Tubes Asylum: REVIEW: Dynaco ST 70 Amplifier (Tube) by thoriated

Model:	ST 70
Category:	Amplifier (Tube)
Suggested Retail Price:	$199.00 New/ vintage 300 to 800 US
Description:	35 Watt/ channel Vaccum tube power amplifier
Manufacturer URL:	Dynaco
Manufacturer URL:	Dynaco

Review by thoriated_tiger ( A ) on September 14, 2004 at 10:05:42
IP Address: 208.35.159.6

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for the ST 70

This is my first review of any real importance. I feel there is entirely too much bunkus surrouding this amp, some of if being deserved, some of it being just sheer ignorance and parroting of other's opinions.
The Dyna Stereo 70 is an EL34 amp, using 2 EL34s per channel in an ultralinear configuration. The front end employs RCAs 7199 pentode/triode driver tube: the pentode is a voltage amplifier, the triode is a phase splitter. There's a phase-compensation network which theoretically doesn't allow the sound to change as the drivers wear (7199s of old vintage can last decades, so..) These amps were $99 when they came out in late 1958. Today that's a thousand-dollar amp. Today, this humble little piece of nickel-plated steel can stand head straight and look MUCH more expensive amps right in the eye.
Mine is a very early example, nickel-plated chassis, cloth-lead output iron. The driver board was removed and replaced with an exact electronic copy from Curcio Audio. It retains the original Ed Laurent / Bob Tucker design. I'd like to stress this is a copy of the original circuit, just executed on a fiberglass board of great quality, with mostly metal-film resistors and Jensen copperfoil PIO coupling caps. The driver tubes are black-plate RCA 7199, the rectifier is a JJ GZ34, the outputs are Svetlana (SED -- winged =C=) EL34's. The only 'real' mod is the use of Sheldon Stokes' SDS Cap Board instead of the 4-section can filter cap. This 'mod' also removed the original selenium bias rectifier and replaced it with a safer, more stable silicon diode, as well as replacing the original bias supply electrolytics.
I got this unit about 4 years ago, and I listened to it in tired, stock form for about 1.5 ~ 2 years. In tired, stock form it was indeed a little shy on top, very shy on bottom, with a healthy dollop of midrange magic. Still better than mid-fi solid-scrape, tho.
2 years ago I completely re-built it to the specs mentioned before. The amp immediately changed character. It was shy no more. It is open, airy, delicate, powerful, refined, transparent, punchy, and even thundering when it needs to. My ears tell me there is great extension, from bottom to top. The bottom end is authoritative, with pretty good control of the Klipsch's 12" woofer/drone combo. The high end is liquid, detailed, crystal-clear. The midrange is indeed pure effin' magic.
This amp doesn't get between you and the music. At all. It is like it's not even there.
Leading-edge transients (attack) are superb, one hears the stick hit the cymbal, then one hears the cymbal sing. With a brassy, full-bodied flavor to it. You get the real bulk of the actual *tone* of a cymbal. Likewise, snares startle one. A very sharp *rrap* followed by the tone of the drumhead. Tympani are probably the yummiest through this amp. Another great example of the humble Dyna Stereo 70's attack prowess is the harpischord: One hears, or rather *feels* the plectrum butt up against the strings, then one hears the strings sing. And when they sing, it isn't the dry, thin, thready 'audiophile' harpsichord, it is the full-bodied metallic symphony of plucked strings of the modern piano's grandfather.
Soundstaging is astounding. For example, in a large orchestra, one can pin-point with ease where each fiddle, where each oboe, where each percussionist is. With utmost stability. There is no 'left-right' sliding of a solo violinist, for example. One gets the full visual of an orchestra with this amp. If the recording has the information, the little Dyna Stereo 70 will deliver it fairly intact to your speakers.
Details are what can make or break an amp. The Stereo 70 aquits itself most memorably. The scrape of a fiddle's bow against the strings. The rustle of pages being turned in a score. The parting of lips before a singer sings. The squeek of a chair. The sound of someone walking around the recording venue. The little mechanical chuffing of a tracker pipe organ. All this low-level information is retrieved by the Dyna and shipped to your speakers.
Decay is an oft-overlooked item in most reviews. The Dyna's decay is completely natural, and tied directly to the recording. Best example I can think of is the recent EMI recording of Bach's Magnificat, recorded at King's College. The sound of the choir's voices reverberating those old stone walls is wholly convincing.
Dynamics are quite strong. Pianos truly show up as they should -- a very percussive instrument with a very pronounced attack. Marimbas, xylophones and other similar hard-surfaced instruments startle. Rimshots are particularly well reproduced by the little 70.
Bass is an area where the Stereo 70 gets the most negative comments. To that I'll have to say this: With a NEW quad-cap can, or a suitable under-chassis replacement, this amp has a bottomless pit for bass. The bass is quite solid, although not as 'dry' as a solid-state amp. The extension is remarkable, reaching far down into large pipe-organ territory. A double-bass of the orchestral variety is absolutely no challenge for the 70. Electric bass is even less of a challenge. Large bass drums come across with their tone and taste un-changed. The quality of the bass is what I would term 'fast' or 'neutral'. One doesn't get 'just' the bass, one gets the fundamental, the harmonics, everything. You *taste* the instrument in a very real, palpable manner. I love the bass this amp produces. Superlative, actually. The complete opposite of what some will spout regarding this rather capable vintage piece. Of course, the BIG caveat is -- CHANGE THE FILTER CAPS TO NEW, or even better, up the filter capacitance a bit -- but not too much, the 5AR4 has a hard limit on how much it can take.
The midrange, after all this restoration and light modification, is unchanged. A liquid, silky, smooth, non-grating midrange which conveys the bulk of the music's nuances. Voices are spooky-real. On very well-recorded source material and eyes closed, well, the singer is there. Or the choir. This amp is *not* limited to little-girl-with-guitar music. This amp will take a 40-piece choir and put 'em in your room -- and you will see just about each and every singer in their own space.
The top end is pure effing magic as well. The upper harmonics of violins, harpsichords, organs (especially the mixtur-laden Baroque German organs), trumpets, cymbals, etc are reproduced with such reality that one questions one's locale -- am I in a music room, or am I at the live event? Close your eyes, and you will be at the event.
Power is plentiful, if one has moderately hi-efficiency speakers. My Klipsch Fortés are rated at 96db / 1w. I can play big, big big music with peaks *well* into the triple-digits without any kind of noticable distortion, clipping or other nasties. Just absolute, rock-solid clarity. No need to make excuses. This amp, through suitably sensitive speakers, will play anything you throw at it. Including the much-vaunted, much-maligned DSD Telarc 1812. I've heared people describe the cannon shots as "thuddy" or soft. People, get better speakers and amps. Them are muzzle-loading six-pounders, and they *sound* like it. Loud, crisp, clear, percussive and effing LOUD. The Dyna does 'em right.
The most startling feature of this little amp? I swear it provides an almost triode-like clarity, with pentode-class power. It's a little paradox in a steel box! (keep the tubes biased right on the money, or that clarity can degrade.. but biasing this pup is super-easy!)
The Dynaco Stereo 70 was made to the tune of just over half-a-million copies over 30 years. It sold for a pittance. It looks humble, and it is. It was a *working* man's amp. But in *RESTORED* condition, with good driver tubes, and a decent power supply (in other words, brought up to the level it was when it was new) it is anything *but* humble. It will take the fight to gear from fancy factories, costing much more. And in some cases, triumph over them. I will take my little 70 with its lightly pitted chassis, dusty transformers and ugly-duckling looks over some offerings i've heared a local hi-fi boutique or some friend's systems. it is, and will remain, my reference -- until I hear something better.
In the 4 years I've had it, and especially in the 2 years after the restoration, I've yet to find something that would immediately urge me to upgrade. I've heared SET, I've heared PP KT-88, 6550's.. I've heared a lot of gear. I'm still with my little 70. It's really that good -- once *properly* restored.
What can it do better? It could have a *smidge* more bass control. It could have a better biasing system. It could have a triple-regulated power supply. But soundwise? I don't think it lacks anything, sound-wise.
In closing, I hope this review helps to open the reader's eyes, ears and mind towards this amp. It is my sincerest hope that after reading this, those owners considering modding it beyond recognition will instead restore it carefully, modify the power supply to something a little less stone-age, and then *listen* to it -- *before* shelling out bucks to 'modify' and 'improve' what was stellar then, and is still today.
One last thought -- this amp truly deserves the cleanest speakers possible. My choice is compression horns. The 70 delivers *so* much it is a shame to strangle it with inhererently-dirty 'conventional' speakers.

