Dynaco ST70 Stereo Tube Amplifier:
Here is a tube amp that is easy to own, easy to enjoy.
HE Dynaco or more aptly the Dynakit ST70 is a wonder. Aimed at
a budget price point and premised on delivering the most value
for the buck, its parts quality was average. The quality of assembly
was erratic and varied with the dexterity of the do-it-yourself
kit assembler's hands. Yet, even after 30 years of service, many
ST70s continue to provide musical satisfaction to their second,
third, or even fourth owners. One would think that these ST70's
would have died long ago. Their reliability is a testament to
conservative design and execution. It also makes one wonder why
some of the high end components of recent times don't have the
same longevity, despite the use of higher quality parts, improved
manufacturing techniques, and advanced circuitry.
Ubiquity
Many tube audio enthusiasts have had at least a brief acquaintance with the ST70. It was like a rite of passage before moving on to greater (meaning expensive) and more sophisticated equipment. I frankly believe it is a great starting point to learn tube audio maintenance such as biasing, tube replacement, troubleshooting and modification, if one were so inclined. I've listened to least half a dozen of these amps. Some I have owned for a time, others were owned by friends. It's the amp that gave me the confidence to dabble into tube audio. I found out that tube components are more "friendly" and easier to troubleshoot than the silicon-based units.
Packaging
The ST70 will not win any aesthetic design award--it was not designed to be decorative. It has a nickel-finish chassis with vinyl charcoal brown cover. With its HWD dimensions of 6.5 x 9.5 x 13 inches, the amp looks like a small box. Not to worry, this boxy look afflicts many tube amps. I frankly prefer not to have the cover on and marvel looking at the tubes as they glow in the dark. But then, I have no pets to worry about. The shipping weight of the unit is 32 lbs. The ST70 has an output impedance of 4, 8, and 16 ohms. There are socket provisions to power an external preamp. I have never had any use for these however. The bias points are conveniently located in each of the sockets. You do not have to open the unit to bias the output tubes like some tube amps. There is also a stereo-mono switch.
The amp has a tube complement composed of: 4--EL34's, 2--7199's, and 1--GZ34 selenium rectifier. The 7199's are no longer in production but one can still find these from tube dealers and individuals. For reliability, the output tubes are said to be operated at 65% of capacity, the filter capacitors at 85% of rated voltage. The ST70 is said to deliver 35wpc. I have seen many reports contradict this power rating, stating instead that this is in the 20-25wpc.
Associated Equipment
The following components were used to evaluate the ST70: Audible Illusions Modulus 3 tube preamplifier, JVC XL-Z1050 compact disk, Sound Dynamics 300Ti, Linn Kan II and mostly with the Rogers LS3/5a speakers. Prior to reviewing the unit, I had the system playing for about two hours.
Listening pleasure
I had just recently been listening to transistor amps. Right away I could distinguish the difference. The ST70 provided the bloom, fuller sound, and body reminiscent of tube equipment. One does not have to search for these. They are there to be enjoyed.
The amp like most of its glowing siblings excel in the midrange and vocals. Joni Mitchell's singing in her Blue CD (Reprise 2038-2) was a joy to listen to. There was not a residue of dryness. Kung Wha Chung's rendition of the Beethoven Violin Concerto (Decca 400 048-2) became much more involving. The violin was so eloquently captured as was its interplay with the rest of the orchestra.
It is not that the amp shed more light or provided more illumination. No, there were not many more players that could be heard. From the darkness of the soundstage, there was the illusion of space among the players. This spacing among the players provided dimensionality but also less detail and shading as a consequence.
Weaknesses
Against better tube and transistor components, the ST70 shows its age and compromises. It lacks dynamism, high frequency detail, quickness and low-end wallop. True, one can hear the highs but not the extended highs. And the highs do possess some granularity.
It does not also have a deep soundstage. I've heard units with beefed up power supply and expensive passive parts with virtually no improvement in soundstaging. It's bass is also deficient. It cannot reach deep down and provide viscerally moving bass.
A must own tube amp
I'm afraid I've been too hard on this amp. I am comparing the ST70 with those costing over ten times as much. In the context of today's sky high prices, its almost a miracle that one can find solace with a humble amplifier that costs between $150-$200 at current street prices. Aren't we lucky that we still have the Dynaco ST70 to kick around and enjoy. I almost always keep one in the house because these amps are too cheap and too useful to be without. They are getting rarer by the year so keep what you can get. Recommended. Two clefs.
by Judy Cipriani
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