Dsl 100h bias setting low

Discussion in 'The Workbench' started by lshindel, Nov 22, 2020.

  1. lshindel

    lshindel New Member

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    Hi everyone,
    New to the forum,
    Working on a 2014 DSL 100H. It was just purchased. Going over it for our guitar player.
    Wall voltage set to 120v with variac.
    Bias measured at the test point pins were 28.1mv and 89.3mv with voltage at 348 measured at plate.
    After adjustment reading are 44.4 and 44.3 with voltage at 369.
    Can not get higher then 44.4mv. Doubled checked with bias probe and had 18.1ma @ 377v on V8 and 17.1 @ 377v on V5.
    Voltage at red leads from PT is 298v AC. Heater voltage is 6.2v AC
    Bias should be double according to Marshall as far as I can tell. All tubes are JJ's and all test good and have been switched around with no difference.
    Where do I look to increase voltage and ma's.
    Thanks
     
  2. pedecamp

    pedecamp Well-Known Member

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    Ive always heard Marshall's recommendation is high, youre better where you have it.
     
  3. anitoli

    anitoli Well-Known Member Gold Supporting Member

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    What type of tubes are in there?
     
  4. Matthews Guitars

    Matthews Guitars Well-Known Member

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    Is the bias stable over time? That's really an important question to ask when dealing with that model. Some revisions are notorious for bias drift due to the circuit board becoming conductive over time.
     
  5. Gunner64

    Gunner64 Well-Known Member Gold Supporting Member

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    The original Jcm2000 dsl/tsl are known for the drift issue. The "revisions " or next incarnations of the dsl are not the drifters. 2014 is well into the new incarnation era, and not a drifter.
     
  6. 2L man

    2L man Member

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    I assume it uses four power tubes. How many measuring sockets it has? Take power cord out of wall socket and measure bias socket voltage and if there is none measure their resistance against ground. I am sure there is no voltage when power is off but verifying it before resistance measurements is good practise when measuring strange circuits. For example working amp uses power from capacitors when it is put Off but failed possibly does not and capacitors can have hundreds of DC volts which is dangerous.

    Something over one ohm but close should be normal. Then put multimeter leads together and you can read how much the leads resistance is and this should approximately be the difference there comes over one ohms.

    Esa
     
    Last edited: Nov 22, 2020
  7. anitoli

    anitoli Well-Known Member Gold Supporting Member

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    The amp has test pins for the left and right pairs of tubes and adjustment pots for each side. It's not that complicated. You measure the left pin/center then right pin/center and this gives you the ma for each side of the ot. Amp on, plugged in, standby off, tubes warm with a load connected.
     
  8. william vogel

    william vogel Well-Known Member

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    Less than 400 volts seems low. I don’t know what it’s supposed to be though.
     
  9. anitoli

    anitoli Well-Known Member Gold Supporting Member

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    Usually 430 and up. 377 is way low for these amps.
     
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  10. Dogs of Doom

    Dogs of Doom Moderator Staff Member

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    not quite...

    Marshall recommends 90mV

    Most people recommend 72-78mV

    He's getting ½ of that & can't get within range...
     
  11. lshindel

    lshindel New Member

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    The tubes are running at 8 watts or around 32%. That's to cold. I would think they should be 50% or higher.
     
  12. lshindel

    lshindel New Member

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    Tubes are JJ EL34
     
  13. lshindel

    lshindel New Member

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    Amp has been on over 2 hrs with no fluctuations.
     
  14. william vogel

    william vogel Well-Known Member

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    The low current is most likely due to the low plate voltage. You need to find out why the voltage is low. The heaters are correct voltage but less than 300 volts AC is low.
     
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  15. lshindel

    lshindel New Member

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    Yes I agree that this is a newer version. The board saya "Marshall DSL 100-60-0 ISSUE 1. D1 94V-0
     
  16. lshindel

    lshindel New Member

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    The tubes are running at 8 watts or 32%. I think they should be closer to 12.5 to 15 watts
     
  17. lshindel

    lshindel New Member

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    JJ's EL34
     
  18. lshindel

    lshindel New Member

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    I agree, I wes hoping somebody had run into this so I did not have spend time troubleshooting. If it's a bad PT he has a 30 day return.
     
  19. lshindel

    lshindel New Member

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    It is measuring 1.6 ohm but I'm curious where you are going with this.
     
  20. lshindel

    lshindel New Member

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    Yes I know, that's where I got the readings.
     

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