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Unread 08-14-2011, 03:40 PM   #1 (permalink)
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incorrect resistance in volume potentiometer?

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!
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Unread 08-14-2011, 03:44 PM   #2 (permalink)
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Re: incorrect resistance in volume potentiometer?

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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Unread 08-14-2011, 04:35 PM   #3 (permalink)
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Re: incorrect resistance in volume potentiometer?

Check out Antique Electronic Supply

CTS pots are $3.25, real Gibsons are $8.50. Orange drop caps are 60 cents.
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Unread 08-14-2011, 05:32 PM   #4 (permalink)
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Re: incorrect resistance in volume potentiometer?

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.
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Unread 08-14-2011, 07:44 PM   #5 (permalink)
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Re: incorrect resistance in volume potentiometer?

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
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Unread 08-14-2011, 11:46 PM   #6 (permalink)
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Re: incorrect resistance in volume potentiometer?

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Check out Antique Electronic Supply

CTS pots are $3.25, real Gibsons are $8.50. Orange drop caps are 60 cents.
+1 on this store. I ordered enough parts for a complete upgrade from them. Their prices are going to be hard to beat.
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Unread 08-14-2011, 11:49 PM   #7 (permalink)
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Re: incorrect resistance in volume potentiometer?

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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.
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?
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Unread 08-15-2011, 07:17 AM   #8 (permalink)
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Re: incorrect resistance in volume potentiometer?

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Originally Posted by lespaulguitarguy View Post
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.
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.
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Unread 08-15-2011, 07:33 AM   #9 (permalink)
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Re: incorrect resistance in volume potentiometer?

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.
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Unread 08-15-2011, 10:06 AM   #10 (permalink)
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Re: incorrect resistance in volume potentiometer?

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.
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Unread 08-15-2011, 05:46 PM   #11 (permalink)
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Re: incorrect resistance in volume potentiometer?

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.
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Unread 08-15-2011, 06:03 PM   #12 (permalink)
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Re: incorrect resistance in volume potentiometer?

Quote:
Originally Posted by lespaulguitarguy View Post
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!
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?
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Unread 08-15-2011, 06:16 PM   #13 (permalink)
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Re: incorrect resistance in volume potentiometer?

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!
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Unread 08-15-2011, 06:21 PM   #14 (permalink)
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Re: incorrect resistance in volume potentiometer?

490s ARE muddy...imo.
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Unread 08-15-2011, 06:41 PM   #15 (permalink)
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Re: incorrect resistance in volume potentiometer?

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!
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Unread 08-15-2011, 07:54 PM   #16 (permalink)
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Re: incorrect resistance in volume potentiometer?

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.
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Unread 08-15-2011, 08:55 PM   #17 (permalink)
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Re: incorrect resistance in volume potentiometer?

Quote:
Originally Posted by lespaulguitarguy View Post
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!
I hope you give that guitar shop an earful, that's a bonehead move on their part.
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Unread 08-15-2011, 10:06 PM   #18 (permalink)
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Re: incorrect resistance in volume potentiometer?

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.
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Unread 08-16-2011, 12:11 PM   #19 (permalink)
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Re: incorrect resistance in volume potentiometer?

Quote:
Originally Posted by lespaulguitarguy View Post
they went out of business a few years ago
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!
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Unread 08-16-2011, 12:17 PM   #20 (permalink)
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Re: incorrect resistance in volume potentiometer?

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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?
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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Unread 08-16-2011, 01:32 PM   #21 (permalink)
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Re: incorrect resistance in volume potentiometer?

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Originally Posted by Ermghoti View Post
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.
LP wiring has a high-pass filter to ground connected in parallel with the volume control. A lower cap value will indeed have a higher frequency rolloff, which will clip less of the bright part of the signal......

Since OP is an Electrical Engineer, he can appreciate the effect of different capacitor construction and quality on the behavior of a simple low-pass filter circuit (i.e. our guitar tone control). Some capacitors have very high tolerance (for example +/- 20% for the ceramic disc capacitors), which means that the actual capacitance can be quite different from what is printed on them and the tone controls can be "harsh sounding". Others are tighter (for example +/- 5% for orange drops) which means they are more consistent and predictable in value. Most modern capacitors are just that: pure capacitors. That brings us to the mysterious paper in oil (PIO) version - originally designed for RF tube amps (the oil helps them magically "heal"), these not only have dubious tolerance and capacitance value, but there is some "resistance" there as well due to the construction (particularly with age) which modifies the filter circuit design, albeit accidentally - that's where the "warm smooth tone" comes from.
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Last edited by pknoot; 08-16-2011 at 02:15 PM.
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Unread 08-26-2012, 06:12 AM   #22 (permalink)
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Re: incorrect resistance in volume potentiometer?

Quote:
Originally Posted by pknoot View Post
LP wiring has a high-pass filter to ground connected in parallel with the volume control. A lower cap value will indeed have a higher frequency rolloff, which will clip less of the bright part of the signal......

Since OP is an Electrical Engineer, he can appreciate the effect of different capacitor construction and quality on the behavior of a simple low-pass filter circuit (i.e. our guitar tone control). Some capacitors have very high tolerance (for example +/- 20% for the ceramic disc capacitors), which means that the actual capacitance can be quite different from what is printed on them and the tone controls can be "harsh sounding". Others are tighter (for example +/- 5% for orange drops) which means they are more consistent and predictable in value. Most modern capacitors are just that: pure capacitors. That brings us to the mysterious paper in oil (PIO) version - originally designed for RF tube amps (the oil helps them magically "heal"), these not only have dubious tolerance and capacitance value, but there is some "resistance" there as well due to the construction (particularly with age) which modifies the filter circuit design, albeit accidentally - that's where the "warm smooth tone" comes from.
Boy, it's nice to find somebody who understands the electricals in guitars. Wish I did better than I do.

It sounds like you are talking about OLD (literally, very old) oil and paper caps, and I presume that's why their values wander. If you buy new oil and paper caps, will the dubious values still be likely, and will they have the resistance that gives the "warm smooth sound." And why does resistance give that?

I'm into high end stereo - hifi - and one can pay hundreds for a handmade cap, say a Hovland Musicap or a Teflon Vcap
(see The Great Capacitor Shoot-Out if you are interested).

I play jazz guitars, neck pickup only, and tone matters. Any idea if one of these really high end signal path caps would make my guitar sound better? I have often wondered about this because some luthiers' jazz guitars cost $15K but they are using $5 electronic parts in the circuits.

Thanks for any insights here.....

Jim
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