Subject: Dynaco ST 70
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Date: Thu, 12 Feb 2015 13:11:37 -0800

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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.
ù 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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