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humbucking dead tone followup (long)

12/22/1997 11:25 AM
Ken Drottar	humbucking dead tone followup (long) The original problem was that the tone rolled off my Les Paul as I reduced the volume. I then went off to try and figure out why it happened and what could be done about it. I had a lot of help. Thanks. Technical background: The tone rolloff of humbuckings is due (primarily, there are many secondary effects) to an interaction between the pickup and the volume control. This system behaves as a LR low pass filter. The pickup acts as the inductor, the resistor value is the volume control, the tone control, and the destination's input grid resistor in parallel. With this type of filter, the cutoff frequency varies linearly with the resistance. So as the volume control is turned down the filter cutoff frequency lowers. Many thanks to customer technical support at Dimarzio, Gibson, and Stewart MacDonald websites. Problems and fixes: 1. Large inductance of humbucking pickups require a large resistance in the volume pot to offset it. Unfortunately for the volume function, there is a lot of volume change in a small amount of knob travel. Fix A> Reduce the size of the volume control from 500k to 300k. This was first done in 1973 by Gibson. Fix B> Add a 150k resistor between legs of the volume pot to simulate 300k taper. Fix C> Learn to live with the 500k volume taper. 2. Smaller volume control means more high frequency rolloff. Fix A,B> Add 1 to 5 nF cap between legs of volume pot. Fix C> Add 500pF to 1 nF cap between legs of the volume pot. Less high frequency compensation is required if the volume control remains large. What I did: I chose what I believed to be the least intrusive path, Fix C. I connected seven 5% tolerance silver mica caps ranging from 510 pF to .001 uF to a rotary switch and spent some time with each value to select one which sounded best. Nothing sounded like the pickup at full volume. This gave me pause to reconsider the validity of this solution and insight as to why the guitars don't come from the factory with a fix. This realization did not remove the problem I needed to fix. I compromised between between too much high midrange and too much treble. "Trooper testing" this solution live demonstrated it works better than stock and better than expectations that were set in the hyper-critical bench test environment.

12/22/1997 11:25 AM

Ken Drottar

humbucking dead tone followup (long)

The original problem was that the tone rolled off my Les Paul as I reduced the volume. I then went off to try and figure out why it happened and what could be done about it. I had a lot of help. Thanks.

Technical background:

The tone rolloff of humbuckings is due (primarily, there are many secondary effects) to an interaction between the pickup and the volume control. This system behaves as a LR low pass filter. The pickup acts as the inductor, the resistor value is the volume control, the tone control, and the destination's input grid resistor in parallel. With this type of filter, the cutoff frequency varies linearly with the resistance. So as the volume control is turned down the filter cutoff frequency lowers. Many thanks to customer technical support at Dimarzio, Gibson, and Stewart MacDonald websites.

Problems and fixes:

1. Large inductance of humbucking pickups require a large resistance in the volume pot to offset it. Unfortunately for the volume function, there is a lot of volume change in a small amount of knob travel.

Fix A> Reduce the size of the volume control from 500k to 300k. This was first done in 1973 by Gibson.

Fix B> Add a 150k resistor between legs of the volume pot to simulate 300k taper.

Fix C> Learn to live with the 500k volume taper.

2. Smaller volume control means more high frequency rolloff.

Fix A,B> Add 1 to 5 nF cap between legs of volume pot.

Fix C> Add 500pF to 1 nF cap between legs of the volume pot. Less high frequency compensation is required if the volume control remains large.

What I did:

I chose what I believed to be the least intrusive path, Fix C. I connected seven 5% tolerance silver mica caps ranging from 510 pF to .001 uF to a rotary switch and spent some time with each value to select one which sounded best. Nothing sounded like the pickup at full volume.

This gave me pause to reconsider the validity of this solution and insight as to why the guitars don't come from the factory with a fix. This realization did not remove the problem I needed to fix. I compromised between between too much high midrange and too much treble. "Trooper testing" this solution live demonstrated it works better than stock and better than expectations that were set in the hyper-critical bench test environment.

