Tube Power Amp Reviews

Manufacturers' Comment

Editor: Thank you for your time and effort put into the measurements of our amplifiers for this review. Your measurements are correct, but we must point out that the Margules U280-SC Black is definitely not a conventional tube amplifier. It has a different design approach, so it should be treated as such, and your comments are a good opportunity to explain the engineering behind the amplifier and to shed light on why Jason Victor Serinus's auditory evaluation and your technical measurements have differing outcomes.

Respectfully, and in the interest of accuracy to best serve your readers' understanding, we submit that we consider this to be a case of misunderstanding our type of amplification technology that has led to the utilization of incorrect or inappropriate methodologies and metrics to measure our amplifiers, naturally yielding materially erroneous results that are misleading for the proper evaluation of our amplifier design and performance.

Please allow us to explain. The Margules U280-SC Black is a full class-A amplifier that tracks the input and output signals. Having said that, if this amplifier is fed with a continuous and constant signal, it will run, as [John Atkinson] commented, "unacceptably hot." But if you use it as an audio amplifier to reproduce music, which is its purpose, with a music signal it will run much cooler than most tube amps. Even after hours of operation at high volume levels, you can put your hand on any of the surfaces without burning yourself, and you can even touch the tubes without harm (of course, they will be hot). This in fact provides important benefits; among others, it will prolong the life expectancy of the tubes and equipment.

The U280-SC Black is the only tube amplifier that we know of capable of offering full class-A operation with almost the efficiency of class-AB while not requiring any adjustment, even when changing tubes or tube types. That is why no users or reviewers anywhere have mentioned any concerns with the heat output—quite the opposite.

To achieve low output impedance, distortion, and improved frequency response, most tube amplifiers use negative feedback; this technique has its drawbacks, as it tends to sound unnatural, and can imprint a "tube" sound lacking richness and detail. The Margules U280-SC Black is a full class-A amplifier that has no negative feedback. The control to achieve proper performance is taken care by the proprietary active servo controls, which dynamically change the tube characteristics to optimize power transfer and distortion.

The changes in the output impedance change not only the power transfer but also the damping control; our manual states that, for speakers of 4 to 8 ohms, we recommend that this amplifier be set to use the 4-ohm taps, and for speakers of 8 ohms and up we recommend that this amplifier be set to use the 8-ohm taps; we recommend that users optimize the taps to their systems. The Margules U280-SC Black amplifier, when set to be used as a monoblock, parallels the channels—because you double the current capability, the taps should be used differently, as stated in our manual.

It has to be noted that the specifications of the U280-SC Black are rare for this type of amplifier. In engineering and design work you deal with a large number of variables, and to make a proper product you are required to balance them to achieve the design goals. This amplifier was designed, first and foremost, as an audio amplifier with the sole purpose of music reproduction; the emphasis was not on the static signal (continuous signal), but rather on the dynamic behavior like music reproduction. It was designed to be able to operate with all its capabilities with any conventional loudspeaker and make it sing, even highly capacitive speakers like electrostatics, thereby covering most of the speakers in the market.

We believe that our products should be steady and reliable as a rock for decades of satisfying performance. Therefore, all of our amplifiers and preamplifiers have been and will be upgradable to our new technological improvements.

We do hope that this response, which is written in good faith and in the most constructive spirit, is of use to you for what is understandably an unusual design that incorporates proprietary elements. Therefore, in the interest of providing your readers with an accurate representation of the technical characteristics and performance of our amplifiers, we must respectfully request that you either reevaluate said technical aspects based on the corresponding and correct criteria, or publish our full explanation, again in the best interest of serving your readers. Thank you.

Note: The specifications, especially distortion and power, are obviously affected by the tubes. When products leave the factory, the measurements stated must be fully met. The supplied tubes undergo double tests, one by our supplier and one in our factory; even so, we offer a two-month tube warranty if failure is suspected. That is the nature of tubes; however, we will be happy to supply a new set to you for testing, as we do for our customers.—Carlos Smith, Sales & Service, Margules Group