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Looking for help setting the bias on my Scott 222B

Discussion in 'Tube Audio' started by Onebean, Nov 8, 2018.

  1. Onebean

    Onebean AK Subscriber Subscriber

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    I just bought a Scott 222B and I would like to know how to check and set the bias on the power tubes. I found a post where Tinkerbell linked to the instructions on the Scott website. Here's what it says.

    The Type 222B (also 6BQ5-based) featured an adjustable self-bias design.

    To adjust a 222B's bias: place a DC voltmeter across resistor R-201, 1.2k-ohm, located across the preamp filament heater supply. (Note: Remove the bottom plate cover to gain access below the chassis). Adjust the Bias Control (located between the power transformer and electrolytic cap until the meter reads 38.5 volts DC.

    So how do I locate R-201, and how do I connect my meter?

    Is this telling me to clip my meter on the legs of the resistor? When I bias my Fender Brown Princeton clone, I'm pretty sure I clip my leads between a resistor and ground.

    Is the bias control pot for each power tube accessed from the underside of the amp, or top side?

    A picture would be worth a million words for me. Thanks for the help.

    Onebean
     

     

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  2. radioactive

    radioactive tube gear fanatic Subscriber

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    FE615A76-D09E-4366-A252-81695DC7F323.jpeg according to this picture theyre the ones ontop.
     
  3. Onebean

    Onebean AK Subscriber Subscriber

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    Radioactive, is it the recessed pots? I see 2 of them, each one between 2 power tubes. Are the power tubed biased as a pair? Can you show me where to attach my meter?
     
  4. radioactive

    radioactive tube gear fanatic Subscriber

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  5. radioactive

    radioactive tube gear fanatic Subscriber

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    i scrached my head looking for r-201 but it dont exist .
     
  6. radioactive

    radioactive tube gear fanatic Subscriber

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    tje pots on the topside should be labled if the writing isnt gone.
     

     

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  7. Onebean

    Onebean AK Subscriber Subscriber

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    Yea, the writing is gone.
     
  8. drew_t

    drew_t AK Subscriber Subscriber

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  9. Onebean

    Onebean AK Subscriber Subscriber

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    Ok, on page one of the linked manual, it says to connect one lead of the DVM to point A, and the other to chassis ground. On page 4 ( the amp schematic) I found the "A" point, but it's very blurry, and I can't tell what components are around it. I'm not good at translating schematics to layouts either. Also, I only see one point "A". Should there be 4 places to set the bias? One for each tube???
     
  10. drew_t

    drew_t AK Subscriber Subscriber

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    The PDF linked in my post above is a scan of official HH Scott documentation. I think what you’re calling the “linked manual” is a scan of a “SAMS” document. These were published by a third party company; they incorporate schematics that were drawn by their own personnel, often in a different style and using different component identification numbers, and sometimes not exactly correct. Here, R91 in the SAMS schematic is the same thing as R201 in the HH Scott schematic. (It’s shown as a 10 watt resistor in the SAMS, rather than seven.) Whether the one in your unit is a seven watt or a 10 watt, it shouldn’t be hard to find. It will be bigger than most of the other resistors.

    The 222B does not have individual bias adjustment capacity for each power tube.
     
  11. Onebean

    Onebean AK Subscriber Subscriber

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    Thanks Drew. I'll get the bottom plate off tonight and take a look. Does it matter which side of the resistor I clip onto for setting the bias?
     

     

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  12. drew_t

    drew_t AK Subscriber Subscriber

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  13. Onebean

    Onebean AK Subscriber Subscriber

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    Ok, here is a picture from the bottom side. I think I found the resistor I need to measure across to set the bias. Please take a look at the arrows I added to the picture, and let me know if I've got this right. I appears the adjustment pot is right next to the resistor too. My plan is to clip the leads of my meter onto either side of the resistor, power up the amp, set the bias, then power down and move to the other one. Then let the amp idle for 30 minutes and check it again. I'll have speakers hooked up while doing this. Let me know if I'm on the right track.
    Scott 222B.jpg
     
  14. drew_t

    drew_t AK Subscriber Subscriber

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    2DFC9EA2-058C-4D5A-9525-DCC3E07ED39D.jpeg

    I lose the detail if I try to zoom in too closely on the photo, but I think the resistor you’re looking for is the discolored rectangular cement one I have circled. And the adjustment pot is probably the one right above it.
     
  15. Onebean

    Onebean AK Subscriber Subscriber

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    I'll take a close up shot of that area and post it.
     
  16. Dandy

    Dandy Super Member

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  17. Onebean

    Onebean AK Subscriber Subscriber

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    Here are some close up pictures. Any help on where to clip my DVM leads, and which pots to adjust is greatly appreciated. I live in an area with no qualified techs that I know of. If I can't get this figured out and running correctly, it's going to have ot be shipped to someone. I'd prefer to do it myself, so I can service the amp down the road too.

    IMG_1373.jpg IMG_1374.jpg IMG_1375.jpg IMG_1376.jpg IMG_1377.jpg

    Thanks for looking.

    Onebean
     
  18. triode17

    triode17 Super Member

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    Since this model uses a pot. to balance the cathode currents and unless you want to install 10 Ohm resistors in series with the cathodes, I would purchase two Bias Current Probes from eBay for EL84s (equiv to 6BQ5). Then, you simply use two meters set to milliamps and read each pair's current. Adjust the DC Balance control for equal current in each output tube, then adjust the main bias control R213 to set the output tube bias at 25 ma.each. This will provide a 70% Plate dissipation power at idle which will work well for this tube.
     
  19. drew_t

    drew_t AK Subscriber Subscriber

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    Clip the leads where the red arrows point. Set meter to measure DC volts. Adjust the circled pot so that you get 38.5 volts on the meter. 222B.JPG
     
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  20. Onebean

    Onebean AK Subscriber Subscriber

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    Thank you drew! So there's only one pot to adjust? The tubes are a mix of brands, so I doubt they are a matched set.
     

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