Listening #52:
Page 2
The high-tension line, which has dark stripes on a silvery yellow background—the yellow may have been from age, of course—was shorter. I desoldered one end from the point on the tag board where it connected to resistors R2, R3, R5, and R6 (footnote 1), and clipped the other end from terminal 3 of the Jones plug. All that remained was to desolder the input signal wire from terminal 2 of the input socket—I left the other end connected to the control grid of input tube no.1—and Mr. Jones was made redundant.

Now came the hard part: fashioning a mounting plate for a newfangled phono socket. One could either make such a thing out of phenolic and retain the Quad amp's original grounding eyelet to the chassis, or make it out of metal and ground it the easy way. I retained the eyelet and made mine from metal, making sure to discard the insulating washers that came with the Kimber phono plugs that I prefer. Back went the 1.5 megohm loading resistor (R1) as well.

After about three hours' work, my new old Quad amps were driving my new old Quad loudspeakers to good effect. There was no hum or noise— zero—and the bass and treble extensions were remarkable.

On the downside, it was apparent that one of the amplifiers was playing a bit quieter than the other. Nor did either of them sound as good as the best Quad IIs I've heard over the years. The breed is known for a rich, deeply textured sound, but these amps had too much texture: treble highlights—on note attacks and the like—were overdone, and natural overtones that should have sparkled were instead gritty. Back to the bench...

Changing times, changing values
Reasons for some of the above sonic attributes were obvious. One of the Quad IIs had come with KT66 output tubes, but the other had arrived with that tube's substitute, the common 6L6. (Both had 5V4 rectifier tubes, a perfectly fine replacement for the GZ32.) A quick call to New Sensor Corporation (800-633-5477) was all it took to get a matched quartet of Sovtek KT66s for just $124. (Other Sovtek tubes, especially their 2A3, had impressed me with their excellent ratio of performance to price, and in any event I couldn't afford a set of new-old-stock GECs. Imagine that.)

An additional fault seemed likely: In an amplifier this old, most if not all of the resistors can be counted on to have drifted in value, typically upward. Although Peter Walker designed the Quad II so that shifting values would minimally affect performance, there's no denying that a push-pull amplifier will underachieve when certain parts values creep beyond the pale. (Still, the Quad II's published specifications state that tube mismatching of up to 25% will result in no more than 0.08% additional distortion!) Moreover, some resistors have the potential for getting just plain noisy—as with R1 (1.5 megohm), which, in parallel with the grid of the EF86 pentode, sets the Quad II's highish input impedance.

Replacing all the resistors gave me peace of mind, if nothing else. I measured them beforehand and found that some were way high. I was also puzzled that the global feedback resistor, which should have been 470 ohms, seemed to have drifted drastically lower in value—until my engineer friend Neal Newman explained that it couldn't be measured in situ without having one of the output-transformer secondary windings in parallel.

With one exception, the Acoustical Manufacturing Company, Ltd., specified quarter-watt resistors for the Quad II amplifier—which is surprising to know, given how physically big those old resistors were compared to those of today. In one of my amps, R5 and R6 appeared scorched, and in the other, those two resistors—and only those—had been replaced. The need for a beefier part in those positions seemed pretty clear. To play it safe—and, I admit, to achieve a less jarring appearance—I chose 1W resistors for all but the KT66 cathodes, which cried out for a big 5W jobbie. Far be it from me to say which resistors sound the best, or to even suggest that such a question is reasonable; suffice it to say that most of the ones I used are Rikens, purchased from the genuinely nice people at the Parts Connexion (866-681-9602).

An additional warning: I've seen two contemporary (redrawn) Quad II circuit diagrams on the Internet that both misidentify the value of R4 as 680k ohms. Coincidentally, and in their defense, I made the same mistake myself when I misread the value I'd copied down from my auto-ranging multitester's display: 0.680k ohm. Whatever the reason, you should trust the original schematic and use a 680 ohm resistor in that position.

I replaced all 13 resistors in each of the two amps, one at a time, working slowly, and with heatsinks where appropriate—such as the choke terminals that hold the cathode bias resistor in place. Even so, a few terminals on the tag boards broke off easily, forcing me to improvise more than I would have liked. A more cautious approach to removing the old resistors, with less wiggling of leads and fatiguing of metal, might have prevented that.

The finish line was in sight. I reinstalled the tubes, powered up the Quad IIs, and the sound was more or less the same as the last time. So I disconnected the amps from my system but left them powered up, propped upside down with their bottoms undone, in order to measure the operating voltages. Low rail voltage on one amp led me to try a new rectifier tube, which brought things back into line—and, in so doing, I noticed that the amp's original 5V4 tube was cracked. That explained that.

But on the other amp, the suppressor grid potentials for the two input tubes were drastically different from each other—which ain't good—and because the two resistors that establish those voltages were identical (not to mention brand spanking new), the fault clearly belonged to one of the input tubes. Another call to Parts Connexion snagged me some new EF86s—two matched pairs, just to make sure.

That, my friends, made the biggest performance difference of all. In fact, if I had to do it all over again, I'd start by buying new input tubes and let everything else slide for a while, if I could get away with it.

No matter: What we're talking about here is a stereo pair of world-class amplifiers for ridiculously little money. In their present condition, my Quad IIs are great fun to listen to. Yes, they sound like tube amps, in the best and worst ways— best meaning that they have wonderful presence, warmth, and texture, especially through the mids, and worst meaning that their bass is somewhat woolly. Rhythm and pacing are not up to Naim 250 levels, but tunefulness and pitch relationships are fine, and the Quads have an excellent sense of momentum and flow.

