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5E7 Webber Kit amp and odd problem

Discussion in 'Amp Tech Center' started by Vonscott, Jan 12, 2018.

  1. Vonscott

    Vonscott TDPRI Member

    Age:
    46
    44
    Jan 10, 2007
    michigan
    I tried my hand at building a Webber Kit for a 5E7 Combo. I got through the main board and got a little intimidated when I got to the power section so I brought it to a local amp tech who finished it for me.

    I had it for a little over a year and gigged with it a few times and sold it to a friend of mine. I ended up buying it back and it has developed an odd issue.

    After playing the amp for 20-30 minutes when I turn the volume pot on my guitars up or down, I get a scratchy sound like a dirty pot... this does not happen with the same guitar on any other amp. If I turn it very slowly it is less noticeable if I turn it fast it is much more pronounced. I have switched out pre-amp tubes which did not fix the problem. It seems like it has a bit less headroom than what I remember but it's possible that may be me and not necessarily the amp.

    So the $64,000 question, why would a sudden change in my volume pot cause a static or dirty pot kind of sound?

    Any help is much appreciated.
     

  2. Bellacaster

    Bellacaster Tele-Holic

    801
    Sep 5, 2010
    Ypsilanti, MI
    Perhaps try new power tubes? Had lower headroom in an amp over time and eventually ended up blowing a power tube. The increased headroom in the amp with the new tube was very noticeable.
     

  3. Phrygian77

    Phrygian77 Tele-Afflicted

    Apr 30, 2016
    Crawfordville, FL
    Seems to me like you may have DC from the grid of the first gain stage flowing into your guitar.
     
    clintj and corliss1 like this.

  4. corliss1

    corliss1 Friend of Leo's Gold Supporter

    Sep 13, 2008
    Lansing, MI
    Yup - what @Phrygian77 said. Make sure there isn't any DC anywhere it shouldn't be in the signal path. A bad/leaky capacitor will cause exactly what you're hearing.
     

  5. Dacious

    Dacious Poster Extraordinaire

    Mar 16, 2003
    Godzone
    Yep. In this. Case it's a cap, almost certainly.
     

  6. clintj

    clintj Friend of Leo's

    Apr 4, 2015
    Idaho
    There's no cap between the input stage and jacks on these. I'd be looking closely at the input jack wiring and soldering, especially the 68k/1M resistors, then checking the bias on V1 to see if it's getting too warm and causing some grid current to flow. Make sure the jacks are still securely tightened, too.

    If you're using JJ for both your current and replacement tubes, try another brand. I just saw where someone in another group had a similar issue, but a JJ tube caused the scratchiness and a Sovtek did not. Food for thought for you.

    If the input stage loses its grid leak path via the input resistor network, the guitar becomes the grid leak path.
     
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  7. BobbyZ

    BobbyZ Doctor of Teleocity

    Jan 12, 2011
    Snellman MN
    Once had a 250pf cap from Weber fail shortly after I put it in a Vibro Champ. It failed within the first few minutes of testing the amp and since it was about the only new part in the amp it was about the last thing I suspected. I assume I overheated it putting it in but it was wierd it worked for awhile.
    Doesn't sound like that's the problem here though. Unless a bad cap is putting dc on the ground but you'd think since the chassis's grounded that wouldn't be it.
     

  8. Vonscott

    Vonscott TDPRI Member

    Age:
    46
    44
    Jan 10, 2007
    michigan
    Thanks for all replies and information. I am sure this will help me get the problem sorted.

    The replacement 12ax7's were JJ's but the current tubes were a mix of Tung-Sol 12AT7 in V1 and EH 12ax7's I believe. I will double check the wiring and solder joints and I have a tech I can take it to to explore the caps and check for DC.

    Thanks again guys... I will keep you posted on the fix and what the problem turns out to be.
     
    BobbyZ likes this.

  9. robrob

    robrob Poster Extraordinaire Ad Free Member

    Dec 29, 2012
    United States
    Is the problem the same on both channels and hi and low inputs? If not that's a clue to where the problem lies.

    When tubes go bad they can generate excessive grid current which can place voltage on the grid and upstream to your guitar controls. Start by trying a new V1 preamp tube because the V1 grids are the only thing connected to your guitar.

    [​IMG]
     

  10. Vonscott

    Vonscott TDPRI Member

    Age:
    46
    44
    Jan 10, 2007
    michigan
    I will check tonight and see if it is the same on the bright channel... I did switch out V1 and it was the same with both tubes through the normal channel.

    Just to make sure I know what I'm talking about, V1 is the the tube all the way to the right if I am looking at the back of the amp right?
     

  11. robrob

    robrob Poster Extraordinaire Ad Free Member

    Dec 29, 2012
    United States
    Yes, that is V1.
     

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