Positive Feedback ISSUE 58
november/december 2011

 

 

A Quick Review of the JWN Amplifier
by Lynn Olson

 

I've been looking for a "backup" amplifier for my rather over-the-top Karna amplifiers for some time now. Why? Well, a matched quartet of 300B meshplates ain't all that cheap, and the matched quartet of 45's (for the driver) ain't that cheap either. And I've found out the hard way that way that transistor amps just don't cut it, at least for me.

No Class D, either. No thanks.

Cheap-n-cheerful Chinese SET's—no, not that either. Despite getting the DHT-triode bug from writing the first American review of the Ongaku [way back in the print days of Positive Feedback, early 1990s vintage], I still have a soft spot for the right PP-pentode amplifier. Yes, it's true that once you get used to the you-are-there clarity and beautiful tone colors of the best DHT amplifiers, most pentode amps sound congested and cloudy, with a shut-in perspective.

But the process of designing the Karna amplifiers taught me a few things—the weak spot of many amplifiers are sketchy driver stages that can't deliver enough linear current to the power tubes—and this is a particular problem for PP-pentode amplifiers, where the phase—splitter frequently does double duty as a driver. Adding another driver stage takes the load off the inverter, but also seriously degrades the overall stability of the amplifier thanks to the additional RC-coupling—this is the most serious problem with the Williamson topology, dating from the original Wireless World article in 1947.

The sound quality of PP-pentode amplifiers is strongly affected by the phase inverter, which comes in three main flavors (and I mean "flavors" literally, since they affect the overall sound of the amplifier). The split-load inverter (or "concertina") is used in the Williamson, Dyna Stereo 70, and many other "Golden Age" amplifiers, but to be honest, I don't like the sound of this inverter at all.

Maybe it's the difference in distortion spectrum coming off the plate and cathode, maybe it's the weak drive capability, I just don't know.

But they're not my cup of tea.

The next most common flavor is the long-tail pair, or "differential" inverter. This was first used in the Mullard amplifier of the late Forties, Leak, Radford, and a number of other high-end amplifiers of the Fifties, including the Marantz. To my ear, these have more refined and modern sound, and sound more open and spacious.

There's a third flavor that is quite rare in amplifiers from the Fifties onward—the floating paraphase inverter. On paper, it is easy to see why it went out of favor—the balance isn't as exact as the other two, but it has the intriguing property of a stronger driver capability, which was an important parameter for the Karna amplifier.

I came across Dick Olsher's Enjoy The Music review of the JWN amplifier, and it attracted my interest. His comment about the 3D capability was very interesting—depth and width is a very desirable property in an amplifier, since it indicates the amplifier is recovering subtle room reflections, and preserving the important time relationships. Amplifiers with good 3D are usually good at other things, too. And it's not like amps have reverb circuits in them; they have to recover reverb, not make it up. The lack of spaciousness in many transistor amplifiers is a strong indication that low-level detail is being suppressed or masked, even if it isn't apparent in most measurements.

So I contacted Jim Nicholls, and he suggested some interesting improvements to the "standard" design. Separate B+ supplies for the input/driver and the output section—no argument there, it's a key design feature of the Karna amplifier. I suggested a separate heater transformer for all the heater of the audio tubes, and a few other wrinkles. We went back and forth a few times, improving a bit each time. And the starting point was pretty good; all vacuum-tube rectification, all point-to-point wiring, choke-fed filtering (far better than the usual giant caps used in transistor amp), and my favorite octal tubes for the drivers, the 6SN7, which is one of the most linear tubes ever made.

SND Tubes has a smokin'-hot special on a quad of Philips/ECG 6BG6GA's—48 bucks!!! For a matched set of military-grade NOS tubes! So what the heck is a 6BG6GA? Actually, it's just a good ol' 6L6 inside, as used in millions of guitar amps worldwide. Specwise, the separate top plate connection increases the HV rating (good), and decreases Miller capacitance (really desirable). Unlike the 807 cousin, the 6BG6GA uses a standard octal socket, so rewiring the amplifier to use conventional 6L6 tubes is just a matter of warming up the soldering iron and moving two wires per tube. And it makes the amp look cool - infants and small animals can just keep their distance, as they should for any hifi system. These are toys for adults, not children.

I think you'll agree from the pix it looks good. No, it doesn't have a 1/2" anodized aluminum faceplate. Good. Those massive faceplates remind me of an Escalade cruising the Strip in Las Vegas. Not my thing, man. I like tube amps to look like tube amps.

