This classic tube amplifier used to be the best value on the block. Fifteen years ago you could buy one of these for $25. This amp was designed to be inexpensive and to get maximum performance for dollar paid. At the end of it's sale it sold for about $100 in kit form. David Hafler, the amp designer, made the cost containing compromises in a very clever way. For example, the power supply is wimpy by modern standards, but he biased the output tubes into Class A, so the current demands on the power supply are fairly constant regardless of program material. The input section is very simple; an input pentode, followed by a direct coupled split load inverter based on a triode. This topology was very popular at the time, and used the 7199 tube. There is an outer feedback loop that encloses the output transformer and input pentode. There is also a secondary feedback loop that is taken from the ultra-linear screen tap. Even with these two feedback loops, this amplifier very stable. From my measurements and listening tests, I believe that the Dyna ST-70 output transformers are of very good quality (at least the ones on my ST-70).
There are many modifications available for this amp. There are two problems that most modifications fall into. First there are the total mods. Basically, everything in the amp is removed except the iron, and the amp is rebuilt. This is all well and good, but don't forget that this is a budget amp, and the chassis is not so great either. If you are going to dump that much money into a tube amp, why not start with a new chassis, and design it from the ground up instead of making compromises to fit the mod into this chassis. The second type of modification class is just to replace the input board which contains the hard to find 7199 tubes. This can be a good way to go, but if the topology is changed significantly, the stability of the amp can be compromised. And if the original design is changed in one way, it can show the compromises in another area. For example, if the bias current is reduced on the output tubes to lengthen their lives, the power supply modulates more with the musical signal, and the IMD goes through the roof.
Based on my listening experience with various ST-70 mods. The most satisfying mods change the amp the least. These mods have kept the pentode voltage amp followed by a triode split load inverter. Changing the 7199 tube to a 6GN8 TV tube is popular, and requires socket repining. But triode/pentode tubes generally don't sound all that good, I believe that is because only the 7199 was really designed for audio. The other method is to use a third tube, and use two pentodes and a dual triode. This method allows the use of readily available, good sounding tubes.
ST-70 Circuit Schematic
A Picture Of My ST-70
Dyna ST-70 Input Board With EF86's
Dyna ST-70 Input Board With 6AU6's
I've designed a replacement capacitor board for the ST-70. This board replaces the large can capacitor in the power supply section of the Dynaco ST-70 amplifier. Exact replacements for the original can capacitors are difficult to find. The values needed can be built up with various replacement caps, but the installation of all the needed capacitors tends to make the amplifier very messy. This circuit board uses readily available circuit board mounted capacitors. There is a large range of values and voltages available in this form factor. The board easily holds similar capacitor values to the original can caps with room to spare. The capacitor values can be increased substantially and still fit on the board and in the amplifier. As an added bonus, the cost of all the caps needed is almost the same as a single modern twist lock capacitor.
ST-70 Capacitor Board (396 Kb)