Tube Power Amp Reviews

It would appear that there are still people out there who are unaware that this is the age of the transistor. Not only are tubed amplifiers not vanishing from the face of the earth, they are proliferating. Audio's equipment directory for 1977 listed three tubed power amplifiers. The same directory for 1984 lists over 30 of them, and the Quicksilver amplifier is not even included!

This very hefty (95lb) stereo power amp uses eight EL34 tubes to deliver a rated 95 watts per channel into 4 or 8 ohms. The unit comes with a plug-in milliammeter for setting bias, and detailed adjustment instructions are provided. But as is so typical of American designs, this adjustment is made more difficult than necessary. Bias is supposed to be periodically readjusted during the first 50 hours of operation, and should probably also be trimmed from time to time during the "many years" of the tubes' life. But bias adjustment requires removing the perforated metal cage, which is a minor pain, but becomes a major pain if the amplifier is normally stowed in an inaccessible location, simply because the thing is so hard to move!

The preliminary owner's manual supplied with our sample did not inspire confidence. There are various and sundry warnings about how the amp should be turned on (through two switch settings), during what phase of its life, and in every case the bottom line seems to be that not following the instructions might cause the output tubes to break down. This didn't happen while we were using the amplifier (passing it from staffer to staffer for their individual comments), but it had all of us a little hesitant about turning the thing on for fear of forgetting the warmup countdown. Ultimately however, our sample unit proved more reliable than several other tubed power amps we have tested in recent years: nothing blew up.

Sound Quality
Describing the sound of the Quicksilver is a little more challenging than usual because it comes in two incarnations: triode and pentode (footnote 1). Not triode or pentode outputs, as one might guess, but triode or pentode voltage-amplifier/driver sections. The amp is now normally delivered with the triode boards in it, while the pentode boards are available on special order for $165 per stereo pair. At the time I tested the amp, the pentode board was "standard," so I spent most of my time listening to that version of the amp.

Sonically, the most descriptive thing that can be said of the Quicksilver (with its pentode boards) is that it is tube-like. It has the typical good tubed amp's aliveness, richness, warmth and almost-holographic depth. Inner detailing is superb but, for those to whom spaciousness is the be-all and end-all, I must report that soundstage presentation from the Quicksilver is not quite as wide as I have heard from some solid-state power amps—the Eagle 7a and the Threshold S/500. Bass is deep and full, tending slightly towards heaviness through the midbass region. LF impact is good but not in the same league as that of the best solid-state amps, nor indeed as good as the Berning 2100 or Conrad-Johnson Premier One from the tube world. Highs are rather soft but exquisitely sweet and musical—comparable in quality to our top-rated Premier One.

Driver-board replacement is simple and straightforward. You remove four anchoring screws (two per board), unplug the original boards, plug in the alternate boards, insert the tubes and replace the screws. There are adjustments on each board to optimize performance, but they require the use of a distortion analyzer, which I do not own, although Quicksilver president Mike Sanders maintains that the adjustments are not critical, and that the boards can be used as set up at the factory. The triode boards effected an interesting change in the sound. The high end became noticeably sweeter yet with no apparent loss of detail or "snap," the overall sound became subtly warmer, and depth (perspective) increased almost to the point of exaggeration. This increase in depth was a remarkable effect, and not at all unpleasing.

Not surprisingly, the Quicksilver is at its best driving electrostatics, which tend to complement perfectly its own characteristics. With Acoustat 2+2s for example, the sound with the triode boards (which I preferred) was about as good as I have heard from those speakers, although not quite as velvety-smooth at the top nor as tautly controlled at the bottom as with the Paoli S.O.B. Although I had not had a chance to try the Quicksilver with Quad ESL-63s by the time this report was written, I would be willing to bet that this would be one of the best driving amplifiers around for that speaker system. Its power rating is almost exactly what the 63s call for, and the Quicksilver's lush liveliness should flesh out the ESL's slightly dry sound with solid-state amps. I am not nearly so enamored with the sound of the 63s with solid-state amplifiers as are a lot of other critics.

Conclusion
The Quicksilver Model MS190 has been on the market for 3½ years, and the manufacturer reports only two amplifiers returned out of 85 produced; probably we can assume that the current model, the MX190 (footnote 2), will be also very reliable. Sonically, it must be ranked at the top among tubed amplifiers, along with the Paoli S.O.B., the C-J Premier One and Premier Four, the Berning EA-2100, the Audio Research D-160B, and the EAR 509. (The EAR and the Paoli sound rather un-tubelike in some respects, and should be considered on their own merits rather than as "tube amplifiers.")

I can heartily recommend the Quicksilver MS190/MX190 for use with electrostatics, but I feel that its suitability for dynamic speakers will depend very much upon the individual speakers. Those which produce good musical balance with your average solid-state amplifier may well sound rather heavy and slightly dull with either configuration of the Quicksilver, although less so with the Pentode boards. However, a dynamic speaker like the Thiel CS3, with its extended high end and tightly controlled bass, could do very well with this amp.

In short, this is a very fine amplifier at a price which a year ago I might have thought was steep, but which is just average these days for a 100-watt tube amplifier. The choice between triode and pentode boards increases substantially your ability to make the sound of the amp appropriate to your system.

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Footnote 1: See "As We See It" in this issue for a description of the standard triode tube. A pentode tube inserts two additional grids; a screen grid, which steps up the amount of current change occurring with a given control-grid voltage change; and a suppressor grid, which limits electron re-radiation from the anode. This results in more gain per tube; in the Quicksilver, the pentode driver boards have only one amplifying stage (one amplifying tube per channel), while the triode boards have two.—Larry Archibald

Footnote 2: The model actually auditioned was the MS190, although the specifications we were given cover the MX190, a slightly later version. The primary differences in the MX version are larger output transformers and heavier gauge wiring (10 as opposed to 16). The manufacturer assures us that the audible differences are minimal.—J. Gordon Holt