Need help from the amp wizards. Please. SV20H no sound.

Discussion in 'Marshall Amps' started by TXOldRedRocker, Sep 21, 2020.

  1. TXOldRedRocker

    TXOldRedRocker Well-Known Member Gold Supporting Member

    Joined:
    May 15, 2020
    Messages:
    388
    Likes Received:
    876
    Location:
    Cypress, TX
    I have an SV20H. I was moving some cables around. I made THE cardinal sin. The speaker cable going to the cab was not pushed in all the way when I powered the amp on, and I did attempt to play through it. (I know, I know, I know. Mad at myself.)
    Now I have no sound from my amp, with one exception. If I strike a chord and turn the power to the amp off, as it powers down the chord can be briefly heard before the power is completely gone.

    I have the chassis out. Not visible burns on the board. Fuses do not appear blown, but haven't checked them. Tubes look OK, but looks can be deceiving.

    Does anyone know 100% what's going on here?
     
    tce63 likes this.
  2. black knight

    black knight Member

    Joined:
    Sep 17, 2014
    Messages:
    49
    Likes Received:
    74
    Start simple. Check the speaker cable, if you haven't already, with a meter (if you have one). Connect it to your cabinet and take the ohms reading from the other end (black to sleeve, red to tip). Is the reading stable? Correct?
     
  3. Gunner64

    Gunner64 Well-Known Member Gold Supporting Member

    Joined:
    Nov 16, 2013
    Messages:
    6,323
    Likes Received:
    8,253
    Location:
    Midwest, USA
    Check the ht fuse..with a meter.
     
  4. TXOldRedRocker

    TXOldRedRocker Well-Known Member Gold Supporting Member

    Joined:
    May 15, 2020
    Messages:
    388
    Likes Received:
    876
    Location:
    Cypress, TX
    Thank you. Cable is good. I have more than one amp. Used the cable to connect to a different head and works perfectly.
     
    tce63 and Mitchell Pearrow like this.
  5. TXOldRedRocker

    TXOldRedRocker Well-Known Member Gold Supporting Member

    Joined:
    May 15, 2020
    Messages:
    388
    Likes Received:
    876
    Location:
    Cypress, TX
    Thank you. Both fuses at the Mains Input are good.
     
    tce63 likes this.
  6. TXOldRedRocker

    TXOldRedRocker Well-Known Member Gold Supporting Member

    Joined:
    May 15, 2020
    Messages:
    388
    Likes Received:
    876
    Location:
    Cypress, TX
    What should I test next? I have plenty of tubes and can replace them, but not sure I want to charge it all back up if there's something else I should look at before new tubes.
     
    tce63 likes this.
  7. TXOldRedRocker

    TXOldRedRocker Well-Known Member Gold Supporting Member

    Joined:
    May 15, 2020
    Messages:
    388
    Likes Received:
    876
    Location:
    Cypress, TX
    There are 3 other fuses on the PCB, FS1, FS2 and FS3. All are good.
     
    tce63 and Mitchell Pearrow like this.
  8. malice95

    malice95 Active Member

    Joined:
    Feb 24, 2016
    Messages:
    68
    Likes Received:
    146
    Probably totally unrelated but do you have pedals in the loop or it hooked up at all? I had this happen before when I had a cable go bad in my loop. "powers down the chord can be briefly heard "
     
    tce63 likes this.
  9. TXOldRedRocker

    TXOldRedRocker Well-Known Member Gold Supporting Member

    Joined:
    May 15, 2020
    Messages:
    388
    Likes Received:
    876
    Location:
    Cypress, TX
    Thanks for your input. I absolutely should have been more clear in my initial post. Pedals were involved. However, before I jumped on here, I plugged the guitar directly into the amp input. I use a Weber attenuator with this amp as well. But, I plugged the amp directly into the cabinet to remove the Weber as a possible source for compete tone loss. So, guitar->amp->cabinet only, before asking for help.

    Thanks again. Sorry I wasn't more descriptive. Juggling 3 things at once at home, and that's how I got sloppy.

     
    tce63 and Jethro Rocker like this.
  10. Matthews Guitars

    Matthews Guitars Well-Known Member

    Joined:
    Aug 17, 2019
    Messages:
    2,667
    Likes Received:
    3,915
    Sounds like you need to take your amp to a qualified tech to confirm a probable diagnosis of a failed output transformer.

    However...while I'm not personally that familiar with the SV20's circuit, I know from experience that a DSL100 can exhibit exactly the same problem yours has,
    and the problem turned out to be that fusible resistors connecting the output tube cathodes to ground had fused open, thus resulting in no output. So this needs to be diagnosed
    by a tech who can diagnose the problem and not just throw parts at it. If I'd failed to check those resistors I might have put an output transformer in the amp that wasn't
    going to fix the problem.
     
