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REVIEW: Four 6AS7G-based home tube headphone amps reviewed and compared

post #1 of 51
Thread Starter 

After getting a fourth tube headphone amp which uses the 6AS7G power triode, I decided it was time to sit down and do some comparisons. A little background: ever since getting a Darkvoice 336i (not part of this review), I have been enthralled with the sound of this tube. I prefer it by a wide margin over EL84-based designs, of which I own 2 and have owned 2 others. The EL84 is a very “tubey” sounding tube. The 6AS7, on the other hand, while definitely delivering that special touch of midrange magic that I personally only experience from tubes, it is a far less syrupy tube. In fact, in the better amps, it’s a gutsy, powerful, neutral, very wide range amplification device. It does have a lot of power. While somewhat prone to microphonics, the 6AS7G also has the advantage of being an inexpensive tube. Given that the starting point of a decent pair of EL84’s is $50, and a pair of something like an EL34 or 6550 is $200+, the 6AS7G has a significant advantage in being affordable, since they can be purchased as NOS for $12 each and used tested for $6 each at various online tube sellers.

The 6AS7G:

Black plate, top halo getter

Grey plate, top halo getter

Black plate, bottom getter

Of the US Makers, RCA-branded 6AS7G’s are by far the most common, and they sound good. Tung-Sol also made 6AS7’s, and they are a little better sounding IMO than the RCA’s (they are also constructed a little differently), but they are much harder to find, and not worth paying an enormous premium for. GE, Sylvania, Chatham and Raytheon also made 6AS7G’s, but they also don’t seem to offer anything the RCA’s don’t sonically, IMO. There are other brands available, but I am pretty sure they are just branded versions of the other manufacturers listed. The JAN (Joint Army Navy) versions were supposedly somehow selected specially (tighter tolerances, etc), but there are no construction differences with the non-military versions. I have seen JAN-6AS7G’s from RCA, GE, Tung-sol, and Chatham (which at one point was bought by Tung-Sol).

The GEC UK-made 6AS7G (A1834) are my favorite of the 6AS7G - they have brown bases and a different plate structure than the US types.

Finally, there are the Russian equivalent 6N13S, but I really don’t care for these. I have seen Amperex 6AS7G’s that say "made in Holland", and Ultron "made in W. Germany", but both of these were almost certainly Russian tubes rebranded.

The 6080 is identical in rated performance/specification to the 6AS7G. The 6AS7G is a ST/"Coke Bottle" glass type, whereas the 6080 is straight bottle. I almost universally prefer the 6AS7G. I have about 100 6AS7G/6080/6520/5998 tubes.

The primary construction differences in a 6AS7G are that some are made with a top halo getter (and the resulting chrome top flashing appearance) and some with bottom-mounted D getters (where the flashing is at the case and the tube has a “clear top” appearance). RCA also made “Grey Plate” and “Black Plate” versions – all other makers 6AS7G’s have black plates. Supposedly the Black plates sound better, but I believe this is either untrue or extremely subtle at best. Some also have an additional metal “skirting” shielding the wiring as it travels from the base to the grid, but I have not been able to attribute any real sonic differences there either. Bottom line – any Western 6AS7G is a good choice, as long as it’s in good shape and isn’t horribly microphonic.

There is also the 5998 tube, made only by Tung-Sol regardless of branding, which does have different operating characteristics, but can be used in any of the amps I have. In general the 5998 offers slightly more neutral sound, slightly better transparency, and are less likely to be noisy. They are approximately 2-3x the price, generally, and I have seen some idiots paying $50 per tube on Ebay when you can buy them NOS from reputable online tube-sellers for $29 each. Still, this is more than double the 6AS7G price. And there is also the 6520, which is equivalent to the 6AS7G. The 6520 was also only made by Tung-sol. The 2399 is also a Tung-Sol only tube, and I have never scored one myself, but supposedly it is also just a high-quality 6AS7G.

This review, however, refers to the sound of the amps using 6AS7G’s.

ALL of the amps in this review use different driver tubes, and as such different circuits. The driver tube choices do have some real practical implications which I will discuss. I am not going to describe each amp fully – there is plenty of description available on this forum for each. Also, I bought all of these amps with my own money for my own use – none were loaned for review. With the exception for the SpaceTech labs, which I acquired fairly recently, I have owned all of these amps for quite a while, and I have used them with a fair variety of headphones. I followed my usual review practice, which is in fact my general listening practice, which is to use an SPL meter and pink noise to set levels to 80db, such that they are matched.

