, near the rear panel, controls the Power Save circuitry. When this switch is in the On position, the MC275LE turns off after 30 minutes of having detected no signal. The MC275LE can be turned on and off by compatible McIntosh preamps and surround preamplifier-processors. The new Sentry Monitor protection circuit shuts down the amp in the event of malfunction, to prevent further damage.

The power-tube sockets have been slightly lowered, for cooler operation. A change from V.V that's partly functional and partly just fun is that, on turn-on, an LED incorporated into the socket of each of the MC275LE's seven small tubes glows first amber, then green if everything is okay—or flashes red if there's a malfunction. This feature, apparently a one-off prepared for an audio show, proved such a hit with audiophiles that McIntosh decided to put it into production.

The most important changes in the MC275LE are under the hood, and represent some significant technical improvements. According to Ron Evans, VP of engineering for McIntosh and chief design engineer for the MC275LE, they include:

• A new output-transformer winding process has resulted in wider bandwidth and better consistency.

• Because of the wider output-transformer bandwidth, the other gain stages could be optimized without sacrificing overall stability.

• Unlike in previous MC275s, the negative feedback is now taken directly from the output transformer's speaker winding. This has resulted in a significantly increased damping factor.

• In the main amplification stage, the power-supply voltage has been increased from 485 to 720VDC. This lets the 12AT7 tubes operate with increased linearity.

• Evans told me in an e-mail that "Careful attention to every other circuit detail has also improved linearity with less dependence on individual tube characteristics. These small but important details have together reduced the amount of tube thermal noise that actually reaches the speaker output."

Comparing the two models' published specifications reveals that: the V.VI's bandwidth is 10Hz–100kHz, a higher top extension than the V.V's 10Hz–70kHz; the damping factor has been increased from 14 to 22; and the signal/noise ratio has risen from an already-impressive (especially for a tube amplifier) 100dB to 105dB.

I had experience of two samples of the MC275LE. The first worked fine for a day, but when I powered it up on Day 2, the LEDs of three of the small tubes flashed red rather than steady green. I turned off the amp and, as per instructions in the owner's manual, turned it on again. Again the lights flashed red. I could see nothing that would account for this problem: the speaker cables weren't shorted, and the preamp worked fine with other amplifiers. This wasn't the first time I'd experienced a problem of this sort with tube amps—and, as manufacturers will attest, no matter how statistically rare a problem is, it's bound to occur with a sample sent to a reviewer. McIntosh quickly arranged with a local dealer (Audio Excellence, of Vaughan, Ontario) to get me a new sample of the MC275LE, which functioned flawlessly throughout the review period.

The MC275LE has both balanced and single-ended inputs. My Convergent Audio Technology SL-1 Renaissance preamplifier is single-ended only, so I used only the Mac's SE inputs. There are three sets of output terminals, to match speakers rated at 4, 8, or 16 ohms. I played around with these, and while there were some sonic differences, I ended up mostly preferring the 8 ohm terminals with the speakers I used.