Specifications

ManualsBrandsDYNACO ManualsAmplifierStereo 70

The Dynaco ST-70 surely must represent the greatest amplification

paradox of all time: It was arguably the best selling power amplifier of all

time, and is arguably the most maligned such device as well. For a

generation who adored it, there have been those in following generations who are quite vocal about it, to the point that the only

useful component they claim it has, is its output transformers. Therefore, it is likely the most modified amplifier of all time as well.

As always however, the vast majority of these claims are solely subjective in nature, with little or no measured performance data

to back up their claims, or to show the improvement a given modification affords to all its ills. Ultimately then, the sales pitch boils

down to the best small lot used car salesman's tactics: Trust me! With so many modification options, and so many new folks

arriving at the doorstep of our hobby daily, the plan of attack in restoring these units can be bewildering to say the least.

I own a rather pristine, early model example of this unit, that I keep as absolutely stock as possible, to allow the unit to act as a

reference against all challengers of its original humble existence. To that point, everything is completely original in my unit

(except for tubes), from the fuse and power cord, to the coupling and power supply caps. At this time, all components are well

within spec, with no leakage or signs of physical age at all. In this day then where the restoration battle cry is "Replace all of it!"

(which I hardly argue against for long term dependability), this unit stands in stark contrast all on its own. Therefore, it serves its

INTENDED purpose for me very well indeed.

I have been asked on numerous occasions to develop an EFB™ modification for this unit (adding to the great pile of

modifications already in existence for it!). Such an effort always starts with baseline testing of a properly operating stock unit, so

as to have a bar set to measure any achievement against. I am offering up my reference example for that purpose here. And

while the purpose of the testing was originally to develop yet another modification for this unit, I thought that with so many, many

other modifications out there -- and all the claims about what you should and shouldn't do to achieve good performance from

this unit -- it would be a good idea to do a reset of the much discussed ST-70, and publish just how good -- or bad -- the original

product really was. Along the way, some of the more popular modifications will be examined as well.

This will include the power supply, output stage, driver board, and overall measured performance as well. Of course, this will all

ultimately be related back to the original goal of developing an EFB™ modification for this unit also. However, of a more general

nature, the results will also serve as a good documented baseline for any and all on their own particular ST-70 journey.

To get the ball rolling then, this was a kit model, and I won the lottery with whomever assembled it. It is neatly built, with the

underside shot almost mirroring the pic provided by Dynaco in their assembly manual as representing a quality build. Those are

the famous "cloth" version of the A-470 output transformers, with everything still clearly being in its original build form.

The tubes supplied with the unit when received were also primarily lotto winners as well, with the output tubes being well

matched pairs of low hour Zenith branded genuine Mullard tubes. The weakest pair of these tubes produced 98% of Average

New NOS Power Output, while the strongest pair produced 102.6% of this value. The worst pair was matched both statically

AND dynamically (i.e. collectively) within 3.65%, while the best pair was matched to within 2.1% under the same criteria. The

two pairs then were matched within 2.55% of each other. Clearly, these are superb tubes to use in a reference Dynaco ST-70.

The driver tubes were also relatively new Zenith labeled un-carbonized 7199s, of unknown (American) manufacture. One of

these tubes has been replaced, for reasons which will be discussed further in the section on the driver board. But clearly, some

one or some tech went through and re-tubed this unit from their Zenith stock before it was placed on the (in)famous auction site

nearly some 5 years ago now, from which I snagged it. The rectifier tube was a brand new JJ GZ34 S. This tube has also since

been replaced (with a very good large bottle Sylvania 5AR4/GZ34 tube), which will be discussed further in the post on the power

supply. Suffice to say however, the tubes now installed in the unit are equally up to every bit of performance that the build and

condition of this unit is, allowing it to represent all that the Dynaco ST-70 was intended by David Hafler to be.

In all then, I have no wish to pile on to any of the popular notions out there, but rather, to let the chips fall where they may

regarding the true performance of the original design, and that of some popular alterations as well.