The Amps:

Darkvoice 337 - $800 approx

SinglePower Extreme - $1,000 base price

SpaceTechLabs SE-6AS7G - $880 base price

WooAudio WA3 - $495 base price

Darkvoice 337

The Darkvoice 337 is a truly dual mono design – 2 power cords, 2 volume controls. Kind of a pain sometimes to have 2 volume controls, but VERY handy at other times. It uses the 6SJ7, a sharp-cutoff pentode, as the driver tube (one per channel). It’s the only amp here that uses a pentode driver tube. This is a very good sounding tube, but they are very prone to hum, and take a full 24-48 hours to even become usable if NOS. As such, getting a completely quiet set of tubes for the 337 was a bit of a challenge. And fortunately, the 6SJ7 is much cheaper than the driver tubes used by any of these other amps.

Having finally done so, the 337 is a terrific sounding amp. It has an engaging, dynamic, organic, and slightly dark sound. It has an ultra-wide and deep soundstage, which is well defined. Bass is powerful and punchy Better detail than everything other than the SP Extreme. Female vocals are just spine-chillingly beautiful through it. I use it mostly with the JVC DX1000, and this is a heavenly match, providing truly world-class sound.

In general the 337 is a little darker than the Extreme, but it ranks very close to it sonically overall. However, the Extreme does get the nod. It gets a strong second place among these 4 amps.

SinglePower Extreme

I got the Extreme after the 337, and I wasn’t sure that it had a chance to better it, but indeed, it does. The Extreme uses a single 6SN7 dual-triode as the driver tube (although 6SL7’s are also usable). This helps with cost, since the premium 6SN7’s can get pretty pricey!

The Extreme is a terrific amp. Better than any other amp I have owned, tube or SS, and better than some amps I have heard costing 2x the price. It’s detailed and transparent, and essentially neutral, while still retaining that magically engaging midrange. It’s the best soundstage of any headphone amp I have ever owned. It has amazingly deep, punchy, and well-define bass. Its treble is so shockingly sweet and pure it’s just a joy to listen to. It is hard to imagine a more open window to the music than provided by the Extreme. Female vocals are just luscious, but at the same time very busy large ensemble jazz and heavy rock still sound excellent. The Extreme sounds just as good with Holly Cole and Diana Krall as it does with Nightwish and Leaves Eyes. And certainly not every amp does.

It’s easily worth the extra $200 you pay over the DV 337 in my opinion, as good as the 337 is. It also simply refuses to hum with any tube I have tried – it’s the quietest tube amp I have ever had, in addition to being the best sounding. For me, it represents very strongly the inflection point in the price performance curve of tube headphone amps – you have to spend absurd amounts more money to get marginally better performance, whereas the Extreme offers significantly better performance than the WooAudio WA3 (which delivers good sound for the money).

SpaceTech Labs SE-6AS7G

This is the only one of these amps that is designed to drive speakers as well as headphones. Its default driver tubes are a pair of 6SL7’s. It can use 6SN7’s, but 6SL7’s are cheaper, and in this amp they sound better (although in the Extreme I felt the 6SN7’s sounded better).

The SpaceTech sounds the least like the other amps. It has the most “midrangy” sound. It sounds tubier than any other 6AS7G based amp I have owned or heard. It also has the weakest bass – it has noticably less bass than any of the other amps, and that isn’t due to the tube compliment. I tried several different driver tube sets, all with the same basic results. I also tried 4 pairs of 6AS7G’s just to make sure that it wasn’t a problem with the specific pair of tubes I was using. It wasn’t. The presentation gives the amp kind of a lightweight sound, and makes the midrange pop, which it definitely does. The midrange presentation is very nice, though, as is the high end. It’s got great soundstage depth, and really nice image detail. Highs are sweet, and still detailed. And the bass, while lightweight, is very agile.

After trying a bunch of 6SL7 types I had, I felt the Tung-Sol Round Plate VT-229’s sounded best. A pair of Tung-Sol Round plate 6SN7’s will run you close to $500; 6SL7’s cost less than $100/pair NOS. So the fact that this amp sounds best with the 6SL7 is a really good thing for the wallet!

So while I like the SpaceTech, it’s definitely not in the same league as the DV337 or Extreme, even though it’s in the same price range. I bought mine used, fortunately, and that I can recommend. But at the new asking price, I don’t recommend this amp – the 337 and Extreme are both better.

WooAudio WA3

The WA3 differs from the other amps in several significant ways. First, it uses just a SINGLE 6AS7G power tube, which is possible since it’s a dual-triode. It also uses 6DJ8/6922’s as the driver tubes (it does use a pair of these). The 6DJ8 is a good sounding tube, but the better NOS ones are very expensive, and so for an inexpensive amp, this is a somewhat expensive tube choice (but no worse than the 6SN7). I use a pair of Amperex 6DJ8’s, which is logical since they made most of them (RCA, for example, never actually manufactured 6DJ8’s, they just relabeled them, mostly from Amperex).

