by dcgillespie » Sat Jun 18, 2011 5:43 am
WJW -- While some physical details of the ST-70 did change over the years, the actual circuit design of the unit never did.
Your unit appears to be a slightly modified stock unit, with the 230 volt power transformer "option". I place the word option in quotations to distinguish it from being a "version". There was really only one version of the ST-70 produced under David Hafler, in that it's circuit design never changed during that time. Being a product of the USA, the vast majority of those produced were supplied with a power transformer suitable for operation from a 117 vac power source, but an alternate power transformer was available to accommodate 230 vac operation. It could have been built like that at the factory as an offering for the European markets, special ordered as a kit in the states, or later changed out by someone to suit their needs. In any event, the alternate transformer was available, but I doubt that Dynaco would change the silk screening on the chassis to accommodate it. They would most likely employ a sticker to be placed appropriately on the unit to indicate it has the alternate power transformer option installed.
As for changes over the life of the ST-70's production run, they were very minor, never electronic in nature, and typically related to either component availability, or slight cosmetic changes.
The specifications of the OPT transformers never changed, but the manufacturers did. The differences were most often noted by different type wires used, and even different part numbers being used. Of course, that spawned a world wide debate over which transformers were better, but to date, any evidence of any differences is opinion based only, as nobody has been able to show any measured performance difference between any of the transformers based on which manufacturer it came from. The transformers on your unit appear to be the original item produced for Dynaco, with the A-470 part number stamped on them.
The front sticker changed in size and styling, as did small details of the cover. Older units used two screws on each side of the unit to hold the cover and bottom plate in place, later units used three on each side. The address of Dynaco as silk screened onto the chassis changed over the years as well.
Capacitors were provided by different manufacturers over time, and of course, everybody has their favorite maker, but the values never changed over the course of its run.
Some of these changes were a product of Dynaco moving its location, and some from it eventually being sold to Tyco. Even later, a ST-70 II appeared from the buyer of all remaining Dynaco stock (Stereo Cost Cutters) after Tyco formally shut operations down. However, the bottom line is that the original ST-70 basically remained unchanged in all that was important over the entire course of its production run, whoever was producing it.
Other than that, your unit has had the original coupling caps bypassed with (obviously) larger (physically and electrically) values at some point by a previous owner. Other components appear to be bypassed or changed as well. Only you can determine if you like the effect of this or not. From a technical standpoint, it is always a concern to raise the value of coupling capacitors significantly in a feedback amplifier, as then low frequency stability can become an issue. Dynaco amplifiers were always known as being quite stable, as this was an issue that Hafler openly championed in the day, and built into his designs. Those with a lesser understanding of these concerns who hap-hazardly change these values in the quest of better sound can unknowingly create more problems than they solve. This is simply offered for you to beware of. Of course, the original schematic is widely available to convert the unit back to its original form, and numerous aftermarket boards with all different manner of tube configurations are available as well.
The ST-70 is widely reported to be the highest selling stereo power amplifier of all time, surpassing many other marks put together. If you get this one, congrats on expanding your Dynaco collection!
Dave