How covers inpact the inductance, and then some...

[RedHouse]

....But as I've said before, look how people started taking the covers off back in the 70's. It wasn't just for looks....

...People putting the covers back on now is part of the retro "vintage" fad....

Yeah I was around back then too (70's) and I do recall that, and did it myself, and I'd tend to disagree with that a bit.

I'll be the first to admit that some of the things (many/most?) we did back then was a lot of sillyness and ruined some good guitars, like for instance putting Brass (everything) on our guitars, Brass nut's (guilty) and Brass bridges (guilty), and Brass tailpieces and pickup rings (not guilty), and drilling/adding those mini-toggle-switches (God bless push/Pull pots!) adding metal plates to the back of the headstocks for "more sustain", stuffing our Marshall cabs with acrylic-down or foam material then installing JBL's with aluminum dust covers, and the coup de gras; installing those stupid sealed Bourns conductive-plastic "computer" pots in everything, sheesh, what were we thinking?. The Floyd Rose bridge has survived but only because the tuning stability help tends to out-weigh the tone robbery ...but not by much.



Here's me in '78 with a brand new SG:


same guitar two weeks later:


Geez, I'd just kick my own ass now-a-days if I did that, sheesh!.

My favorite old days mod was those 1/2 covers, or "open covers" which I later installed on my (then) new DiMarzio Super Distortion (dual sound) humbuckers, oh yeah baby!.



(I still have some of those and I'm often tempted to put them back on my '04 R9)

I think today we can pretty much say that most of those "mod's" were really not tone "improvements" but only tone alterations that didn't stand the test of time, perhaps the experimentation de jour, many of those thing we did back in those days, you think?.

{EDIT}

I wonder what effect those half covers have as far as the eddy current thing?

[Mike Sulzer]

I wonder what effect those half covers have as far as the eddy current thing?

I have not tried listening to those, but based on the measurement above, they probably have a lot less effect than full covers. The metal that is present follows a path that encloses approximately equal amounts of flux from both out of phase magnetic circuits. Thus they tend to cancel.

That video does show a huge effect. Why would anyone use such high conductivity covers? Could it be a corporate effect? "When some dude with money in his pocket walks into a store, I want the Gibson LP to blow away the epi."

[RedHouse]

...That video does show a huge effect. Why would anyone use such high conductivity covers? Could it be a corporate effect? "When some dude with money in his pocket walks into a store, I want the Gibson LP to blow away the epi."

I keep going back to that video, it seems to me, somethings not right.

The more I look and listen to that clip, I'd swear the guy did something additional than just a cover removal. For instance when the covers are off there is also less bass in the tone of the pickups which I'd not expect with only a cover removal, bass tone and midrange tone should (usually) stay pretty much the same.

Perhaps moved his tone controls or change his tone caps, might not be the same pickups who knows, but as a second-third opinion I'm thinking it's even more change than the Epi I described in an earlier post.

[Peter Naglitsch]

Been off-line for a few days and a lot of replies. Thanks guys, it helped me understand a few things.

The cover acts as a shorted turn, so it lowers the inductance and lowers the Q of the resonant peak somewhat. Because it's non-magnetic metal, it doesn't have any effect on the magnetic circuit that would increase the inductance. The rule is that non-ferrous metals lower the inductance and ferrous ones increase it.

That really explained a few things for me. Thanks Steve

Does your LCR meter measure low Q inductors with good accuracy? The inductance reactance of 9.23 H at 250 Hz is about 15,000 ohms. The resistance of your pickup is, maybe, 8000 ohms

I have no real idea of the accuracy. But it measures from 2 mH to 20 mH so it is indeed intended for pretty low inductance measurements. The pickups DC resistance is 16.09 Kohms (8000 + 7800 turns of AWG43 on each coil)

This implies that it is primarily the top plate of the cover that is causing the losses.

More on that later.

I wonder what effect those half covers have as far as the eddy current thing?

I have a set of those open covers from Guitar Jones. Don't know about the Eddy currents but I measured the inductance with and w/o the open covers. Those "covers" doesn't change the inductance much. From 9.23 to 9.28 mH.

