Marshall DSL 20 head crackling on high output

Discussion in 'The Workbench' started by heave1, Feb 20, 2021.

  1. heave1

    heave1 New Member

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    My DSL 20 head has a bad crackling noise when I switch to the high output (down position). The low output (up position) is okay, maybe a little hum. I ran a bias check on the two stock Marshall EL34 tubes and they are badly unmatched. From 21 mA on one tube, up to 60mA on the second tube.
    I am assuming that the problem is bad tubes. Any other suggestions?
     
  2. KraftyBob

    KraftyBob Well-Known Member Gold Supporting Member

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    Can you adjust the bias back or are the tubes not responding? I don’t know if the bias being off like that would cause crackling, but I would try that first.

    If that doesn’t solve the problem then new tubes might be in order.
     
  3. pedecamp

    pedecamp Well-Known Member

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    Pretty sure this amp self biases so it doesnt really matter the tubes are matched or not, the amp adjusts itself as needed. Plug your guitar into the return effects loop and see if the problem goes away, that will at least tell you if you have a bad preamp tube, if the problem still exists I'd look at replacing power tubes. You could spray the switch with Deoxit first before replacing tubes to see if the problem goes away.
     
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  4. heave1

    heave1 New Member

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    Cleaned pots and tubes and tube sockets. Tried plugging into FX loop and still crackled if in the high output setting (down position on switch). The amp is self biasing, there is no bias pot. I've order new Mullard EL34 power tubes and JJ ECC83 tubes. Also found out that the foil tape on the inside of the amp head rubs off by the screw holes. Covered them all with copper tape and will put copper tape on the chassis screw holes.
     
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  5. heave1

    heave1 New Member

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    Is there a chart with testing points for the Marshall DSL 20 head and the schematic? I cannot find them anywhere.
     
  6. Max Gahne

    Max Gahne Active Member

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    The hi/low power switch selects hi/low B voltage for the phase inverter, the power tubes and the output transformer. So of the tubes it could also be the phase inverter which is also after the FX loop. If it isn't a phase inverter or power tube I'd look at the power switch itself and the hi power filter caps etc. Could be a bad solder joint in the hi power circuit. There's some DSL20 schematics around, I found some somewhere. They're too big to upload.
     
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  7. Max Gahne

    Max Gahne Active Member

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  8. heave1

    heave1 New Member

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