| [ Dynaco ST 70 ] |
Technical specifications
The Dynaco ST-70 is one of the most famous tube amplifiers in audio history. Dynaco headquarters were in Philadelphia and in the 50's, 60's and 70's there were built a variety of great quality audio products. The most famous product was the ST-70. Almost 300.000 units of this amp were built in this period of time. It was sold as assembled units or in kit form. In our days the Dynaco name belongs to Panor Corporation who still builds valve amplifiers under the Dynaco name. Even there's a Series II of the ST-70.
The ST-70 is one of the favourites amplifiers of vintage tube lovers. In the USA is well known its great quality. Not a long time ago it was very easy to get a ST-70 for only a few $ (garage sales). Now it is more difficult because of the fever for audio retro equipment. One of the best things in the ST-70 are the output transformers. Those units equipped with transformers named A-470 are excelent. Everybody believes that these transformers are very, very good. Even Joe Curcio from Curcio Audio, is using these transformers to built his new designs. Why? The first units were hand wired!
There's a lot of information about this amplifier in the internet. It is very easy to find info about the ST-70 using a search engine. There are schematics, upgrade kits, info about restoring, etc, etc. The most common advice is to change some parts of the units (capacitors, selenium rectifier, driver stage...). There are kits to do this available from The Parts Connections a subsidiary of Sonic Frontiers, Curcio Audio (see link above), Audio Van Alstine and others.
One of the most common problems with the ST-70 is the driver stage due to the tube used on it. The original 7199 triode-pentode is scarce and expensive. A possible solution is to change the driver stage with one of the kits available from the specialists shown above. These kits changes the 7199 for a most common tube. But if you want to mantain the 7199, Sovtek (the russian valve manufacturer) is distributing worldwide a new 7199 that can be used in the ST-70. This tube is available from The Parts Connection or Sovtek web site.
My Dynaco ST-70 is in good condition, in fact is the best tube amp I have got until now. Dust and that's all. Electronically speaking there're some resistors out of tolerance but with the Parts Connection upgrade kit it will be in perfect working order very soon.
This restoration process is now my main objective, even before than the Fisher X-101D I adquired some months ago. Keep your eyes in these pages to get all the information about the process.
In a near future I will give you more information about the restoration process and some schematics. Perhaps somebody reading these lines will need this info someday.
By the way, the british magazine HI-FI News and Record Review has selected the ST 70 one of the best american audio components of the 20th century. I agree with them.
January 2000, Last News!. The ST 70 restoration process has finished. The unit is working in perfect condition. We have used the modification proposed by The Parts Connections, a subsidiary of Sonic Frontiers. All components in the driver stage have been replaced. We have used Sprague Orange Drop Caps and Solen Fast Caps, etc. The 7199 valve has been manteined because the modification from TPC also mantains it. The power tubes habe been replace with an EL34 matched quartet from Groove Tubes (Tesla). All the restoration process has been made by the expert hands of Jorge Bueno of Amptek . The ST 70 is driving my Quad ESL 57 and the combo is great. Here are some pictures of the ST 70 and his partners.
If anybody meeds the complete users manual of the Dynaco ST 70, it can be found at Curcio Audio web site. It is a complete manual with even schematics.
More info can be found also at Gary's Tube Page.