Manley Laboratories 175 monoblock power amplifier By Dick Olsher   •   January, 1995 Neither its rather pedestrian name nor Manley Labs' own literature gives much of a clue as to the 175 monoblock's special pedigree. Where are the bands, the fanfare?! After all, the rolling-out of a 6L6–based high-power audiophile-grade tube amplifier definitely qualifies in my book as a momentous occasion. Deplorably, such happenings are rare indeed; the 6L6 has been unjustly neglected in high-end circles.

The 6L6 was the first-ever beam power tube to be designed, and a certifiable American classic. Thousands upon thousands of guitar amplifiers, from Fenders to Mesa Boogies, have used the 6L6 family of tubes over the years. This is no accident or fad, but the direct result of the 6L6's essential musicality. Understand that guitar amps are routinely driven hard, much of the time into clipping—even a moderate twang on an electric guitar string is almost sufficient to drive an amp into overload.

I asked Randall Smith of Mesa Engineering (home of Mesa/Boogie) why he thinks the 6L6 is so appealing for musical-instrument use. His answer, not surprisingly, related to the manner in which it clips. Unlike an EL34, which clips with a blaze of raspy odd-order distortion products, the 6L6 yields a distortion spectrum rich in even-order harmonic products—a benign and musically attractive tinting of tonal color that gives a rounder, more liquid tone. A million guitar players can't be wrong—they've esteemed the 6L6 as a sonic talisman because of its ability to perform passionately under pressure.

I believe that a domestic power amp, too, must be able to perform under pressure; 100-watters are often called upon to drive loudspeakers that barely scrape a sensitivity spec of 84dB. An amp in such an application will red-line more often than you might think.

The 6L6 also happens to be a rugged tube—as it has to be for musical-instrument use. Guitar amps are designed to withstand being knocked over during bar fights, tripped up by lounge lizards, and dropped off the backs of band vans. In the 1950s and '60s, the British, bless their hearts, routinely used KT66s—close cousins of the 6L6. In this country, the 6L6 has been snubbed by high-end designers—at least until just recently. What a collective blind spot! The Manley 175 should change all that.

Technical details
The input signal of the 175 is direct-coupled to the grids of a 12AT7WA dual-triode, both halves of which are connected in parallel. Following the input stage, the signal is RC-coupled to a 6350 dual-triode connected as a cathode-coupled phase splitter. Note that the 6350 is the American military version of the 12BH7A, but with a different pin-out. Be warned: there is no substitute.

The output stage is dominated by an octet of Russian (Sovtek) 5881/6L6WGC beam power tubes operating at a B+ voltage of 525V. According to David Manley, the "W" in the tube designation denotes highest mil-spec to Russian protocols. These tubes are said to be rated by the factory at a maximum plate voltage of 600V—considerably higher than the ratings of ordinary 6L6 types.

The power supply is of conventional design, using solid-state rectification and capacitor-based filter networks. The output transformer is factory-wired for a 5 ohm load—a reasonable guesstimate for most dynamic speakers. But this prohibits the 175 from being used with loads that dip below 3 ohms in the bass. However, these type of "short-circuit," juice-hungry speaker loads aren't really tube territory anyway—they'd force any tube amp into cardiac arrest.

Individual bias pots are provided for the output tubes. The trim pots and test points are conveniently and logically laid out on top of the chassis. A multimeter with a DC voltage function is required for the bias adjustments. The trimpot is adjusted so that the DC voltage across each 10 ohm cathode resistor measures 260–275mV, which, by Ohm's law, corresponds to a bias current of 26–27.5mA. The corresponding plate dissipation is a conservative 13W. Manley Labs recommends that the bias be checked every few months. They estimate an output tube life of three to four years with average usage.

Sound
My first listen to the Manley 175s was with the Audiostatic ES100/SW100 speaker system; I immediately felt as though the good Lord had plucked me out of my listening seat and slam-dunked me into an ocean of liquid harmonic textures. The music, quite fittingly, was good ol' Bach's Violin Concertos 1 and 2 (Itzhak Perlman, Daniel Barenboim, English Chamber Orchestra, EMI/Angel CDC 7 47856 2). Violin phrases were silky-smooth, the mids heavenly suave. Imagine biting into the soft, juicy flesh of a Georgia peach, the nectar dripping down your chin. That's what the 175s sounded like. On a scale of 1 to 10, my first impressions of the Manley's mids rated a stunning 10. My first coherent thought was, "Holy triodes! These have got to be the world's best tube amps!"