12/22/1997 2:49 PM
Doc	Ken: So what final value do you prefer? Or do you have a rotary selector mounted in you guitar now? The ending of your post didn't really say. Thanks, Doc

12/23/1997 12:35 AM
Ken Drottar	I didn't want to say because my choice was highly subjective. The rotary selector is a cool idea! Just like "Lucille". However, lacking the vision, daring, or imagination, I ended up installing just one; 750 pF.

12/26/1997 9:08 PM
Steve A.	Ken: So what year and model LP do you have, and what pickups does it have? From your post I wasn't sure if you have the high-output distortion type or the the lower-output vintage style PAF's. A DC resistance reading will give you some idea if you're not sure what you have. The value I usually use is around 220pf or 250pf, and that is in parallel with a resistor to smooth out the response (usually 220k). PRS usually uses a 180pf cap in their guitars. What I've noticed (with PAF-style pickups, nothing hotter than a SD JB bridge) is that any value over 250pf causes the lower volume signal to be much brighter than the full volume signal. With you selecting a 750pf cap, I think maybe you have high output pickups. Have you tried changing the value of the tone cap? Dan Torres and Matt Griblin strongly recommend .015uf/630v caps for the Eric Clapton "Woman" tone control. I usually test out different values with each guitar and generally settle on .015uf to .022uf. With these values, when you turn the tone pot down, you don't lose all of your high frequencies and definition. With a .015uf cap turned down to 0 and your OD kicked in, a LP sounds just like Cream's "Sunshine of Your Love" (and I always thought that sound was from a wah-wah pedal kicked back). Your discussion of LC circuits brings to mind a L-5 copy I rewired a few years ago. I wanted the cleanest sounding pickups I could get to capture a very true guitar sound and went from PAF's to Alnico's and finally to SD Jazz pickups. I wanted to be able to blend the pickups freely so I switched the two hot leads on the volume pots (which grounds out the signal from the pickup, not the input from the amp). I wanted to be able to "dirty" the sound up a bit, too, so I installed a rotary selector switch with series linkages on the two outside positions. For the "super neck" position, the neck pot served as master volume and the bridge pot would bypass the bridge pickup in the CCW position and engage it fully in the CW position. For the "super bridge" position, the bridge pot was the master volume and the neck pot was a bypass pot as described above. When I got the guitar put back together (not as easy task with an L-5 copy!) everything seemed to work as expected until I tried adjusting the tone control for the bridge pickup with the selector switch set to the "super neck" position. Surprise! When I turned the bridge tone pot down, the tone got BRIGHTER. I figured I must have wired in the tone pots backwards, but they worked as expected for the 3 normal selector switch positions. Explanation: The tone caps pass high frequencies more efficiently than low frequencies, so as I'd turn down the opposite tone pot, more of the high frequencies would bypass the series linkage through the other pickup (series linkages tend to swallow up high frequencies) and thus produce a brighter sound. An alternate explanation would be that the inductance of the series pickup in combination with the tone cap produced an LC network, but I never got around to measuring the inductance of the pickups to complete the mathematical formulae. While that wiring setup produced some interesting sounds, there was one drawback: when I turn either volume pot down in the normal selector switch positions, you can hear a scratchiness (even after cleaning the pots a few times). What I haven't gotten around to doing is adding push-pull pots for the two volume controls, so when the knob is down, the pots are wired like your LP and when the knob is pulled up, the two hot leads on the pot are reversed for the weird blend/bypass effects described above. I haven't drawn up that scheamtic for my web site yet, but you ought to check back in a month or two... http://www.geocities.com/BourbonStreet/Delta/3281/homepage.htm" target="_blank">http://www.geocities.com/BourbonStreet/Delta/3281/homepage.htm">http://www.geocities.com/BourbonStreet/Delta/3281/homepage.htm Steve Ahola

12/26/1997 9:08 PM

Steve A.

