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In Reply to: RE: I had a nice talk with Doc Hoyer. posted by Michael Samra on February 25, 2015 at 12:38:05
1. I think the crappy little (read "moderate resistance") choke in the Dyna ST-70 is beneficial if you beef up the filter caps, as we do, as the resistance damps oscillation.
2. You mentioned that Doc Hoyer understands the McIntosh circuit. I've spent about 3 hours staring at the schematic. I partially understand it. Here's what I *think* I understand:
a. The cathode output winding is mostly for feedback and secondarily for output power.
b. Wiring the output tubes Plate 1 to Screen 2 and Plate 2 to Screen 1 gives ultra-high gain in the output stage which is then beaten into submission with feedback. The result is less required grid drive, higher power sensitivity, and maybe overall lower distortion. That took me a while to wrap my brain around, if I got it right.
I also think that the two McIntosh amps we have right now, a 240 and a pair of 30s, are the best sounding tube amps I've heard, but I suspect they have a bit of 2nd harmonic distortion, as they sound a bit euphonically good.
Follow Ups:
"a. The cathode output winding is mostly for feedback and secondarily for output power."
Both cathode and anode windings contribute equally to output.
"b. Wiring the output tubes Plate 1 to Screen 2 and Plate 2 to Screen 1 gives ultra-high gain in the output stage which is then beaten into submission with feedback. The result is less required grid drive, higher power sensitivity, and maybe overall lower distortion. That took me a while to wrap my brain around, if I got it right."
This arrangement causes each output device to operate in true pentode mode by maintaining a constant voltage betw g2 and k under all normal operating conditions even though k voltage is varying at audio frequencies. g1 drive requirements are abnormally high: g1 AC voltage = anode AC voltage = cathode AC voltage.
...they're bootstraped too.
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