They can also sound big when they want to—which is to say, the Quad IIs let the music sound big if that's how the microphones heard it. The bass drum in Sir Adrian Boult's recording of Edward Elgar's Dream of Gerontius (EMI SLS 987) commanded the stage believably enough—surprisingly so, in fact, for a 16-lb, 15W tube amplifier designed more than half a century ago. Percussion was also the unexpected star of the show when I played Cisco Music's brilliant LP reissue of Benjamin Britten's Young Person's Guide to the Orchestra (with the Concert Arts Symphony Orchestra conducted by Felix Slatkin, Capitol SP8373). The drums resonated believably—no more—and spoke with surprising force.

Incidentally, I'd enjoyed that record through my Quad amps before I remembered that Cisco's Robert Pincus, who was the recording supervisor for the remastering sessions, uses a pair of vintage Quad IIs in his everyday system. One presumes he also heard those wood sprites singing, each to each.

Q, R, S, and T
On undramatic music played at very low levels—impractically low, I'd imagine, for most people—my single-ended Fi 2A3 Stereo amplifier sounds cleaner, clearer, and more open through my Quad ESL loudspeakers. On most all music, my single-ended Lamm ML2.1 monoblocks have more of a sense of drama and an even greater sense of presence. On rock'n'roll music, my vintage Naim 110 has the tightest bass of all.

Nonetheless, the Quad IIs— my Quad IIs—have qualities I cherish. They pull voices out of the mix almost as well as a single-ended triode. Their spatial performance on good stereo recordings is shockingly good, with gobs of believable wholeness and depth. Notwithstanding their lack of speed, the Quad IIs portray bass instruments as having a kind of warmth, texture, and humanity that I don't hear through other amps. And I don't mind admitting that the little Quads look cool, and that I simply like the idea of owning them.

There's something here for everyone: I brought my Quad amps to Neal Newman, whose test bench includes a signal generator, dummy load, Heathkit audio analyzer, and other niceties. Both amps performed to spec—neither exceeded 0.1% harmonic distortion, measured at 400Hz, all the way up to 15W. That wave looked really nice, too. (When the amps did distort, the onset was gentle, and the negative swing lost its shape before the positive swing—which I think may relate to the paraphase arrangement of the first gain stage, whereby tube no.2 doesn't get precisely the same signal as tube no.1 in terms of waveform phase.) I suppose this fine test performance, along with the fact that the amps are still in service at all, would please Peter J. Walker more than anything else.

A few miscellaneous bits of advice for those who might consider this route:

• Quad II amplifiers were made with one of two different mains transformers: one for household current between 100V and 120V AC, the other for current between 220V and 240V. Both allow the user to choose a more precisely correct voltage range by means of a screw-type primary jumper, accessible from the rear panel. (Contrast this with the Quad ESL speaker itself, which is user-adjustable for virtually any household current.) If you're considering buying a Quad II, make sure it can be used in your locale.

• On a related matter: The Quad II amplifier predates today's three-conductor AC sockets and plugs. Its detachable two-pronged power cord can be flipped one way or the other, but one of those ways is better than the other. Find out by plugging in the amp (it lacks an on/off switch), then using your multitester to measure the voltage of the stray current traveling between the amp's chassis ground and a known good ground, such as the third conductor of a nearby unused AC outlet. Flip the power cord plug and measure again: The lower reading is the righter.

• As manufactured, the Quad II was set to drive speakers of relatively high impedance (nominally 15 ohms), such as Quad's own ESL. The amp can, however, be modified to deliver optimal power into a 7 ohm load. Inside the Quad II, just below the resistor tag board and off to one side, you'll see a row of output-transformer secondary taps, each identified with a white stick-on letter (here's hoping the letters are still there). For 15 ohm operation, there should be a jumper between the taps marked R and S; for 7 ohm operation, you must clip or remove that link, and install links between Q and R and between S and T—another of those procedures that must be carried out with a heatsink, preferably with a good-quality, temperature-adjustable soldering iron.

• As mentioned above, the 5V4 rectifier tube is a fine substitute for the original GZ32. I am not, however, comfortable with the idea of replacing the rectifier with a GZ34 or its American-name counterpart, the very popular 5AR4—examples of which have been known to draw more than 2 amps of filament current, and may stress the mains transformer. As your father may have told you when you whapped your sister and she had the audacity to whap you right back, Don't come crying to me...

• I haven't mentioned the subject of capacitors, and I don't want to. Will the power supply be "faster" if you replace the twin 16µF caps with Black Gates, assuming you can get them? Maybe. Will the amp sound different if you replace the coupling caps with modern, polypropylene ¸bercaps? Probably. Does the 25µF cap at the bottom of the cathode windings have any effect on the sound? Debatable. At this point in time, I remain incurious: happy with my amps the way they are. (But if I ever change my mind, in my "good" parts bin is a quartet of 0.22µF Sprague Vitamin Q oil caps that I've been hoarding for some special project...)

The point remains: I could live the rest of my life with these amps and these amps only, and never want for much. On the other hand, there's nothing wrong with owning and using a variety of different-sounding amplifiers for the sheer fun of it. Depending on your intentions, that needn't be an overly materialistic point of view—and it has nothing to do with snobbery. Especially not when your amp collection is centered around things that are small, or affordable, or reliable, or abundant.

Half a century after its introduction, the Quad II is all that and more. Small wonder.

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Footnote 1: R2 and R3 are the resistors that establish the correct potential on the suppressor grids of the input tubes; R5 and R6 are the same tubes' plate resistors. All of these details and more can be found in the Quad II circuit diagram, which appears on a great many Quad-related sites on the Internets—itself made up of tubes.