incorrect resistance in volume potentiometer?

Discussion in 'Gibson Les Pauls' started by lespaulguitarguy, Aug 14, 2011.

  1. lespaulguitarguy

    lespaulguitarguy Junior Member

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    I have a les paul studio that's getting terrible tone through the bridge pickup. it is paired with a Marshall AVT 275 valvestate amp. The best way i can describe it is sounding more like the neck pickups, but very muddy. it's not a "hot" tone like most les pauls get through a Marshall. I've always thought it was the amp, but the amp sounds good with my buddy's guitar. Also, there is a definite volume drop in the bridge pup vs. neck pup (weaker signal).

    Years ago i developed a crackling noise in the volume pot that controls the bridge pickup, and I had a local guitar shop replace it. I honestly don't remember it sounding bad at the time, but I was less of a tone head and had only been playing for about 2 years.

    I measured the resistance of the guitar's output jack through a 3 inch cable and im getting around 9.18K for the bridge pup and 7.83k for the neck pup. From the research I have done, I probably have a 498t/490R combination for the pups where the neck pickup resistance should be between 7k-9k and the bridge pickup should be between 12k-14k. If this is the case, it would take a very low impedance volume pot (around 25k-30k) to bring the bridge pup's output resistance down to 9.18k. Is it possible that the tech that worked on my guitar grabbed a 25k when he meant to grab a 250k or 500k? or am i crazy? or is my bridge pup dead? I think my math is good because I did the same calculations on the neck pups and it checks out(roughly 8k pup resistance with a 500k pot) and I'm an electrical engineer :p I just dont have much experience working on electric guitars, but i have faith that some of you do :) Please help guys!
     
  2. Pinkie

    Pinkie Senior Member

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    Get an rsguitarworks pot upgrade kit that will make your L.P. sound the way it should.
    Gibson uses either 500k or 300 k vol pots and 100 k tone.RS kits are 500k + vol and 400 k tone and you get better caps as well in the kit.
    One of the best things you can do for a L.P.
     
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  3. straybeat

    straybeat Senior Member

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  4. KenG

    KenG Senior Member

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    It really depends on the year. My Trad has 300K Vol and 500K Tone pots.
    I think your math is good as well. If you have a soldering iron and a DMM, unsolder the ground lug on the volume pot (the lug soldered back to the pot body) and measure across the two outside lugs. This will remove the rest of the circuit and you should be able to measure the value clearly.
     
  5. lespaulguitarguy

    lespaulguitarguy Junior Member

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    That's a good point KenG. That would probably be the least invasive way to find out for sure. Although, as some of the others pointed out, it would be pretty cheap to just buy a new 500k pot made by gibson since that seems to be the most likely culprit :)
     
  6. SJM

    SJM Premium V.I.P. Member Premium Member V.I.P. Member

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    +1 on this store. I ordered enough parts for a complete upgrade from them. Their prices are going to be hard to beat.
     
  7. straybeat

    straybeat Senior Member

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    You got them eh? I was wondering if you ever made it down here? I still haven't gotten over there myself? I need my Carvin redone, everything's 30 years old and getting dirty, but I can't decide on doing it myself or sending it back to the factory?
     
  8. Ermghoti

    Ermghoti Senior Member

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    I can't picture a guitar leaving a factory with the bridge and neck pickup mis-matched that much. Unless other factors are at play (ie divergent pickup build philosophies like wire guage and magnet type), I'd expect there to be a difference of about 1kohm, yours seem well matched.
     
  9. lespaulguitarguy

    lespaulguitarguy Junior Member

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    Ermghoti,

    according to what i've found online etc... there are two different pickups in the les paul studios. the 490R goes on the neck position and the 498t goes on the bridge. This accounts for the difference in resistance. I could be wrong though. I've only just started researching this, so if anybody has a better understanding of stock pickups installed in les paul studio guitars please step in and correct me.
     
  10. Ermghoti

    Ermghoti Senior Member

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    Right, there are often/usually different model pickups, but they are selected so that the bridge is 10-15% higher output than the neck. All things being equal, the resistances you measured would suggest that is the case in your guitar. That said, the 498t is listed at 12-13k, there is definitely a "hotter" magnet, maybe the wire is markedly different than the 490r.

    Are you certain the pickup is stock?

    Even with a hotter bridge pickup, I always end up with the neck a lot farther from the strings to get the balance I want.
     
  11. lespaulguitarguy

    lespaulguitarguy Junior Member

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    yes. i am certain they are stock pickups. I bought the guitar in 2000. The only thing that has ever been modded is that volume pot I mentioned. That's why i'm fairly sure it is the suspect part. The only other thing i could think of would be my bridge pickup. Though, i'm not sure how often those go bad if ever.
     
  12. pknoot

    pknoot Senior Member

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    It's not possible to measure the pot resistance through the output jack. Follow KenG's suggestion and measure the pots out of the circuit.

    Have you eliminated pickup height as a possible reason? Your bridge pickup may be too low?
     
  13. lespaulguitarguy

    lespaulguitarguy Junior Member

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    pknoot,

    I agree with your statement :) "It's not possible to measure the pot resistance through the output jack". I measured the total DC resistance of the whole circuit and calculated the POT resistance based on estimated resistance values of the 498t bridge pickup (roughly 12k-14k). However, I am going to do what KenG recommended. It's the only way to know for sure. Thanks guys!
     
  14. AngryHatter

    AngryHatter Senior Member

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    490s ARE muddy...imo.
     
  15. lespaulguitarguy

    lespaulguitarguy Junior Member

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    ok. so I desoldered the ground lug on the suspect potentiometer to break the circuit and measure the true resistance. It confirms my calculations, and I have a 25k pot for a volume knob. The pickup is 14.23k ohms which i would expect. no wonder my tone is dead. looks like I'm buying a new 500k volume pot lol!
     
  16. Lipinhu

    Lipinhu Senior Member

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    If you can, buy the 4 500k pots and 2 decent caps, orange drop .022 will be way better than the ceramic discs.

    Pretty affordable and a mod that goes, not only shows.
     
  17. Ermghoti

    Ermghoti Senior Member

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    I hope you give that guitar shop an earful, that's a bonehead move on their part.
     
  18. lespaulguitarguy

    lespaulguitarguy Junior Member

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    lipinhu,

    I have heard a lot about those orange drop caps lately. what is the overall effect of this mod? more clarity? how noticeable is it?

    ermghoti,

    I would, but they went out of business a few years ago lol. I agree though. That should never have happened.
     
  19. Ermghoti

    Ermghoti Senior Member

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    Coincidence? I think not.

    Hm, I'm out of 500k, I'll just throw in whatever I have laying around. And... done! Let's plug 'er in... sounds like crap. Good enough!
     
  20. Ermghoti

    Ermghoti Senior Member

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    Caps are pretty subtle, IMO, the differences are among the slope/Q/whatever of the rolloff. Cap value is a bigger difference, but I did notice differences among cap construction when I was swapping around. Paper in oil types seem a little smoother, ODs seem to have a little high-mid peak, ceramics run bland to harsh.

    I also tend to like lower than typical cap values, which have a higher frequency rolloff, clipping the bright part of the signal, but leaving more of the mids intact. I end up with 0.015 or 0.010 on the bridge, 0.015-0.022 on the neck on humbuckers.
     
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