Ken:

    So what year and model LP do you have, and what pickups does it have? From your post I wasn't sure if you have the high-output distortion type or the the lower-output vintage style PAF's. A DC resistance reading will give you some idea if you're not sure what you have.

    The value I usually use is around 220pf or 250pf, and that is in parallel with a resistor to smooth out the response (usually 220k). PRS usually uses a 180pf cap in their guitars. What I've noticed (with PAF-style pickups, nothing hotter than a SD JB bridge) is that any value over 250pf causes the lower volume signal to be much brighter than the full volume signal. With you selecting a 750pf cap, I think maybe you have high output pickups. Have you tried changing the value of the tone cap? Dan Torres and Matt Griblin strongly recommend .015uf/630v caps for the Eric Clapton "Woman" tone control. I usually test out different values with each guitar and generally settle on .015uf to .022uf. With these values, when you turn the tone pot down, you don't lose all of your high frequencies and definition. With a .015uf cap turned down to 0 and your OD kicked in, a LP sounds just like Cream's "Sunshine of Your Love" (and I always thought that sound was from a wah-wah pedal kicked back).

    Your discussion of LC circuits brings to mind a L-5 copy I rewired a few years ago. I wanted the cleanest sounding pickups I could get to capture a very true guitar sound and went from PAF's to Alnico's and finally to SD Jazz pickups. I wanted to be able to blend the pickups freely so I switched the two hot leads on the volume pots (which grounds out the signal from the pickup, not the input from the amp). I wanted to be able to "dirty" the sound up a bit, too, so I installed a rotary selector switch with series linkages on the two outside positions. For the "super neck" position, the neck pot served as master volume and the bridge pot would bypass the bridge pickup in the CCW position and engage it fully in the CW position. For the "super bridge" position, the bridge pot was the master volume and the neck pot was a bypass pot as described above.

    When I got the guitar put back together (not as easy task with an L-5 copy!) everything seemed to work as expected until I tried adjusting the tone control for the bridge pickup with the selector switch set to the "super neck" position. Surprise! When I turned the bridge tone pot down, the tone got BRIGHTER. I figured I must have wired in the tone pots backwards, but they worked as expected for the 3 normal selector switch positions.

    Explanation: The tone caps pass high frequencies more efficiently than low frequencies, so as I'd turn down the opposite tone pot, more of the high frequencies would bypass the series linkage through the other pickup (series linkages tend to swallow up high frequencies) and thus produce a brighter sound. An alternate explanation would be that the inductance of the series pickup in combination with the tone cap produced an LC network, but I never got around to measuring the inductance of the pickups to complete the mathematical formulae.

    While that wiring setup produced some interesting sounds, there was one drawback: when I turn either volume pot down in the normal selector switch positions, you can hear a scratchiness (even after cleaning the pots a few times). What I haven't gotten around to doing is adding push-pull pots for the two volume controls, so when the knob is down, the pots are wired like your LP and when the knob is pulled up, the two hot leads on the pot are reversed for the weird blend/bypass effects described above.

    I haven't drawn up that scheamtic for my web site yet, but you ought to check back in a month or two...

http://www.geocities.com/BourbonStreet/Delta/3281/homepage.htm" target="_blank">http://www.geocities.com/BourbonStreet/Delta/3281/homepage.htm">http://www.geocities.com/BourbonStreet/Delta/3281/homepage.htm

Steve Ahola

12/27/1997 7:48 PM
Ken Drottar	Steve, The guitar is a 1971 Standard, 6th gold-top version. A previous owner was nice enough to route out the pickup cavities for humbuckings (nice job, too, although I haven't met a vintage guy who liked it much). The pickup in question is an SD Antiquity bridge humbucker. Wound slightly hot (8.56k), but nothing like the usual high output distortion type. You are correct that as volume rolls off 750pF unbalances the highs to lows. I preferentially chose consistency thru the midrange instead. I expected everybody's mileage to vary on selected cap value. It was a tough choice. Got the Blue Guitar Page in my bookmarks! -ken