  11. Mitchell Pearrow

    Mitchell Pearrow Well-Known Member VIP Member

    Joined:
    Jul 2, 2018
    Messages:
    11,910
    Likes Received:
    21,355
    Location:
    Moreno Valley CA
    Have you tried to plug your guitar into the fx return ??
     
  12. mirrorman

    mirrorman Well-Known Member

    Joined:
    Mar 30, 2014
    Messages:
    970
    Likes Received:
    1,512
    Location:
    Canada
    Did you also remove any pedals from the loop? I have found that an unpowered pedal in the loop will result in no amp output.
    Also, you tried the speaker cable, did you also test the instrument cable?
    Did you try more than one guitar (could be a guitar issue, not the amp)?
     
  13. TXOldRedRocker

    TXOldRedRocker Well-Known Member Gold Supporting Member

    Joined:
    May 15, 2020
    Messages:
    388
    Likes Received:
    876
    Location:
    Cypress, TX
    Thanks to all of you for your input!

    I am back up and running. I replaced all 5 tubes. I was on the verge of doing some rolling anyway, because I noticed a recent drop in clarity. But my free time is limited. This incident caused me to stop and make time. If I had more time, I'd have replaced one at a time, but alas I just went for all of them. My guess is that I had a tube, or tubes, not working so well, and this killed it/them. When I get some time I'll test to see which one, or ones, were damaged. I'll set them aside until I can. And... the best part, the SV20H (which is only about 5 months old) sounds incredible! Better than right after opening the box. Woohoo!

    I have some great NOS tube inventory, but like I said, little free time right now, and didn't want to deal with decades old tubes not ever used before. So I went with newly made and matched Shuguang EL34's, as well as newly made Mullard clone 12AX7's. I had these in an amp I sold earlier this year and were known to be good.

    Thanks again! :cheers:

    :dude:
     
  14. Mitchell Pearrow

    Mitchell Pearrow Well-Known Member VIP Member

    Joined:
    Jul 2, 2018
    Messages:
    11,910
    Likes Received:
    21,355
    Location:
    Moreno Valley CA
    Bravo brother
    Cheers to getting your amp back
    Mitch
     
    tce63 and mirrorman like this.
  15. TXOldRedRocker

    TXOldRedRocker Well-Known Member Gold Supporting Member

    Joined:
    May 15, 2020
    Messages:
    388
    Likes Received:
    876
    Location:
    Cypress, TX
    Thanks Mitch! :cheers:
     
    tce63 and Mitchell Pearrow like this.
  16. Matthews Guitars

    Matthews Guitars Well-Known Member

    Joined:
    Aug 17, 2019
    Messages:
    2,667
    Likes Received:
    3,915
    Two possibilities: The phase inverter died. This will certainly make the amp quiet.

    Or...the cathode follower died. (IF there is one.) Be aware that not every 12AX7/ECC83 tube is good for cathode follower duty. Most Russian made tubes can't handle that application, one exception being, very specifically, the Sovtek 12AX7WB.

    Glad you're back without having to spend money on an actual repair.
     
  17. marshallmellowed

    marshallmellowed Well-Known Member

    Joined:
    Jun 26, 2009
    Messages:
    4,043
    Likes Received:
    5,308
    Glad to hear
     
    Last edited: Sep 21, 2020
    tce63 and Mitchell Pearrow like this.
  18. Mitchell Pearrow

    Mitchell Pearrow Well-Known Member VIP Member

    Joined:
    Jul 2, 2018
    Messages:
    11,910
    Likes Received:
    21,355
    Location:
    Moreno Valley CA
    My next post was going to be to check the preamp tubes .
    But you had already fixed the problem.:applause: :cheers:
     
    tce63 likes this.
  19. _Steve

    _Steve Active Member

    Joined:
    Apr 29, 2020
    Messages:
    144
    Likes Received:
    130
    Location:
    Los Angeles
    If the problem occurred when there was no load connected wouldn't it most likely be the EL34s?
     
    tce63 likes this.
  20. Matthews Guitars

    Matthews Guitars Well-Known Member

    Joined:
    Aug 17, 2019
    Messages:
    2,667
    Likes Received:
    3,915
    It's hard to kill power tubes with a no load condition. I'd say that is a very unlikely outcome. Remember, the power tubes are transformer coupled to the speaker loads so there is a degree of isolation.

    But if you short the output, the increased current draw can make the power tubes draw enough current to blow a bias supply fuse (if there is one) or even over-current the cathode resistors, which I think is what happened to that DSL100 I had and fixed.
     
    Mitchell Pearrow and tce63 like this.

Share This Page