ST-70

Base Line Testing

by David C. Gillespie

as previously published on Audiokarma.org

Summary of content (13 pages)

PAGE 1
Base Line Testing ST-70 by David C. Gillespie as previously published on Audiokarma.org The Dynaco ST-70 surely must represent the greatest amplification paradox of all time: It was arguably the best selling power amplifier of all time, and is arguably the most maligned such device as well. For a generation who adored it, there have been those in following generations who are quite vocal about it, to the point that the only useful component they claim it has, is its output transformers.
PAGE 2
Measured Stock Performance While I have individual information on each channel, the information was close enough to simply average the results for reporting. This also helps to minimize information overload. 1. POWER OUTPUT AND DISTORTION: As measured into 8 ohms from the 8 ohm tap, with the Biaset voltage set for 1.56 vdc in each channel under stabilized quiescent conditions.
PAGE 3
In any event, any one of many, many popular amplifiers will exhibit this rising distortion with rising frequency characteristic, with the Dynaco MK III, numerous Heathkit and Eico models, and many others all clocking 20 kHz THD figures at the specified power levels well in excess of 2%.
PAGE 4
The case against the power transformer centers on its current capacity, and the temperature it operates at. However, it still allows its individual per channel power output level to be achieved across the entire audio bandwidth, even permitting it within the majority of the audio bandwidth with both channels driven. So its current capacity is hardly of the grossly inadequate grade that some give it.
PAGE 5
To do a good job of mimicking the voltage drop of a GZ34 tube, there should be a 50 ohm 10 watt resistor installed BETWEEN the output of the SS rectifiers, and the FIRST filter cap. Installing a resistor at this location will control the peak currents drawn and improve regulation, providing nearly the same B+ level that a GZ34 would under any condition of operation. Installing a resistor after the first filter cap, would hurt regulation, and NOT limit the peak currents drawn from the power transformer.
PAGE 6
For example, with a Biaset voltage of 1.40 vdc (45 ma total current flow per tube), 1 kHz THD climbs nearly 55%. If you reduce it further to 1.
PAGE 7
Early "high fidelity" designs (think big E H Scott radios) had but one way to operate the output stage: Triode mode. Why? They were the only tubes that existed at the time. For a vacuum tube configuration, triodes have the lowest plate resistance, and therefore, make for pretty good devices to drive a speaker. But, they aren't very efficient, so they don't produce very much power. As a result, speakers had to be large and use horn configurations to have the efficiency required to produce a usable output.
PAGE 8
As a result, any modification that does not include SOME reasonable level of feedback is almost certainly going to color the sound produced, either by way of its internal resistance, or distortion produced. Therefore, designs that claim that feedback is not needed due to the use of a UL or triode output stage should be held in question, with regards to accuracy produced. The stock ST-70 displays an internal resistance of .
PAGE 9
Fig. 4: 10 kHz square wave in triode mode. Fig. 5: 10 kHz square wave in pentode mode. Fig. 5 10KHz Pentode Mode Fig. 4 10KHz Triode Mode Note: An important point of 2X loading performance with pentode and UL modes is not the actual power drop-off, as that is principally a function Ohm's Law: The source generator can only develop so much output voltage, so if the load resistance attached to it is doubled in value, then the power developed must fall off. This is true with any amplifier.
PAGE 10
measure, the usual screen bypass cap used in all three models is also connected to the cathode terminal as well, for the same purpose of maximizing stage gain. While both approaches work well for their intended purpose, the addition of the fixed bias effort in the ST-70 makes it susceptible to low frequency oscillation (@~ 3 Hz), if specific amounts of low voltage DC is applied to the input jack.
PAGE 11
3. As Screen Grid voltage increased between the tubes, OLG decreased accordingly between the tubes, in keeping with theory. 4. The tubes with the most even split of voltage across the plate and cathode resistors and tube (HH Scott), produced the least distortion.
PAGE 12
6. A very good Damping Factor, 7. Relative insensitivity to fluctuations in load, and 8. Credible performance with both channels driven The ST-70 delivered a ton of performance for very little money, from a young upstart company that had no business producing such a good product -- all of which made the big boys blink, and set standards that many amplifiers manufactured today could only dream about achieving. The biggest failing this amplifier has, is that it was too good for the price.
PAGE 13
EFB™ is simply a way to achieve virtually all of what regulated power supply operation has to offer with regards to distortion reduction, with the possibility of correcting other ills along the way as necessary. In the ST-70, EFB™ would not add extended tube life to speak of, as the operation of the output tubes is already quite conservative to begin with. It cannot be used to correct for original design ills in the output stage, because they basically don't exist.