December 1998Parts Connection Assemblage ST-40 Amplifier Upgrade Kit
by Todd Warnke
Movin on up
Upgrades are the bane of audiophiles. You know how it goes. Borrow from next years vacation fund (with a silent promise to pay it back before the spouse notices) and sell this piece to buy that one, hoping you dont go backwards in the transaction. Well, throughout their history, the good guys at Sonic Frontiers have tried to take the edge off the upgrade process. With roots as a parts and kit company they understand that the best upgrade is the one that doesnt require you to sell; that is, a little solder and the right parts are a lot cheaper than swapping your gear for someone elses. Their catalog is full of upgrade kits for both classic and current preamps, amps and speaker crossovers. So when their Assemblage division offers a kit, you know that an upgrade is also in the offing.
In the case of the 40Wpc, EL34-based Assemblage ST-40 stereo amp, the $249 upgrade kit consists of replacement resistors (six Caddocks and eight Vishays) and capacitors (four MultiCaps), Kimber silver wire to replace the output wire, Kimber KC1 to replace the input cable, and EAR isolation feet. Input tubes are also upgraded with the single 12AX7 being swapped for a Tesla gold-pin ECC803S, while the two 12AU7 tubes are replaced by the Mullard CV4003 (a rugged military variant of the 12AU7).
Because I care so damn much (OK, because my mental-health provider says I have masochistic tendencies), and because the kit allows me to test it like this, I did the upgrade in three steps. First, I put in the tubes alone to see how much of an effect well-thought-out tube rolling can have. Second, to test the power of improved parts, I put the old tubes back in after doing the caps, resistors and wire upgrade. And lastly, the parts change with the new tubes. Oh, one thing before we start. If you havent read my review of the basic ST-40, following this link will get you there. Come on back when youre done; Ill be waiting.
Over the last four or five years my preamps have primarily used variants of the 6DJ8 (6922, 7308, E88CC, ECC88 or CCa), so I have a menagerie of those on hand (Sovtek, Amperex, Ediswan, Sylvania, Siemens, GE, Mullard and Telefunken). From such a cache, its obvious that I believe in tubes. To me it comes down to simple math: For less than the cost of a cheap pair of interconnects I can have a larger impact on the sound of a system by rolling tubes.
For tube rollers the good news is that many of the new tubes are quite good. In fact, if you avoid most of the Chinese tubes, you can get very good glass at fantastic prices. The bad news is that as good as the new tubes are, the best NOS tubes are even better. Still, this bad news has a silver lining, namely, that the ultra-expensive NOS tubes are not the only good ones. In the 6DJ8 family (which I know well), the Ediswan is a great tube, perhaps the best balanced 6DJ8 variant out there. Kevin Deal at Upscale Audio deals them for about $55 a pair. Expensive? Sure, but not as expensive as a mint set of gold-pin Amperex or Mullards. Even better, some mid-to-late-60s Sylvania tubes go for about $20 a pair and offer an easy 90-95% of the sound of the Ediswan. Finally, as with all things audio, system matching is the key. For example, as good as the Telefunken 12AX7 is, at $100 or more per tube, its subtle strengths would be lost in the ST-40. When rolling tubes it pays to look around and test in your system, which is exactly what Sonic Frontiers/Assemblage has done with this kit. The NOS Mullard and the gold-pin Tesla are excellent tubes (Ive seen the Tesla referred to as best affordable 12AX7 replacement), but not overkill. Theyre well chosen, high quality, and yet affordable and available.
Putting them in, Im happy to say, brought no surprises. Better said, every benefit I expected was there. They brought an instant increase in detail, harmonic richness and refinement. The top opened up, but without any glare or harshness. Bass was slightly more controlled, no deeper, but more correct and less plumy. The mids were slightly more defined, more exact in timbre. Using the Cowboy Junkies album The Trinity Sessions [Classic Records RTHCD8568], before the tube swap the sound of Margos voice had been the main focus of attention; with the new guys in place the balance between her and the rest of the band had a decidedly more natural feel. The hall also was a more of a contributor to the overall sound. A moderate and nice step in the right direction.
Hot solder and cold beer...or is that hot beer and cold solder joints?
Now for the real bad news. If swapping the tubes was a small step for the good, the parts replacement was giant leap for tube-kind. Why is this bad news? Because the parts upgrade takes far more effort than just turning the amp off, swapping tubes, turning it on and checking the bias. Not that the upgrade is hard, its just that Im a lazy so-and-so and have given myself over completely to the American ideal that manual labor is code for Made in the Far East.
Actually, the upgrade would have been easier if I had been more careful with the original kit. With four of the resistors and all four of the caps I failed to use the alternate mounting locations and so had to de-solder several connections. Dumb. The moral? Listen closely to Mom -- always take your time, read directions carefully and wear clean underwear in case you get in an accident.
