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New Dynaco ST 70 series II vs original ST-70

Part I

[Italian version]

If there is one name in Hi-Fi which is synonomous with excellent sound, and at the same time with excellent price, that name is Dynaco. Of all the audio manufacturers, both past and present, Dynaco is admitedly my favorite. The engineering is clean and simple which lends itself to a special sound. In addition, I have found Dynaco equipment to be reliable - no doubt due to its military-like construction.

Dynaco was based in Philadelphia, Pennsylvania. They manufactured several audio products in the 1950's, 60's and 70's. The equipment was available as both factory wired and as kit form. The most famous product was the ST-70 amplifier of which over 300,000 units were manufactured (factory wired and kit combined). Later during the 80's Sound Valves (Columbus, Ohio) bought the entire remaining inventory of original Dynaco parts.
Even today this inventory is being auctioned off at ebay on the internet. Pan Orient Corporation (Panor) [Hauppauge, New York] bought the Dynaco trademark in the 90's, and today continues the heritage with their own line of Dynaco equipment.

Various incarnations of the ST-70 were produced over the many decades of manufacture. They all have the same circuitry, except for Series II, but the part type, chassis and cage type were all different. The very first and original ST-70 can be identified by the chassis (nickel plate), cage (brown), and output transformers (Dynaco A 470 with cloth covered leads).
The next version had a chrome plated chassis. Another version had A 470 transformers with plastic leads. The early transformers were wound by hand in the USA. Then the transformers underwent a part number change to 454326, and were outsourced to Japan for manufacture. Even later versions of the output transformers had a part number change again - to Z-326.

The original output transformers (Dynaco A 470 with cloth covered leads) are fantastic transformers. This is one good iron. You would be hard pressed to find anything better even today, some 40 years later (you see, some things just can't be improved upon!).
The later versions made in Japan are probably just as good, but they were machine made (some claim that they don't sound as good - I think they are just fine).

As the chassis finish changed over the years, so did the cage colour. After the brown cage came a grey cage, followed by finally a black cage. In addition, the Dynaco logo changed over the years. The early Dynaco logo said "dynakit stereo 70" on the chassis and cage. Later versions, especially with the black cage had a fancy "Dynaco" logo on the chassis and cage.

To add confusion to all of this, over the years the power transformer also changed. The early versions had part number PA 060. Then the transformer was increased in size. Early versions were made in the USA, and later versions were made in Japan. So you can see that many permutations of the ST-70 from the original Dynaco company in Philadelphia were possible. Between the chassis type, cage colour, logo, output transformer type, and power transformer type, many, many different ST-70's are circulating out there.

But wait, it gets even better! The Series II from Panor Corporation was a revamped and reintroduced ST-70. It looked a lot like the original ST-70, but in so many ways it was different. The chassis was a gorgeous bright chrome (even shinier than the chrome on the original ST-70). The cage was black, and had no Dynaco logo on it. The chassis had the "Dynaco" label on it in fancy letters. The output transformers were the Japanese 454326. The power transformer was larger with more iron than the original.
The Series II was actually manufactured by Sound Valves under subcontract to Panor. The circuitry of the Series II is copyrighted by Sound Valves. Sound Valves has their own line of vacuum tube amplifiers, and their VTA70i looks very similar to the Series II, but is not identical (the circuitry is different).

I wanted to compare both the physical features, circuitry, and sound of the Series II, and the original ST-70. I happen to have both. It would make a good listening contest! The basic difference in circuitry between the Series II and the original ST-70 is in the power supply. The Series II used solid state rectifiers (diodes), whereas the original ST-70 used vacuum tube (Mullard 5AR4/GZ34).
The debate over these two types of rectification continues even today, but each has its advantages and disadvantages. There is no doubt that solid state rectification gives fast response to transient highs and lows. The vacuum tube rectifier is slower to respond.

The power supply filter capacitors in the original ST-70 totalled 90mfd. In the Series II the capacitance amounted to more than triple that. Greater capacitance in the power supply provides more bass and energy to musical passages. Also to consider is that the choke in the power supply of the Series II was lower than that in the original ST-70. This could be done because of the increased capacitance. What it allowed was a slightly greater power output from the power supply.

Both amplifiers used cathode bias. Series II bias was controlled by a solid state circuit using LEDs to achieve a correct bias. This bias circuit had a solid state rectifier. The original ST-70 used a variable potentiometer to control the bias. It was set by connecting a voltmeter to the circuit and then adjusting the potentiometer until the correct bias voltage was achieved. The bias circuit utilized a selenium rectifier.

In Part II of this article, I will continue with the comparison between the two amplifiers in terms of circuit differences. Then, I will describe the results of the listening tests. Is there an audible difference between these two amps? Wait until Part II for the answer!

Go to Part II

Copyright © 1998 Harvey A. Kader for TNT Audio, http://www.tnt-audio.com

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