Burnt resistor, having trouble finding the value to replace it with

Discussion in 'The Workbench' started by iron broadsword, Jan 30, 2020.

  1. iron broadsword

    iron broadsword Well-Known Member

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    Hey guys, long time since I've been here and need some help. Blew an el34 in my JCM900 4100 and replaced them but wasn't getting any bias reading, so went looking and fuses were fine but found a burnt resistor on the last power tube shown in pic. I'm having trouble finding the value of it. Here's a schematic: https://www.thetubestore.com/lib/th...rshall-JCM900-Dualrev-100W-4100-Schematic.pdf

    Also, anything I should be concerned about before firing it up after replacing it? 0.jpeg
     
  2. South Park

    South Park Well-Known Member

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    Do not power up with tubes until the bias is working . If you don’t get this stuff . It is all right I don’t ether
     
  3. iron broadsword

    iron broadsword Well-Known Member

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    I've been working with modifying gear for about a decade, but if it gets over my head I know when to ask. Unless that resistor being burnt is a clear indication that something else might be wrong, replacing it firing it up will be fine with new tubes, I just need to be sure and not so great with schems. When tubes fail they can take out other shit, I'm expecting it's just that.
     
  4. Seanxk

    Seanxk Active Member

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    Is it actually touching ground?. Marshall wouldn't be that sloppy would they
     
  5. iron broadsword

    iron broadsword Well-Known Member

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    it's just the angle, it's a good 1/2 inch away from that bolt
     
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  6. iron broadsword

    iron broadsword Well-Known Member

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    Lol, just realized there's an unburnt one on the first power tube that is 15.2ohm 0.02%, not sure the wattage. I'm assuming it's probably going to be the same thing
     
  7. ampmadscientist

    ampmadscientist Well-Known Member

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    Yes.
    If the bias split resistors are 220K, then the resistor should be 1500 ohm 1/2 W.
    If the bias split resistors are 150K then the resistor is 2200 ohms 1/2 watt.

    However look at the other 2 power tubes, and there will be an identical resistor.

    "Also, anything I should be concerned about before firing it up after replacing it?"

    Yes the 1 power tube could be bad, and the resistor will burn up again.
     
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  8. ampmadscientist

    ampmadscientist Well-Known Member

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    It's supposed to be 1500 ohm Brown Green Red...
     
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  9. Micky

    Micky Well-Known Member VIP Member

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    Measure one of the others to possibly find the value
     
  10. Matthews Guitars

    Matthews Guitars Well-Known Member

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    The value will be the same as the equivalent resistor on the opposing power tube. The tube shorted and took out the resistor which functions as a safety device, a thermal fuse.

    Replace with the same type of safety resistor. You want that resistor to burn open in the event of a hard tube short, rather than have a winding burn open on your transformer.
     
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  11. iron broadsword

    iron broadsword Well-Known Member

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    Excellent, thanks guys.
     
    Last edited: Jan 30, 2020
  12. iron broadsword

    iron broadsword Well-Known Member

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    Hey, so i replaced the resistor but now I get no reading on the center tube pair when biasing them. They're nearly cold to the touch so i'm guessing it's a bias supply issue. Where should I start looking for this?

    All fuses are good btw
     
    Last edited: Feb 19, 2020
  13. ampmadscientist

    ampmadscientist Well-Known Member

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    Set meter for DC volts.
    Connect black probe to chassis.
    Connect red probe to pin 5 of output socket.

    Take the output tubes out.
    Turn power on but leave standby in warm-up position.
    Measure DC voltage on pin 5 of each output socket.
    Write down the voltages for each socket.

    Turn standby to operate position.
    High voltage is now turned ON.
    Measure DC voltage on pins 3 and 4 of output tube sockets.
    Write down readings of each socket.

    report all your readings.
     
  14. ampmadscientist

    ampmadscientist Well-Known Member

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    Turn off power and un-plug mains power.
    Take the output tubes out.
    Set meter for ohms.
    Connect black probe to chassis.
    Connect red probe to pin 8 of each output tube socket.
    What is the ohm reading pin 8 for each socket?

    2. You will see that there is some large white ceramic 5 watt resistors mounted on the rear circuit board.
    With power off / the output tubes removed, measure each of these large resistors with your ohm meter.
    Make sure that all the ceramic 5 watt resistors read good.
     
  15. iron broadsword

    iron broadsword Well-Known Member

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    Nope, i was a dummy and one of the fuses were blown. Looked ok but when I metered it it was open. So, replaced that and biased it up and sounds great, but the inner pair is reading out at 28ma when the outer pair is set at 36ma. This is a new issue, i've never ran into a discrepancy like this before. I mean it sounds good, can't really complain but obviously something is off. Should I be concerned?

    It was a matched quad from tube depot, they've always been on point. I can try swapping them around but seems like that's probably not going to solve it
     
  16. Exojam

    Exojam Well-Known Member

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    Well according to page 4 of the pdf the bias should be set to -38v to -40v so maybe set it according to the documented spec and see how she sounds.
     

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