Anybody know what is happened to my AMP

Discussion in 'Marshall Amps' started by HiSaY, Aug 7, 2020.

  1. HiSaY

    HiSaY New Member

    Joined:
    Apr 10, 2020
    Messages:
    6
    Likes Received:
    13
    Hello all Marshall lovers,
    This is my JCM800 2203 in the video. The YouTube link below is will show my AMP error.

    I heard the sound like that a few days ago when I turn on my AMP. There is a noise whenever I knock with drum stick to the amp body and/or the circuit and no guitar sound is out. Like a loose connection, but I think it is not loose connection as the sound is there any where I hit with the stick.


    Please tell me if someone know what's happened?

    Thanks.
     
    Mitchell Pearrow likes this.
  2. SmokeyDopey

    SmokeyDopey Well-Known Member VIP Member

    Joined:
    Mar 9, 2011
    Messages:
    11,464
    Likes Received:
    7,122
    Location:
    Argentina
    Could be a possible bad pre amp tube, but I'm not sure. And I don't even know if just a bad pre amp tube can cause that type of issue. I also hear a 100 Hz hum (something with the filter capacitors as well perhaps?)

    I'm not a tech so I can't really diagnose that.
    How do the tubes look? Any red?
    Do you have an extra preamp tube to swap out to see if that can be the culprit?

    Hopefully some techs will chip in. Good luck
     
    Mitchell Pearrow and HiSaY like this.
  3. rickc

    rickc Member

    Joined:
    Jul 7, 2020
    Messages:
    6
    Likes Received:
    34
  4. rolijen

    rolijen Well-Known Member

    Joined:
    May 19, 2019
    Messages:
    182
    Likes Received:
    538
    Most likely a bad tube.
     
    Mitchell Pearrow and HiSaY like this.
  5. marshallmellowed

    marshallmellowed Well-Known Member

    Joined:
    Jun 26, 2009
    Messages:
    3,962
    Likes Received:
    5,122
    Yes, sounds like a bad tube. Only other thing I can think of, that would cause that type of noise, is dirty jack or tube socket contacts. I'd start with tubes, unplugging/plugging, then replacing (preamp first, then output).
     
  6. Jethro Rocker

    Jethro Rocker Well-Known Member Premium Member

    Joined:
    Mar 11, 2011
    Messages:
    8,464
    Likes Received:
    10,494
    Location:
    Saskatoon, Canada
    I'm betting a microphonic tube as well.
    Does it do this when the chassis is back in the shell? If not, why worry about it?
     
    HiSaY and tce63 like this.
  7. South Park

    South Park Well-Known Member

    Joined:
    Sep 14, 2019
    Messages:
    2,880
    Likes Received:
    2,377
    You have a grounding problem . When you tap the amp you are going to ground . Check all grounds
     
    Mitchell Pearrow, HiSaY and tce63 like this.
  8. marshallmellowed

    marshallmellowed Well-Known Member

    Joined:
    Jun 26, 2009
    Messages:
    3,962
    Likes Received:
    5,122
    Try tapping "lightly" on each tube with a wooden pencil or chopstick. Do not tap the tubes hard, like you were tapping the chassis in the video.
     
  9. Maggot Brain

    Maggot Brain Well-Known Member

    Joined:
    Dec 13, 2011
    Messages:
    949
    Likes Received:
    1,585
    Location:
    Seattle, Washington
    Sounds like a microphonic/bad tube to me.
     
  10. Jethro Rocker

    Jethro Rocker Well-Known Member Premium Member

    Joined:
    Mar 11, 2011
    Messages:
    8,464
    Likes Received:
    10,494
    Location:
    Saskatoon, Canada
    With a wooden drumstick?
     
    cozmacozmy, Trelwheen, HiSaY and 2 others like this.
  11. Matthews Guitars

    Matthews Guitars Well-Known Member

    Joined:
    Aug 17, 2019
    Messages:
    2,472
    Likes Received:
    3,550
    Put in your spare set of tubes and check it again. You DO have a spare set of tubes, right? If not, why not?

    If it's not the tubes or tube sockets then I'd give the filter capacitors a tap with a stick. If they're original to the amp then they're due for replacement.

    Being an early 80s amp they'd be LCR caps, apparently, and those seem to not have the same sort of ultra long life as the Daly caps used in the JMP era.

    But I'd also check every ground connection before changing out ANY parts.
     
    Mitchell Pearrow, HiSaY and tce63 like this.
  12. South Park

    South Park Well-Known Member

    Joined:
    Sep 14, 2019
    Messages:
    2,880
    Likes Received:
    2,377
    First time I Sean a microphonic tube like that . Taping on the chassis not the tube . It would be hard to ground out a drum stick
     
    HiSaY and tce63 like this.
  13. Chris-in-LA

    Chris-in-LA Well-Known Member

    Joined:
    Jul 19, 2018
    Messages:
    1,547
    Likes Received:
    1,273
    Location:
    Los Angeles
    Who is this Sean person you keep mentioning?
     
    Trelwheen and tce63 like this.
  14. Neil Skene

    Neil Skene Well-Known Member

    Joined:
    Oct 3, 2011
    Messages:
    952
    Likes Received:
    1,591
    Location:
    Australia
    It wants to be the worlds most EVIL theremin.
     
  15. maxxi

    maxxi Active Member

    Joined:
    Oct 9, 2011
    Messages:
    465
    Likes Received:
    249
    Location:
    belgium
    I'm thinking grounding issue as well.
    change tubes first(the easy job), if not solved take it to a tech to get the grounding checked( or do it yourself if you're qualified).
    I find it somewhat weird that a microphonic tube would produce sound/noise even when the chassis/board is lightly touched with the drum stick, wouldn't it just amplify the vibrations of the chassis if tapped somewhat harder?

    just my thoughts, I'm in no way a qualified amp tech :hmm:
     
    South Park and HiSaY like this.
  16. purpleplexi

    purpleplexi Well-Known Member

    Joined:
    Feb 14, 2011
    Messages:
    946
    Likes Received:
    1,477
    You're not a drummer are you?
     
  17. HiSaY

    HiSaY New Member

    Joined:
    Apr 10, 2020
    Messages:
    6
    Likes Received:
    13
    I am not :)
     
  18. Seanxk

    Seanxk Well-Known Member

    Joined:
    May 14, 2018
    Messages:
    435
    Likes Received:
    373
    I have been known to be Microphonic, I sometimes repeat too.
     
    Mitchell Pearrow and HiSaY like this.
  19. maxxi

    maxxi Active Member

    Joined:
    Oct 9, 2011
    Messages:
    465
    Likes Received:
    249
    Location:
    belgium
    :rofl:
     
  20. pedecamp

    pedecamp Well-Known Member

    Joined:
    Dec 7, 2009
    Messages:
    15,132
    Likes Received:
    12,636
    Clean all jacks, pots and tube sockets with Deoxit, see if that helps. Then take a known good preamp tube and try it in each slot to isolate which is the bad tube. If still no good replace the power tubes and rebias. If still no good tech time. Good luck with it and welcome to the forum, let us know how you make out. :yesway:
     
    Mitchell Pearrow and HiSaY like this.

Share This Page