Tube Power Amp Reviews

The existing N.O.S. inventory is undoubtedly polluted by such rogue tubes. And with the high prices commanded by the likes of M-O Gold Lions, the temptation to falsify a tube's true origins becomes great. The problem, however, extends to new production as well. Bad batches of new tubes are just as likely to be sold as is the plant's premium production. Prospective buyers may get hurt, not only because they got ripped-off in the first place, but also from subsequent physical damage to the amp—a tube arc could easily take out a whole circuit board, causing extensive damage.

Manley argues that manufacturers also stand to get hurt when, for the sake of public relations, they're obliged to absorb extensive repair costs. My answer is to emphasize that it's imperative for tubes in general (especially power tubes) to be comprehensively tested and graded. As long as audiophiles avail themselves of such reputable tube services as Gold Aero, ARS, Tubes by Design, or RAM, I see no reason to suspect the quality and reliability of new or, for that matter, N.O.S. tubes.

It's also essential for manufacturers to use common sense in selecting the power-tube operating point. The Manley 175's Russian 5881s are run at a plate potential of 525V, which rules out quite a few possible substitutions. Note that this represents a nominal operating point. What if the AC mains voltage increases by, say, 10%? The (unregulated) plate voltage will drift in concert with the line voltage, and will climb to over 570V—very close to the tube's stated maximum plate rating of 600V.

The original 6L6, with its metal-can body, was rated at a maximum of 395V. Later versions, such as the 6L6G ("coke bottle" glass envelope), the 6L6GA (smaller "coke bottle" glass envelope), and the 6L6GB (straight glass bottle), all shared similar maximum plate-voltage ratings and similar screen- and plate-power dissipation ratings. The 5881 was one of the first 6L6 equivalents with uprated specifications—at least as far as screen and plate dissipation. Visually, its compact, 3.5"-long body (with straight glass envelope) gives it a much less imposing presence than a 6L6, a 6L6G, and certainly a KT66.

With the introduction of the 6L6GC, the 6L6 reached a new level of power handling. Maximum ratings escalated to 500V for the plate and 450V for the screen, plate dissipation increased from 20W to 30W, and screen dissipation almost doubled, to 5W. The KT66 and its US cousin, the 7581, were also rated at 500V max plate voltage.

Several STR versions of the 6L6GC were built for some big customers. For example, Fender's STR 387, I'm told by Manley, was required to run safely at 550V on the plate. As you can see, therefore, with the possible exception of some STR 6L6GCs, no alternative was ever rated for use with a plate voltage above 500V. Historical spec sheets, however, aren't always reliable or definitive guides to a particular tube type's performance envelope. Practical experience is the bottom line. Without a doubt, Chinese 6L6GCs should never be used in the 175s. The Chinese "GC" version will destruct at plate voltages above 500V and, according to Randall Smith, shouldn't be used above 450V.

Manley maintains that a real 6L6GC would be a fine alternative to the stock Russian 5881. When I asked him about the KT66, he again felt that a real KT66 would work well. He actually sent me (unsolicited) 16 N.O.S. KT66s to experiment with. From this I conclude that Manley Labs will condone certain substitutions for the stock 5881. But be sure to check with the factory before fitting any alternatives.

Output-tube options
Ironically, of all the 6L6 types I tried, the only problem I encountered was with one of the original Sovtek 5881s. One of the stock tubes arced in spectacular fashion, enough to scare my wife Lesley, who happened to be in the listening room at the time. Fortunately, the only damage was a blown B+ power-supply fuse.

I briefly experimented with original metal-can 6L6s. Because the plate dissipation of the 175 is well within the 6L6's 20W rating, this was a thermally safe substitution, and the tube appeared to hold off the plate voltage just fine. The 6L6 came across with more vivid, yet sweeter and more forward, mids—in contrast to the laid-back, exceedingly liquid sound of the stock Russian 5881s. The extreme treble sounded cleaner and more open, with velvety textures, but without the lower-midrange veiling of the stock output-tube complement. At last, hall ambient information shone through with greater transparency. And, most important, bass lines were fleshed out in tighter fashion and with a greater sense of timing. But I could hear David whispering in my ear: "For how long?"—a valid question, as I don't feel comfortable pushing a 6L6 into service over the long haul at a plate voltage of 525V.

The Russian 6L6WGB/5881 fared far worse sonically. The upper mids were grainier and sounded less sweet. Harmonic colors were dulled through the brilliance region, as if bleached out. "Color me gray" was this tube's battle cry.

Gold Aero's GE 6L6GC N.O.S. came very close to capturing the sonic flavor of the 6L6. Harmonic colors were more sensuous, sounding sweeter and warmer than with the stock tubes. Layering of depth perspective was also better. But, above all else, this tube evinced the drive and energy of the 6L6. The music's microdynamics bloomed without reticence. Loreena McKennitt's "The Lady of Shalott," from The Visit (Warner Bros. 26880-2), ebbed and flowed within a believable dynamic scale so that every dynamic nuance was fully resolvable, and with just the right harmonic palette. Amazingly, the 175 was transformed from a heap of jelly into a finely tuned musical instrument.

Out of the 16 KT66s Manley sent (footnote 1), I could only properly bias 11 of them. With the KT66's higher transconductance, there wasn't enough range in the bias pots to set the bias current at 27mA. That number of KT66s sufficed only for about a channel and a half—I filled in the rest with Russian 5881s. Even so, soundstage transparency advanced several notches over the stock tube complement, while harmonic textures took on a distinctly romantic hue, sounding sweet and tropical-forest lush. Neither was the elemental drive exhibited by the 6L6 shortchanged. The KT66 in this application had classic tube sound in spades, but without sacrificing lower-octave resolution and definition.

Final thoughts
Undeniably, the stock Manley 175 monoblocks possess a special gift that, upon first listen, is bound to dazzle and impress even the most hardened transistorphile. The bravura with which it caressed midrange textures was awesome. When I closed my eyes, I was beamed into a palpable, living, breathing soundstage, where I could swim laps in an ocean of liquid harmonic textures.

Unfortunately, the 175 is handicapped by a laid-back, languid upper midrange and an overly ripe, somewhat veiled bass reproduction, with the effect of robbing the music of drive and dramatic spark. A refined Dynaco Mk.III on steroids is what the stock 175 ultimately reminded me of.

Configured with either Gold Aero's N.O.S. GE 6L6GC or genuine KT66s, the 175s gained sufficient bass-line precision, lower-midrange transparency, and timbral accuracy to challenge any high-power tube amp I've heard to date. The sonic gains vault the 175 to the head of the Manley line—the 175 was, in essence, the most musically incisive Manley or VTL amp I've ever heard.

The sonic potential of the Manley 175 clearly rests on its output-tube complement. The choice is yours, but be sure to check with the factory before you roll-in any tube alternatives.

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Footnote 1: I did consider using the Chinese KT66, which is very much in production and available as a premium-grade, fully tested Golden Dragon product. Tubes By Design's Kevin Hayes was kind enough to provide me with the requisite complement of these. However, the Chinese KT66 looks like a slimmed-down version of the original. Gone is the "fat" glass bottle, and the generous electrode structure with its massive plate. I was uneasy about pressing the Chinese KT66 into service, so decided not to chance it with the Manleys.—Dick Olsher