Tube Power Amp Reviews

Then I sobered up enough to form a more objective picture of the 175's performance with the Audiostatics. Stage lighting was darker than with either the Music Reference RM-9 Mk.II or the Pass Labs Aleph Zero amplifiers. The presence and lower-treble regions were a bit subdued, such that violin overtones lacked a full measure of sweetness. Specifically, while the extreme treble sounded open and reasonably airy for a tube amp, the 5–10kHz octave seemed deficient in brilliance. Bass lines weren't fleshed out with convincing definition, sounding a bit full and bloated. More seriously, bass-line punch and pace were diminished. I was left wishing for more rhythmic drive and lower-midrange transparency. The net result was that the Manleys were squeezing out some of the music's fire and drama.

These same impressions were reinforced when I switched to Magnepan's Magneplanar MG-20 loudspeakers (review forthcoming). I like to listen to Judy Collins's rendition of "Pretty Polly" (Wind Beneath My Wings, LaserLight 15 451) at loud volumes. This track, which is blessed with a mean guitar solo, rocks'n'rolls along. The mids sounded glorious as usual, with billowing depth and palpably focused image outlines. The problem, however, was that "Pretty Polly" sounded pretty all right, but not nasty enough. Bass lines didn't boogie, and the feel for the recording venue was somewhat veiled, blunting the music's impact and pace.

I had a similar reaction to Tracy Byrd's No Ordinary Man (MCA Nashville MCAD-10991). Try to imagine country music without the infectious drive—there's little left to write home about. My favorite cut, "Lifestyles of the Not So Rich and Famous," lacked adequate punch and steam; the music was less compelling and involving.

Tube-rolling
It was about this time that I started experimenting with tube substitutions. My sincere thanks go to Gold Aero's Frank Morris, who generously provided me with a variety of 6L6 tubes to experiment with. I quickly discovered that the stock input and driver tubes were sonically about as good as anything else I could muster. However, the performance of the 175 monoblocks very much hangs on the choice of output tube.

The sonic fate of any tube product strongly depends on the final choice of tubes, as differing brands of a given type—even different production runs within a single factory—exhibit unique harmonic signatures. Tube aficionados routinely audition several tube brands in a component in an effort to optimize its voicing in the context of a particular system.

Because the whole tube scene is predicated on tube-rolling, I unabashedly advise tubeophiles to experiment. In my experience, at least 50% of tube products I've reviewed would have gotten a thumbs-down or been seriously downgraded had they been evaluated with their stock tube complements. I see no indication that tube-gear manufacturers are particularly adept at tube selection.

Most of the tubes I substitute are either in current production (eg, of Russian or Chinese origin), or are New Old Stock (N.O.S.); ie, available in quantity, and in tested and graded format from such companies as Gold Aero. Occasionally I'll report on hard-to-find tubes such as Telefunken 12AX7s or Sylvania Gold Brand 5751s, which are worth looking for in particular applications.

Many manufacturers take a dim view of tube-rolling, especially in regard to power tubes. David Manley, who went on record long ago as an ardent opponent of the practice, recently wrote me a 15-page letter in which he made it clear that he regards substituting any tube types other than those specified in the owner's manual as tantamount to modification. He argued that consumers wouldn't dream of substituting transistors in their Krells or Mark Levinsons, and that, although changing tubes requires no soldering, they still shouldn't be messed with. Manley also resents the implication that the designer or manufacturer has failed to offer their best shot in the original parts supplied the consumer.

David, this isn't about good vs bad parts. Often a manufacturer's view of "good" is confined to part selections that are reliable and which leave little chance of failure over the life of the product. Besides, although transistors aren't meant to be replaced routinely, tubes are—if only because of their much shorter lifespans.

By far the most compelling issue is that of reliability. Manley has little confidence that the average person in the street has a fighting chance of landing genuine N.O.S. tubes. He cites three instances of abuse: one company selling certifiable "pulls" as new, or selling nonspecific 12AX7s as 5751s; the M-O Valve Company, after closing down production in the UK and running out of their own specific builds (KT88, 66, and 77), selling imported Chinese tubes in Gold Lion boxes; and ECG-Sylvania, after closing down in the US, likewise continuing to supply dealers with any quality imports they could find—packaged, of course, in their own familiar boxes.

To this list I can add specifics related by Randall Smith concerning their own STR 415—a "Special Test Requirement" 6L6GC that Mesa Engineering requested Sylvania to make for them. The STR 415 was physically more rugged and less microphonic, and included a thicker emissive coating on the cathode for longer life. The last batch of some 200,000 tubes shipped from the factory failed spec and was rejected by Mesa. Randall Smith reports that Sylvania sold these tubes to others; they're still in distributor inventories to this day. Substandard production rarely gets destroyed; rather, it's sold off, sometimes unbranded, to the highest bidder.