From Rain City Audio:
I recently got to work on another vintage stereo kit, the Dynaco ST-70 amplifier. These were powerful, 35W per channel stereo amps typically using a pair of 6L6GC, EL34, or similar power tubes. This turned out to be a very simple, very reliable, and easy to understand circuit design and variants of the ST-70 have continued to be produced by hobbyist companies up to this day. This one, however, is an original from the early 1960s, and uses a pair of 7199 signal tubes driving 6L6GC power tubes. The PCB had the paper capacitors replaced at some point in the past, but otherwise it was original.
Many solder joints on the PCB were very badly cracked. I re-flowed and re-touched almost every point on the PCB to resolve this issue.
Ultimately, one channel was showing a problem in the feedback circuit. I replaced the feedback capacitors and a handful of other components on the PCB which had drifted past their tolerances, 1W and 2W resistors and mica capacitors. The 7199 tubes themselves were also failing, and were replaced with a matched pair of NOS Sylvania tubes.
Afterwards, it powered up successfully and sounded pretty good! Time for some tests. The amp met Dynaco’s published spec (10 Hz – 40 kHz) and while Dynaco didn’t rate the amp’s THD+N performance, it proved to be quite acceptable.
The channels are off by about 2 dB. That’s a little much, but is due to the aging of the output tubes largely, as the rest of the passive components are pretty closely matched. No matter, though, it’s easily corrected with the balance adjustment on any pre-amp.
Good as new!