In any case, it sounds quite good, especially for the money. Where it falls short of the DV337 and the SP Extreme is in having a less deep and wide soundstage, and in being generally less transparent, although again is isn’t bad in either of those regards. Bass is excellent, the mids are very nice, and very lush, and the highs, while a little soft, are basically clean. But compared to the other amps, the WA3 lacks detail, and is a little wooly. I wish the SpaceTech had its bass response, and I wish the Woo had the SpaceTech’s mids and highs…


Well, all of these amps sound good, and for me at least show that the 6AS7G may be the best power tube choice in headphone amps today, due to its excellent sound and low cost. They can be noisy, but so can ANY tube, and tossing out a $7 6AS7 is far less troubling than tossing out a noisy $100 EL34 or a $300 300B! Even the premium 5998’s, which I have never had a noisy one of, are only $30 each NOS.

My favorite of these is the SinglePower Extreme, and I recommend it unfailingly to anyone, due to its excellent sound, and the amazingly large quantity of tube-rolling options.

The Darkvoice 337 is a close second. While its driver tubes can be a little fussy, they are cheap, and patience rewards the owner with a rich, nuanced, organic sound that is unfailingly enjoyable, but still neutral enough to be faithful. Only the fact that it has to be shipped from China, and it is big and heavy, keep this amp from being a runaway hit here in the US, IMO.

The SpaceTech is a tough recommendation at its asking price. It sounds generally good, but it’s sufficiently idiosyncratic that I think the money is better spent on the other two. I admit I have had it less time than the others, and if my opinion changes over the long haul, I will report back.

And finally, for the asking price, I think the WooAudio WA3 is a nice little amp, and gives good measure of good triode tube sound, but I do think that the step-up in price to the 337 or Extreme yields a good payoff in sonics.

Gear mentioned in this thread:

post #2 of 51
Niiiiiice review. I've been looking at the DV and SP Extreme too!!!!

Sweet pics too!
post #3 of 51
Four 6AS7G amps! great review! It'll be nice to have your impressions on different headphones for the respective amps too! Oh, and not to forget, the tube rolling options for the amp (like 5998 and stuff)!
post #4 of 51
It ain't easy. Thanks for the reviews and hard work. Was the Woo the 3 or 3+?
post #5 of 51
Great review Skylab! The one amp I have been dreaming about based on reading the Head-Fi forums is the SP Extreme, and your review just adds fuel to the fire.

You mentioned your preference of the Darkvoice with JVC DX1000's. What is your favorite combination with the SinglePower?
post #6 of 51
great review.
post #7 of 51
Skylab, you are a glutton for punishment aren't you and a hard worker...these reviews don't come without cost and effort.

Thanks for all of your hard work.
post #8 of 51
More great work sky. like the approach of the in-depth look at the tubes, that knowledge is harder to come by.
post #9 of 51
Thread Starter 
Thanks for the nice words, folks!

I like the Extreme with all my cans, but I use it most with the 600 Ohm Beyer DT990's. Wonderful combination, that.

W/r/t 5998's, all these amps sounded better to me with 5998's except the SpaceTech labs, since the 5998 generally has a little less bass quantity than a 6AS7.
post #10 of 51
I can extrapolate that, since I agree with your (carefully-worded and temperate) summary of the Woo3 . . . that the rest of the review is great!

It's reviews like this that help us get a real feeling for what other amps might sound like, and what's an improvement and what's not. Thanks for all your hard work.
post #11 of 51
Great review, thanks

I can really see the Extreme beside me sometime in the future
post #12 of 51
Thank you for an amazing review Skylab, especially the comparison of the Extreme and the DV337. With 220V Extreme amps hardly ever coming up for sale, and the long waiting times for a new one, its nice to know the DV337 can be a good alternative for non-CONUS headfiers. Thanks also for the comments about the tubes themselves and factoring that into the total cost of ownership. Excellent review.
post #13 of 51
Thread Starter 
Thanks again everyone! Really appreciate the nice comments.

Originally Posted by jamato8 View Post
Was the Woo the 3 or 3+?
I think just a 3 - but I bought it used. How would I know? Is there a way to tell by looking? Thanks.
post #14 of 51
Originally Posted by Skylab View Post
Thanks again everyone! Really appreciate the nice comments.

I think just a 3 - but I bought it used. How would I know? Is there a way to tell by looking? Thanks.
The 3+ doesn't have preamp outputs
post #15 of 51
Thread Starter 
Ah! Thanks. Mine is indeed the 3, not the 3+
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