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 be accomplished by making separate posts in this thread for specific design areas of the original unit. It 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, I have provided a few pics of the test volun......er subject. 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 post 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.
More to follow!
Dave
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 be accomplished by making separate posts in this thread for specific design areas of the original unit. It 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, I have provided a few pics of the test volun......er subject. 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 post 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.
More to follow!
Dave