So could it actually still be the change in inductance that take away the treble or is it the eddy currents in that closed loop surrounding the coils or what? I still haven't really got that part yet. Sorry for continuing to ask but I still doesn't really get it?

Thanks for your help guys

[David Schwab]

Yeah I was around back then too (70's) and I do recall that, and did it myself, and I'd tend to disagree with that a bit.

I had a '81 Les Paul Standard about 10 years ago. It had two Shaw pickups with covers. I removed the covers, but ended up putting the cover back on the neck pickup because I liked the tone better with the cover on. But the difference in tone was very evident. With the cover on the neck pickup had less of a mid peak, probably because the cover lowered the Q. I swapped out the bridge with a 70's SuperDistortion.

I'll be the first to admit that some of the things (many/most?) we did back then was a lot of sillyness and ruined some good guitars, like for instance putting Brass (everything) on our guitars, Brass nut's (guilty) and Brass bridges (guilty), and Brass tailpieces and pickup rings (not guilty), and drilling/adding those mini-toggle-switches (God bless push/Pull pots!)

Been there, done that. Blame Alembic and Rick Turner for all that! Then Starz Guitars seling those brass bridges. I put one on my old Sekova LP Custom copy back then and hated it. The guitar had a nice snap, and the brass bridge sucked all the top end out of it. I still have that bridge too.

I've used a lot of brass nuts though, but on my Rics I made aluminum nuts. I had read that aluminum had a similar sound transfer as wood. I'm still not a big fan of brass bridges, and now you see a lot of aluminum bridges on the market.

adding metal plates to the back of the headstocks for "more sustain",

I've never done that, but it's a valid idea. It's supposed to work well on bass to eliminate dead spots. I worked at American Showster guitars in the 80's. If you're not familiar with those, they looked like the tail fin on a '57 Chevy Belair. They were made from basswood, and had cast aluminum bumpers and trim. The metal had a big impact on the tone of the guitar. If you played one with and without the metal, it was like night and day. Near the end of my time there I came up with the idea to insert aluminum bars, or inlay plates in regular Strat style bodies, since the tail fins were so hard to make at the time. I didn't stick around long enough to see those, but Kramer made those guitars for Showster. So sticking metal on the body does improve the tone in some cases.

As a side note, the owner didn't like my design at the time, and shot down the whole idea, so when I left I assumed nothing ever came of it. Then to my chagrin I was reading a Musician's Friend catalog about a year later... and there's my guitar deign! The prick actually got a patent on it too! (#4919029) So always watch what you say to people.

http://www.vintagekramer.com/company45.htm

http://www.vintagekramer.com/Ads/showster.jpg

and the coup de gras; installing those stupid sealed Bourns conductive-plastic "computer" pots in everything, sheesh, what were we thinking?.

Blame Alembic again, but those are great pots! Computers don't use pots...

Nothing wrong with using high quality pots. You see people swapping out their pots for CTS pots all the time. I'd rather used the Bourns sealed pots. Or Vishay.

The Floyd Rose bridge has survived but only because the tuning stability help tends to out-weigh the tone robbery ...but not by much.

Yeah.. they aren't as popular as they once were. They work great though, and I actually like that flutter you get when you flick the bar. I stopped using them though. I don't use trems much, but when I put my Charvel back together I stuck an old ESP Flicker floating bridge I had since Showster days. It's not locking, but it does that flutter thing. Seems to stay in tune OK.

All these changes come about because there's a need for these things. People wanted hotter pickups, and tremolos that stayed in tune. The vintage style stuff is great, but they are really old designs. Guitars that wont stay in tune, pickups that hum, wobbly humbucker mounts that can't be angled, and so on. If it wasn't for the fact that popular music was invented with those tones, who would use these parts?

Here's me in '78 with a brand new SG:

Geez, I'd just kick my own ass now-a-days if I did that, sheesh!.

My favorite old days mod was those 1/2 covers, or "open covers" which I later installed on my (then) new DiMarzio Super Distortion (dual sound) humbuckers, oh yeah baby!.

