Vintage Asylum

• RE: questions about Scott 222B phase inverter - FlaCharlie 12:18:50 08/28/16 (4)
  In Reply to: RE: questions about Scott 222B phase inverter posted by microbe on August 27, 2016 at 20:30:29

  I would definitely make some adjustments even though it may pass a signal with those voltages. While the voltages coming from the can caps are within spec (+/- 15%) the 6BL8 voltages are way off.
  
  Have you tested the 6BL8s? If the tubes are worn out that could have some influence.
  
  I would try using the resistor values listed on the schematic as a starting point and then make further adjustments if necessary. A resistor substitution box is really handy for this if you have one. I just used mine to tweak the voltages on the 7199s in the Sherwood S-5000 I just finished restoring. On the pentode section the plate and G2 voltages are interactive so you may have to tweak both resistors a few times to get the combination of voltages that are reasonable.
  

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  • RE: questions about Scott 222B phase inverter - microbe 13:30:08 08/28/16 (3)
    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.
    

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    • RE: questions about Scott 222B phase inverter - FlaCharlie 19:06:38 08/28/16 (2)
      In Reply to: RE: questions about Scott 222B phase inverter posted by microbe on August 28, 2016 at 13:30:08

      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.
      

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      • RE: questions about Scott 222B phase inverter - microbe 11:08:08 08/29/16 (1)
        In Reply to: RE: questions about Scott 222B phase inverter posted by FlaCharlie on August 28, 2016 at 19:06:38

        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.
        

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        • Keep Us Posted - FlaCharlie 10:53:32 08/30/16 (0)
          In Reply to: RE: questions about Scott 222B phase inverter posted by microbe on August 29, 2016 at 11:08:08

          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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