Grounding Shield Of Coaxial Wire Shorts Input

Discussion in 'Building the Classics' started by mAx___, Mar 4, 2019.

  1. mAx___

    mAx___ Active Member

    Joined:
    Apr 3, 2012
    Messages:
    303
    Likes Received:
    105
    Hi,

    I was installing a shielded wire from Hi Input I to the 68K resistor in the board on a 100w Major build. I did the pigtail ground thing -on one side only- and wired it to ground on the input jack. The central wire goes from the input jack to the 68K resistor. No sound. If I disconnect the shield from ground, sound is back. I don't get it, I did the exact same thing on another build and it works fine. What am I missing?

    Thanks.
     
  2. mickeydg5

    mickeydg5 Well-Known Member VIP Member

    Joined:
    Sep 30, 2011
    Messages:
    19,217
    Likes Received:
    7,229
    Location:
    US of A
    The core wire is probably shorting to the shield and ground somewhere. Amplifier OFF, check things with resistance measurement of a multimeter.
    Or bad wire or bad connections. Again check with multimeter.
    :photos: please
     
  3. ampmadscientist

    ampmadscientist Well-Known Member

    Joined:
    Jun 26, 2014
    Messages:
    10,738
    Likes Received:
    4,926
    Location:
    Whereabouts Unknown
    Somewhere the shield is shorted to the center wire.
     
    mAx___ likes this.
  4. mickeydg5

    mickeydg5 Well-Known Member VIP Member

    Joined:
    Sep 30, 2011
    Messages:
    19,217
    Likes Received:
    7,229
    Location:
    US of A
    What did you just say?
    I do not understand.
     
  5. Micky

    Micky Well-Known Member VIP Member

    Joined:
    Apr 29, 2010
    Messages:
    19,691
    Likes Received:
    15,563
    Location:
    Vermont
    He only reads the titles of threads, not the posts after that...
     
    neikeel likes this.
  6. mAx___

    mAx___ Active Member

    Joined:
    Apr 3, 2012
    Messages:
    303
    Likes Received:
    105
    Thanks so much guys. I tried using 3 different wires and the problem was still there. It baffles me. But the amp is so silent as is that I ended up not grounding that short wire.
     
  7. mickeydg5

    mickeydg5 Well-Known Member VIP Member

    Joined:
    Sep 30, 2011
    Messages:
    19,217
    Likes Received:
    7,229
    Location:
    US of A
    :squint: :scream: :lol:
     
  8. mickeydg5

    mickeydg5 Well-Known Member VIP Member

    Joined:
    Sep 30, 2011
    Messages:
    19,217
    Likes Received:
    7,229
    Location:
    US of A
    You have to be careful with the different types of shielded cable. Depending on the handling and dressing some can short the shield to core if not handled or truncated properly.

    I do not know what type of cable you have used or how you dressed it.
     
  9. mAx___

    mAx___ Active Member

    Joined:
    Apr 3, 2012
    Messages:
    303
    Likes Received:
    105
    I think you found the reason. I just checked and two of the cables had a shorted shield to core! I guess I was bending them too much because of the short distance between the jack and the turret board?
    I had no idea this could happen, I guess I was lucky with my other build...

    Removing the unshielded wire from the amp and trying to fit a new one in that tiny space would be a real pain...is it too bad if I leave it as is?




    IMG_2449.JPG
     
  10. Timbo

    Timbo Member

    Joined:
    Oct 29, 2009
    Messages:
    81
    Likes Received:
    68
    How long is the length of cable?
     
  11. mAx___

    mAx___ Active Member

    Joined:
    Apr 3, 2012
    Messages:
    303
    Likes Received:
    105
    Around 6cm or 2”, more or less.
     
  12. ampmadscientist

    ampmadscientist Well-Known Member

    Joined:
    Jun 26, 2014
    Messages:
    10,738
    Likes Received:
    4,926
    Location:
    Whereabouts Unknown
    Soldering can melt the plastic inside the shielded wire.
    This can short the center to the outside shield.
     
    mAx___ likes this.
  13. mickeydg5

    mickeydg5 Well-Known Member VIP Member

    Joined:
    Sep 30, 2011
    Messages:
    19,217
    Likes Received:
    7,229
    Location:
    US of A
    You know when I read the first post I almost asked "Why a shielded cable there?"
     
  14. mAx___

    mAx___ Active Member

    Joined:
    Apr 3, 2012
    Messages:
    303
    Likes Received:
    105
    The amp has an extra gain stage, I saw some layouts for this mod used a shielded cable so I went for it.
     
  15. neikeel

    neikeel Well-Known Member

    Joined:
    Feb 12, 2014
    Messages:
    2,876
    Likes Received:
    1,339
    - for long runs on the grids it is useful, particularly if running from sockets to front switches or pots, it can dull the sound (cheap high capacitance wires), equally can tame some icepick frequencies too.

    - use it sparingly tho'
     
    mAx___ likes this.
  16. Timbo

    Timbo Member

    Joined:
    Oct 29, 2009
    Messages:
    81
    Likes Received:
    68
    I would use a standard wire and see if that works.
     
  17. mickeydg5

    mickeydg5 Well-Known Member VIP Member

    Joined:
    Sep 30, 2011
    Messages:
    19,217
    Likes Received:
    7,229
    Location:
    US of A
    How about a picture and make of the wire used?
     
  18. mAx___

    mAx___ Active Member

    Joined:
    Apr 3, 2012
    Messages:
    303
    Likes Received:
    105
    I posted a pict of the wires yesterday, look up my post up in the thread. I couldn't see the make but it's the one that Valvestorm sells.
     
  19. mickeydg5

    mickeydg5 Well-Known Member VIP Member

    Joined:
    Sep 30, 2011
    Messages:
    19,217
    Likes Received:
    7,229
    Location:
    US of A
    Ah, haa! I past over that little picture because I though it was just something in your signature.

    I see you pulled back the outer cover and shield on that side but cannot get a good view of the other ends or tell what is under the shrink. That is the ticket with those to separate the core/core jacket from the shield/outer jackets. If you cut or even puncture the core jacket --- start all over.
    I usually prep both ends and then put a multimeter on it to test before installation. Then I test after installation.
     
    mAx___ likes this.
  20. ampmadscientist

    ampmadscientist Well-Known Member

    Joined:
    Jun 26, 2014
    Messages:
    10,738
    Likes Received:
    4,926
    Location:
    Whereabouts Unknown
    High gain extra input stage. The shielded wire keeps the spurious noise lower.
     

Share This Page

  1. This site uses cookies to help personalise content, tailor your experience and to keep you logged in if you register.
    By continuing to use this site, you are consenting to our use of cookies.
    Dismiss Notice