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In Reply to: RE: questions about Scott 222B phase inverter posted by FlaCharlie on August 28, 2016 at 12:18:50
I suspect the voltages are way off due to the change from 100k ohm plate resistors to 220k ohm plate resistors. I guess the question is, is this really what the Scott engineers had in mind? They look original so I assume they were installed there on purpose at the factory. Changing them out would be a pain so until someone tells me this is definitely a poorly designed circuit I'm inclined to just leave it as is. Like I said, the amp does sound quite nice.
Follow Ups:
Well, I can guarantee they didn't have 7 volts on the plate of the pentode in mind! The voltages you have can't be resulting in reasonable operating points.
I would assume the voltages listed in the schematic are correct and going to the 100 ohm plate resistors will get you closer. As I said, you will likely have to play with the resistor values depending on the condition of the tubes.
Replacing all the power supply caps doesn't do much good if the operating points of the tubes are way off.
I think you're right; 7 volts on the pentode plates can't be ideal. Maybe the person who assembled the amp just plain goofed up. I'm really surprised it sounds as good as it does. I haven't really pushed it yet so maybe at higher volumes it'll fall apart. Just to be safe, I think I will change the plate resistors to 100k ohm, maybe it'll sound even better. Thanks for the help, much appreciated.
Please do post the results after you change to the 100 ohm plate resistors.
I have a 222C that I found at a flea market a while back and now that I've finished with the Sherwood S-5000 I'm getting the itch to finally go through it. My 222C is apparently the 3rd version that uses 6U8s (or 6GH8s) in that position so it's bound to be a bit different from your 222B.
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