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New Dynaco ST 70 series II vs original ST-70

Part II

[Italian
version]

In Part I of this
article, we looked at the history of the Dynaco ST-70 and some basic differences
between the Series II and the original ST-70.
In Part II, I will go into
detail about the similarities and differences in circuitry between these two
versions of this very famous amplifier. I will then present the results of my
listening tests in comparing these amps.

First let's look at the similarities in circuitry. Both amplifiers are rated
at 35 watts per channel. In actual fact they are probably class A up to about 15
watts, and then class AB to the rated 35 watts. The output tubes are 6CA7/EL34.
The output transformers are the same ultralinear design. In this design part of
the primary winding is fed back to the output tube. This circuit design reduces
distortion and gives great sound. The original ST-70 came from the factory with
Mullard EL34 (they were actually labeled Dynaco). The Series II had output tubes
by Siemens (labeled Dynaco - made in East Germany).

Both amps had the input located at the front of the chassis, and the output
terminals located at the back of the chassis. They were both fused at the back.

The differences between these two amplifiers shows how 40 years of electronic
development can influence a design, even though both are tube amps! The Series
II was designed in the early nineties whereas the original ST-70 was designed in
the fifties.
The most obvious difference between the two is that the Series
II had the entire circuitry placed on one double sided military spec printed
circuit board made of epoxy fiberglass. This is a very high quality material and
will not go bad over time.
In sharp contrast the original ST-70 was hard
wired, except for the driver circuit which was factory prebuilt on a phenolic
circuit board. The problem with the phenolic board was that over time it became
contaminated with chemical changes. This change altered the electrical
characterisics of the board resulting in changes to the sound.
The change in
electrical characteristics was not predictable from unit to unit, so ST-70s of
the same vintage will probably sound different (other factors being constant) -
great! There are some people who claim that hard wired components sound better
than their printed circuit board bretheren - personally, I don't believe this
claim.

The Series II had a nice metal plate covering the driver board. The original
ST-70 simply had the driver board exposed to the air (not good if someone
accidently pours liquid onto your amp - like at a party!).

The next major difference between the amps was the driver section. The
original ST-70 used the 7199 tube as the driver. The tube acts as a phase
inverter for the output tubes. That worked well for the 50's,60's, and 70's.

However, this tube type ceased production eventually and was only available as
NOS (new old stock) tubes. Only recently has a Russian or Chinese manufacturer
offered this tube. To circumvent this problem of tube availability, but at the
same time recognizing the inherent value of the 7199 as a driver tube for this
type of circuitry, the Series II used the 6GH8A tube as the driver.
However,
since these two tubes are not identical they can not be used interchangeably,
and the driver circuitry for the Series II had to be redesigned to accommodate
this tube.

The third major difference was the power supply. The Series II uses a solid
state rectification (four diodes). In sharp contrast, the original ST-70 used
tube rectification (5AR4/GZ34). This was done for various reasons - some are no
doubt related to sound reproduction, whereas others are for practical reasons.
Solid state rectification gives sound with more edge.
The GZ34 tube supplied
with the original ST-70 was the famous Mullard. This tube was a classic and
lasted for many, many years. However, production ceased - all that is available
now are Russian and Chinese manufacture. The Mullard can be bought as NOS, but
it is very pricey.

The bias power supply in the Series II used a diode for DC bias. The original
ST-70 used a selenium rectifier which over time can go bad as it ages.

Frequency response of both amps is rated at 20Hz-20kHz but is achieved very
differently. The Series II uses an input filter network to limit the frequency
response to specification. The original ST-70 used components within the gain
circuitry to control the frequency response.
The problem with this approach
is that at the frequency extremes gain is reduced, and distortion results as a
consequence.

What is not widely known, is that the ultralinear feedback point from the
primary winding of the output transformers in the Series II design was actually
changed to the 8 ohm tap.
The original ST-70 used the 16 ohm tap as the
feedback point. Since most speakers are typically 8 ohms or less (some are 6 or
even 4 ohms), then it makes more sense to have the feedback point in the
ultralinear design referenced to 8 ohms (instead of 16 ohms which is never used
anyway). The result is less distortion.

In my experience with the Series II is that my sample occassionally blows its
cathode bias resistor (15.4 ohms, 1%,1-3 watts). I have found that by turning
the bias down somewhat so less current through the EL-34s (in other words more
negative bias voltage applied), this is less prone to happen. Yet sometimes by
1500-2000 hours of use either left or right resistor blows.
I am playing my
original ST-70 more to see if the same thing happens. Cathode bias resistors are
known to blow, and in a sense it protects the output transformers and power
supply transformer from blowing also, because if the bias resistor does not
blow, then the transformers will be taken down (and that my friends is very,
very expensive!).
Cathode bias resistors blow either from aging and use, or
from excessive current flow through them for their given power rating.

Occasionally, I have noticed, although not all the time, that the resistor
blowing is proceeded by thermal runaway of an output tube (it glows red).
Thermal runaway may be a characteristic of the tube or the circuit itself, or
both.

I compared the sonic characteristics of the two amps on my stereo system. The
preamp was a Dynaco PAS3 (using Telefunken 12AX7 tubes). Source material was
played on a Dual Dual CS 505-3 turntable (belt driven with an Ortofon OMB 20
cartridge and needle), and a Parasound C/DX - 88 CD player. Speakers were a pair
of Paradigm 11se Mark II.

For source material I played vinyl records and compact discs (classical and
rock'n roll). I tried to listen at the same volume level when comparing the two
amps. I left the preamp bass and treble controls in their flat (neutral)
positions.

I found the original ST-70 to be mellow, smooth, and very laid back in sound.
The bass was not very strong. To get good bass, one had to adjust the bass
control on the preamp. The ST-70 was especially good for classical music.

This amp uses vacuum tube rectification, so as to be expected, it developed sag
(a volume drop due to rectifier voltage drop at high current demands) at loud
and bass-like musical passages. This amp was very tube-like in sound, and it
could be listened to for hours without any fatigue at any volume level!

I was very surprised and impressed with the sound of the Series II amp. It
was dramatic and dynamic (the opposite of laid back). Bass was very strong. You
could feel the bass! This was no doubt due to the increased capacitance of the
power supply, the increased size of the power transformer, and the solid state
rectification.
The sound was wonderful and addictive - one could listen for
days! The amp sounded fast and responded quickly to musical passages (due to the
diode rectification). The Series II had the characteristics of a tube amp, but
at the same time had some characteristics of a solid state amp. I found this
amp to be best for rock'n roll.
At higher volume levels my house really
rocked! It is amazing that the living room bay window did not break. Who knows
what the neighbors thought!

So, which amp do I like better? I have to admit that I like both, and use
both alternately. It all depends on the type of music you play. Both are really
great amplifiers - a treat for the ears!

Neither the Series II nor the original ST-70 is manufactured now, but can be
easily obtained used (after all over 300,000 of the original ST-70s were
manufactured).
I suggest looking at the internet auction web site
http://www.ebay.com. In the search field simply type dynaco. There are always
ST-70s up for auction (the Series II less frequently because much less were
manufactured). The going rate is US$250-450 depending on condition for either of
the amps.
Keep in mind that there were many versions of the ST-70s. The only
ones that I like are 1) the ones where the output transformers were stamped A
470 and have the cloth covered leads - these were the very first ST-70s, and 2)
the Series II amps.

Copyright © 1999 Harvey A. Kader
for TNT Audio, http://www.tnt-audio.com

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