Amp/Preamp Asylum

Posted by petercapo (A) on November 09, 2014 at 21:32:17

In Reply to: RE: "the Stereo 35/SCA-35 is actually the best sounding amp Dynaco ever made" posted by 1973shovel on November 09, 2014 at 19:10:25

You piqued my curiosity, so I pulled up the schematics for the Stereo 35 and the SCA-35. The power amp sections seem pretty similar between the two, with the phase inverter topology the same as far as I can see – a form of cathodyne phase inverter in both cases, correct?

A difference is that the first section of the 7247 uses a straight triode that I believe is for some voltage gain (or is it more of a buffering role?), while the first section of the 7199 is a pentode used for the same purpose, though in the SCA-35 schematic it looks like the 7199 is hooked up in a way that does not use all five electrodes, rather, it looks like it is running in some sort of triode mode(?) with the screen grid connected in some kind of feedback, no?

So, the first section of the 7199 has some differences in the associated circuitry, but this looks to be just what was needed to accommodate the inherent operating characteristics of the first section of the 7199 vs. the first section of the 7247. There are also some differences in the feedback schemes, but the distortion specs are very close – I don't think I could tell the difference between 0.1% and 0.2% IMD "at average listening levels."

I confess that I am not an engineer, but these two circuits look far closer to each other than the difference between the original Stereo 70 two-tube gain plus cathodyne phase inverter vs. the three-tube gain plus long tail inverter Roy uses in his Stereo 70 circuit.

Edits: 11/09/14   11/09/14