A couple of other suggestions. Although the instructions are but two pages long, set aside at least two hours. Youll also appreciate having the original manual by your side as you do the upgrade because youll need to remove the main board to get at the underside of it (besides unscrewing the standoffs, the trick is to de-solder the bias-pot connectors). Youll need to remove the power-supply board; this is not hard, but it has two short wires connecting it to the main board, which means it will flop around while you work. Not a problem, especially for the three-handed among us, but a bit of care goes a long way. Also, watch for the power LED. Bend it out of the way before removing the main board. And lastly, because the entire operation can take up to several hours (at least it can if you failed to use the alternate board locations), keep the six-pack in the fridge so the celebratory beers stay cold.
As for sound of the new parts, clear and controlled describes it all. The Cowboy Junkies, once again, are a great way to illustrate this. The opening track on The Trinity Sessions, "Mining for Gold," has Margos voice highlighting the characteristics of the church used in the recording. It also has traffic and subway noises that are (obviously) outside the church acoustic. The stock ST-40 blurred the difference between the in-hall and outside noise. With the parts upgrade, this distinction was clear and immediate. Further, the hall ambience was more precise, leading to a greater feel for the size and shape of the church. While not quite up to the overall performance of my Blue Circle BC6 amp in this area, the upgraded ST-40 came shockingly close to that exalted level.
Bass, in the best Archie Bell and the Drells manner, was tightened up. Once again, it was not deeper, but had better definition and snap. So did the ST-40 now have transistor slam and leading-edge bass definition? No way, but then thats not why you have a tube amp. What it has (and shares with other good tube designs) is bass that breathes, bass that, down to the lower-30s limit of the Merlin VSM-SE speakers, is rich, reasonably powerful (although it runs out of steam before the Merlins do), and, best of all, sounds real. The harmonic correctness, texture and ease of the bass, in my book, makes up for any slam or extension edge that the solid-state cousins of the ST-40 may have.
Treble was similarly cleaned up. The "Mining for Gold" track is one excellent example of what I mean. Another is the increased detail and bite to the strings and brass in Bernsteins live VPO recording of Mahlers Sixth Symphony [DG 215076]. The first movement opens with two minutes of a surging, powerful bass line with soaring horns, woodwinds and strings that then leads to a horn fanfare that dissolves into inconclusive and forlorn discord. This is followed by a nervous picazzato string passage that leads to bowed strings that try overcome the melancholy and indecisiveness of what precedes. Through the upgraded ST-40 the cymbals, plucked strings and sharply played flutes cut with the appropriate edge, upping the nervous tension necessary to set the mood for the entire symphony.
As you would guess, the increased clarity and detail led to better staging as well. Space, which with the standard ST-40 was localized around each performer, opened up and became communal property. Square rooms had better-defined corners. Depth was, well, deeper. The right-to-left spread was wider as well, with players occupying precisely defined and layered locations.
Best of all, the midrange purity and harmonic rightness that is the soul of this amp was unchanged. This amp has real textures, like no solid-state amp on the short side of $2000 can deliver, and rich harmonics that let -- no, force -- you to feel the musicians. These things are magic, and the upgraded ST-40 delivers them intact.
All together now
With the new tubes in place the parts upgrade really paid off. The increased clarity allowed the sound of the tubes to shine through. The resulting combination -- clear, refined and controlled -- took a solid, affordable, high-value design and made it a solid contender. No joke or hyperbole here at all, I could easily add one more and vertically biamp either the Merlin VSM-SE speakers or (especially) the Dunlavy SC-IIIs and call it a day. With the low-40s bass limit of the SC-III, you get all the good of the ST-40 without facing its low bass limits, which is the only real drawback of this amp.
As odd as it sounds, stealing a metaphor from Thucydides and the Peloponnesian War is the best way to draw out what makes the ST-40 so good. The whale versus the elephant (Athens/Sparta) analogy describes what happens when you put the standard ST-40 up against any solid-state amp up to and even beyond the $2000 mark. The silicon devices are all speed, slam, extension and detail. Theyre impressive sonically, but not natural music-makers. On the other hand, the standard ST-40 is all texture, harmonics, finesse and involvement. Sonically it is adequate, but oh-so musical. These two animals have strengths that are almost contradictory, making head-to-head comparisons very difficult.
The upgraded ST-40, on the other hand, is almost a new animal, a whalephant. It takes all that is good about the ST-40 and adds clarity and control. While it still cant take on the solid-state amps in their territory and win, it can at least put up a respectable fight. The tighter bass gives it a greater sense of speed. The increased clarity opens the stage and makes inner detail more apparent. But, miraculously, the tube joy remains. The richly informed, harmonically correct and emotionally satisfying sound remains -- in fact, it is further refined. Given a reasonable load, the upgraded ST-40 can out-music every sub-$2000 solid-state amp Ive had in my system while giving you few reasons to look for their special virtues. If, like me, you were raised on silicon, you owe it yourself to listen to high-quality, affordable tubes. I know of no better way to do that than the ST-40, and once you try it, the upgrade is mandatory. Its my first choice in amplification at up to twice its price.
A reminder about rolling power tubes. The supplied Sovteks are very well balanced, with superb bass, but a small touch of treble hardness. Teslas are a bit soft in the bass and also a bit too liquid and laid-back elsewhere. On the other hand, the Svetlana is the momma bear of EL34s, just right -- limpid and liquid, warm but not rounded, clear without harshness. And best of all, theyre affordable and available.
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