Product Weakness: Needs restoration to perform to modern standards. Don't even think a tired old 70 will do this. You *have* to bring it up to snuff.. and please.. do so retaining the original ckt *before* modding it to something which no longer will be a Dyna Stereo 70! Another weakness: Modern output valves seem to NOT like running inside the cage -- amp must be run open, else the modern-day output tubes stand a good change of heat-stress death. Last weakness: Must use good, NOS 7199 driver tubes. No one seems to make a good one today.
Product Strengths: PRAT, slam, delicacy, transparency, quietness, will compare favorably to many vintage and modern amps costing a few times over.

Associated Equipment for this Review:
Amplifier:	Easy fella, I'm actually reviewing it..
Preamplifier (or None if Integrated):	Audio Research SP6A
Sources (CDP/Turntable):	Thorens TD145 / Sony DVP-NS755V DVD/CD/SACD
Speakers:	Klipsch Forté
Cables/Interconnects:	Monster / RatShack
Music Used (Genre/Selections):	Classical, Rock, Jazz, Choral, Electronica, and more
Room Size (LxWxH):	15' x 12' x 9'
Room Comments/Treatments:	Carpet.
Time Period/Length of Audition:	2 years
Other (Power Conditioner etc.):	Monster HTS5000
Type of Audition/Review:	Product Owner