Pros: An engaging, organic, highly musical sound
Cons: 6SJ7GT tubes are fussy; doesn't like low-impedance headphones
Darkvoice 337: Introduction
A little background: the Darkvoice 337 uses the 6AS7G dual-triode for its power/output tube which, while definitely delivering that special touch of midrange magic that I personally only experience from tubes, it is a far less syrupy tube than many. 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.
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.
The Darkvoice 337 uses the 6SJ7 for the driver tube. These are available in many variants. The largest difference are between the metal-envelope (which are awful IMO) and the glass-envelope "GT", There are all sorts of 6SJ7GT's, but my favorite are the mesh-shield Tung-Sol's. These can be hard to find, but they sound terrific.
The 6SJ7GT can take a LONG time to get quiet (24-48 hours of burn in), and some never get totally quiet. But once they do, it's a terrific sounding tube, IMO. The 6SJ7GT is used in a number of guitar amps - I'm surprised it hasn't found more favor in audio amps. While it is a sharp-cutoff pentode in its natural state, it is "strapped triode" in the DV337.
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 any case, it sounds quite good, especially for the money. Bass is excellent, the mids are very nice, and very lush, and the highs, while a little soft, are basically clean.
The Darkvoice 337 is a very nice amp. 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.