Noise when jumping channels

Discussion in 'Marshall Amps' started by Mats Raknerud, Jul 16, 2020.

  1. Mats Raknerud

    Mats Raknerud New Member

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    Hi.
    I just got my SV20 head.
    Sounds great, but when I jump the channels i get a loud hum. Even when not guitar is plugged in.
    Have tried different cables. But the only one that I could get to stop hum is a longer cable where I could get the cable itself up in the air awau from the amp.
    As soon as I drop it on the floor it’s noisy like the short patch cables.

    Sounds like some sort of grounding noise.

    Any ideas what it could be?
     
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  2. tce63

    tce63 Well-Known Member Premium Member

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    It doesn't sound good, mine make just a little hum with jumped channels.
    I have short patch cable to connect the Channels.

    You will soon receive suggestions from other members.

    And :welcome: to the forum, a great place.

    Cheers

    :cheers:

    2020-07-16 15-28-11.jpg
     
  3. South Park

    South Park Well-Known Member

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    The ground on the plug may not be hooked up try a different plug
     
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  4. Mats Raknerud

    Mats Raknerud New Member

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    Have tried differen socket combinations. The problem remains.
     
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  5. Mats Raknerud

    Mats Raknerud New Member

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  6. IOSEPHVS

    IOSEPHVS Well-Known Member

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    Just to be clear, the SV20 is a single channel amp with patchable inputs.
     
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  7. Mats Raknerud

    Mats Raknerud New Member

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    Yup
     
  8. junk notes

    junk notes Well-Known Member

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    Move the amp into another room on the opposite side of your home and do the same test. Let us know.
     
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  9. dslman

    dslman Well-Known Member

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    Looks like you have it sitting on another amp, that could be the problem also.
    Try another "small" patch cable, if you haven't already.
    junk notes is on to something.
     
  10. Mats Raknerud

    Mats Raknerud New Member

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    Sitting on it’s cab.
    Have tried 5 or 6 patch cables.

    Also tried moving it to a different room. Same same.
     
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  11. Chris-in-LA

    Chris-in-LA Well-Known Member

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    The second way that you plugged in was the usual way and the hum was less. Guitar should be plugged into the top jack on the treble channel and the jumper should go from the second jack to the top jack on the normal channel. But, only plugging in a cable to the top jack on the treble channel should not cause that much hum, especially with the volumes at noon as you seem to have them.
     
  12. Mats Raknerud

    Mats Raknerud New Member

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    I know. The volumes are both at noon
     
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  13. junk notes

    junk notes Well-Known Member

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    The different room should be on another circuit breaker, and not just another room outlet, of course.
    Also if the room has incandescent or fluorescent. It is good practice to know what your voltage is coming out of the room. Using a multimeter to verify operating range is 120v or less, or whatever running voltages are recommended for the amp.

    The vintage (Plexi) amps had slightly less running voltages at 115v and some at 110v IRRC on the Mains selector. Your SV20 concept is an updated reissue, but there is no schematic as others have stated.
     
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  14. Martin Adams

    Martin Adams Member

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    Noisy V1 preamp tube I reckon

    Try swopping for a good new tube
     
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  15. Mats Raknerud

    Mats Raknerud New Member

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    Have tried to move it to a room at the other end of the house.
    Also swapped preamp tubes on at the time.
    No change

    But I noticed another thing. When playing loud the amp suddenly drops. Like going to 1watt. A good amount more difference than going from full power to 5w.
    But then it comes back to life after some time.
    It’s brand new. So maybe I should get a repair guy to fix it.
     
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  16. junk notes

    junk notes Well-Known Member

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    Yes, good guess, but the amp is quiet. The jumper cable is acting as an RF antenna. ;-)

    edit: any longer, he could pick up 105.5 KNAC
     
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  17. WellBurnTheSky

    WellBurnTheSky Well-Known Member

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    If you tried different patch cables, it definitely looks like an issue with the amp. I used mine jumped most of the time (low 1 to high 2), and even at stage level (with both channels at 7) it's very quiet.

    Edit: I just watched your video, did you try it with soldered cables ? I have zero faith in solderless patch cables...
     
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  18. Mats Raknerud

    Mats Raknerud New Member

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    Have tried with soldered patch cables as well. :)
     
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  19. WellBurnTheSky

    WellBurnTheSky Well-Known Member

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    Ah right, so you covered all bases. I'd guess there's a grounding issue somewhere, something is acting as a RF antenna, might be an issue with the amp. Again, mine is dead quiet in the same scenario.
     
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  20. Mats Raknerud

    Mats Raknerud New Member

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    Yup.
    And my other guitar amps are pretty quiet. They're fender amps, but I record a fair amount at home with gain pedals into the fender amps without any big issues. The good old single coil noise is noticeable, but nothing out of the ordinary.
     

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