12/27/1997 7:48 PM

Ken Drottar

Steve,
The guitar is a 1971 Standard, 6th gold-top version. A previous owner was nice enough to route out the pickup cavities for humbuckings (nice job, too, although I haven't met a vintage guy who liked it much). The pickup in question is an SD Antiquity bridge humbucker. Wound slightly hot (8.56k), but nothing like the usual high output distortion type. You are correct that as volume rolls off 750pF unbalances the highs to lows. I preferentially chose consistency thru the midrange instead. I expected everybody's mileage to vary on selected cap value. It was a tough choice. Got the Blue Guitar Page in my bookmarks!

-ken

12/23/1997 11:04 AM
Dave Harris	<> I had a look at the LP schematic on the Gibson page and the vol pot was wired normally (CW end to pick up Wiper to amplifier). I had expected it to be backwards (Wiper to pu CW to amp) on a 2 vol 2 tone guitar. Is your LP as the schematic ? If it is then the loading on the pu will be less (higher resistance ) as the pot is turned down and the cut off frequency will go up not down. If this is the case then it must be the capacitive loading of the guitar to amp cable on the pot that is reducing the HF. Try using the shortest lowest capacitance cable you can get hold of between the guitar and amp to see if that helps. Don�t try it with both pickups selected because turning either pu vol pot to zero will load down the other pu and eventually kill both pickups when fully ACW. What a mess, I�m glad I only have Strats. Does a LP really work like that ? I will try and be more helpful when I get back next year. Dave H

12/23/1997 11:04 AM

Dave Harris

<>

I had a look at the LP schematic on the Gibson page and the vol pot was wired normally (CW end to pick up Wiper to amplifier). I had expected it to be backwards (Wiper to pu CW to amp) on a 2 vol 2 tone guitar. Is your LP as the schematic ? If it is then the loading on the pu will be less (higher resistance ) as the pot is turned down and the cut off frequency will go up not down. If this is the case then it must be the capacitive loading of the guitar to amp cable on the pot that is reducing the HF. Try using the shortest lowest capacitance cable you can get hold of between the guitar and amp to see if that helps. Don�t try it with both pickups selected because turning either pu vol pot to zero will load down the other pu and eventually kill both pickups when fully ACW. What a mess, I�m glad I only have Strats. Does a LP really work like that ?

I will try and be more helpful when I get back next year.

Dave H

12/24/1997 1:50 AM
Ken Drottar	Feedback at last! Thanks, Dave! Yes, my guitar is wired as the Gibson page indicates. Are you suggesting a half section LC low pass filter as model for guitar response? This is where the inductor (pickup) is in the signal path and the capacitor (cable) is between the signal path and ground? Sigh. This would be a lot easier with pictures... The cutoff frequency of a LC low pass filter varies inversely with the capacitance. I am convinced lower capacitance cable will increase high frequency response, as you said. Lowering the volume should bypass the cable capacitance. If this LC filter were the primary effect, wouldn't this mean a pickup would become brighter as volume was decreased? Also, can you put more words around how the inductor loading is less as the pot is turned down? Look forward to continuing this discussion next year! -ken

12/24/1997 1:50 AM

Ken Drottar

Feedback at last! Thanks, Dave! Yes, my guitar is wired as the Gibson page indicates.

Are you suggesting a half section LC low pass filter as model for guitar response? This is where the inductor (pickup) is in the signal path and the capacitor (cable) is between the signal path and ground? Sigh. This would be a lot easier with pictures...

The cutoff frequency of a LC low pass filter varies inversely with the capacitance. I am convinced lower capacitance cable will increase high frequency response, as you said. Lowering the volume should bypass the cable capacitance. If this LC filter were the primary effect, wouldn't this mean a pickup would become brighter as volume was decreased?

Also, can you put more words around how the inductor loading is less as the pot is turned down? Look forward to continuing this discussion next year!

-ken

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