March 28, 2024: I've just bought an all-new, Dynaco ST-70-inspired, basic amp (for $1,195) that an enthusiast has created for sale (with a Bob Latino input section that he has lightly modded). I bought it on a lark just to see what this much-loved simple tube amp circuit would sound like. I'm fascinated by this antique. I'm 82 years old. When I was a kid I longed to get my hands on the Dynaco ST-70 (in the late 1950s). I couldn't afford $100 then. And it would have needed a pre-amp too, which I could not afford either. But I did buy a 12-watt-per side Heathkit integrated amp featuring four EL84 tubes in push-pull circuits (for $59). I soldered it up, plugged it in, and I relied on it for the next that 13 years---on down to 1973 when I got my first real job as a college teacher. It finally blew a power tube, so I set it aside.
The AudioScience review of an original Dynaco ST-70 that started off this thread a couple years ago, which I found just recently online, was quite off-putting. That amp measured terribly. It was a dog! Could that have been due to the age and wear on the old amp? Installing the Bob Latino input section in that tired old example was not much of a test for the mod in question. Resistors and caps whose values were no longer nominal, solder joints that were questionable, wire insulation that had rotted, corroded tube sockets, transformers past their prime----all these aspects (and more) could account for the reviewer's disappointment.
When I receive the new David Hafler inspired amp from my enthusiast builder, I'll come back to this thread and report. My builder has beefed up the power transformer and obtained bigger, better-wound output transformers. He's put bigger caps into the amp where that might make a difference. He's used high quality resistors with guaranteed, precise values. He's also supplied a connector for a fancy power cord. But he has retained tube rectification in the power supply. Maybe I'll attempt to replace the tube with a solid-state rectifier just to see what such a mod might do for the amp's performance. Report to come.