OK, OK, what's it sound like? I was kind of curious what Jim Nicholls "Colorado Special" sounded like - it has a lot of subtle design details all in one package. Unlike most high-end tube amps (I won't name them, but all the usual suspects appear here), it is not a rehash of 1950's designs. It draws on earlier concepts from the late Thirties and mid-Forties, and predates the Williamson monoculture of the early to mid-Fifties. I didn't expect the JWN to sound like the usual Scott, Fisher, Dyna, Marantz, and latter-day copies, and it doesn't.

Despite the straight-pentode connection of the 6BG6GA's, it sounds like a pretty powerful triode amp—there's about 20 watts/channel on tap, beyond the usual SET range. Yes, it plays in the big leagues, the $10,000 to $15,000 playground of high-end triode amplifiers. The JWN did just fine in a direct comparison to the Serious Stereo 2A3 amplifier, which is one of the best SET amps on the market.

Most immediately noticeable qualities? Well, the 3D spaciousness is clearly there, and is evident even from another room. I frankly didn't hear pentode mush or hash at all—I have to say the 6BG6GA in pentode mode is a seriously under-rated tube in hifi circles. It sounds a lot like the original G.E.C./M-OV KT66 or Western Electic 350A.

Unlike DHT's, it doesn't need to be babied. Think of millions of guitar amps—how gently are they treated?

I was listening for the usual pentode/beam tetrode grunge, and by golly, I just didn't hear it. I sure hear it in plenty of other audiophile EL34/6550 amps, so this amp is really doing something right. The complete absence of any kind of semiconductor rectifier, regulator, source follower, or other kinds of latter-day gizmos probably gets some of the credit, along with the low-noise all-vacuum- tube rectifiers, which have much quieter switching characteristics than conventional solid-state diodes.

Tone quality? Vivid, right-there, and surprisingly strong and deep bass from such a little bitty amplifier (note that it's only a little bigger than an Xbox Slim). Most of all, fast—it follows the most complex beat without getting confused. This is pretty rare, folks—most high-end amps flunk this test. I've heard plenty of $60,000 to $100,000 amps that had a really poor sense of rhythm (tube amps with solid-state regulators, I'm looking at you).

The JWN excels where many high-end amps fall down; it really communicates musical values. You can hear if the musicians are excited and really getting into the performance, or if it's just another studio run-through. Although many audiophiles are into the detail thing, I care more about the expressiveness of the performance.

Another striking quality is a bouncy, alive quality to the sound—like an exuberant puppy, the sound just about jumps right into your lap. This is not the ponderous and overwrought quality of so many high-end amps, particularly the 300-watt floor-standing behemoths. It ain't nothing like that. If the music is any fun at all (and those of you who don't listen for fun can just turn the page right now) you'll be grinning too, because the enthusiasm of the musicians comes right through, no guessing or make-believe. Tone colors are vibrant in a triode-like way, maybe not 2A3 sweet at the top, but the thrill factor carries you right along. The word is FUN.

And the most fun of all? The price for the super-deluxe, tricked-out "Colorado Special" is in the $1K to $2K range, including all tubes.

Yes, you read that right. Made in the USA too, right in New Jersey, handmade by Jim Nicholls. If you were determined to spend more, the only additional improvement I can think would be two monoblock chassis. Jim will be happy to build it that way too—as you can see from the web-page (see References section below), every amp is custom-built to order.

How much do I like it? I'm really glad I took the chance and bought the JWN sight unheard; Jim was really easy to work with, and I ended up with an amplifier with a lot of custom features usually only found in amps costing more than $10,000. Sonically, it's the best pentode amp I've heard, and has better, more natural musical values than many SE-DHT amplifiers.

While I'm at it, I'd like to give another shout-out to the Monarchy DAC; just after the 2011 Rocky Mountain Audio Festival, I had the opportunity to directly compare it to a top-reviewed $5000 DAC, and I strongly preferred the Monarchy—which goes for less than $1100 on the Monarchy web-page. The JWN and Monarchy make an electronics stack for less than $3000—and give better sound than most $50,000 stacks.

References

JWN Amplifiers: http://jwnamplifiers.vpweb.com/

Dick Olsher review of Lance Cochrane and Jim Nicholls amplifiers:

http://www.enjoythemusic.com/magazine/equipment/1010/master_builder.htm

SND Tubes Philips/ECG 6BG6GA: http://www.vacuumtubes.com/6BG6.html

Serious Stereo 2A3: http://www.seriousstereo.com/serious_stereo_2a3_amp.html

Monarchy Audio: http://www.monarchyaudio.com/NM24_Main_Frame.htm

Nutshell High Fidelity and Karna amplifier: http://www.nutshellhifi.com/triode2.html

 

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