They were nice pickups at the time. I guess it was the tone people wanted then, or they wouldn't have been buying them!

Someone just sent me this... this is November 1976. I had just turned 19 ten days earlier.

This is my first '74 Ric bass. Notice the plethora of knobs and switches! That's six toggles and a 6-position rotary, and five knobs. The knobs were just volumes and tones and a stereo master volume. I had installed a carvin humbucker at the neck and a Hi-A at the bridge. Later I added a Fender P bass pickup between the other two, and then changed that to a DiMarzio P. Then I removed the bridge and inlayed a piece of maple to fill the hole left by the bridge, and installed a Badass II. I did more things to that poor bass too. I still have it, but its been disassembled for many years. So more than just replacing the pickups, the whole thing is so far from stock it's not funny!

In original condition it would be worth something now.

{EDIT}

I wonder what effect those half covers have as far as the eddy current thing?

Probably very little. Cut a slit in the side and even less.

[jrfrond]

FYI- the AmProbe LCR55A is also an excellent inductance meter, among other things. It has become a little "miracle worker" here in our shop.

[Mike Sulzer]

I have a set of those open covers from Guitar Jones. Don?t know about the Eddy currents but I measured the inductance with and w/o the open covers. Those ?covers? doesn?t change the inductance much. From 9.23 to 9.28 mH.

So could it actually still be the change in inductance that take away the treble or is it the eddy currents in that closed loop surrounding the coils or what? I still haven?t really got that part yet. Sorry for continuing to ask but I still doesn?t really get it?

From your first post: "The inductance drops 10% with covers on." If the inductance goes down you would expect the high frequencies to go up, that is, the resonant frequency rises. (The exception to this is that with some single coil pickups, increasing the resonant frequency might move the peak above the response of the guitar, leading to less highs, but that is not an issue for humbuckers.) So I do not think that the inductance chages you are measuring have anything to do with the high frequency losses.

I think it is the eddy currents in the top plate rather than the sides that matter with a humbucker. Evidence for this is how the law of magnetic induction works, the measurements I showed above , and the patent that David discussed.

[Peter Naglitsch]

So Eddy currents in the top plate is the thing to pay attention to (I'm not saying staying away from...). And that is the reason the nickle/silver covers doesn't affect the tone as much as brass, as the nickle/silver versions doesn't produce as much Eddy currents. Right?

[Joe Gwinn]

FYI- the AmProbe LCR55A is also an excellent inductance meter, among other things. It has become a little "miracle worker" here in our shop.

I would try the low-Q inductor test before believing any LCR meter with pickups:

Take an inductor of at least few Henrys and wire a 50 K pot in series with it, and measure the inductance while varying the pot. Does the indicated inductance vary as the series resistance increases? If yes, measuring pickup inductance accurately will be a problem.

For example, a pickup with inductance 2.5 H and DC resistance 10,000 ohms measured at 100 Hz (the meter's choice for that inductance): Xl= 2 Pi 100 2.5= 1,571 ohms, which is swamped by the DC resistance of 10,000 ohms.

Depending on the design of the meter, the meter may claim that the inductance is 11571/(2 Pi 100)= 18.4 Henry, which is not just wrong, but impossible for a pickup.

The inductor can be one winding of an audio transformer, or the like, so long as it is built on a transformer frame, with a laminated steel core, and a DC resistance not exceeding 100 ohms or so.

[RedHouse]

Joe, in your Maxwell-Wein impedance bridge, what is Vdet?
(I'm guessing a DMM)

Lx wound be the coil(s) right?

Could you fill in the values for what we here should consider, or could use as a standard test (or tests) across the board? and would you mind walking me through (or link me) using the Maxwell-Wein test jig with pickups to pot some standard resultas that could be used to compare one pickup to another?

[David Schwab]

So Eddy currents in the top plate is the thing to pay attention to (I'm not saying staying away from...). And that is the reason the nickle/silver covers doesn't affect the tone as much as brass, as the nickle/silver versions doesn't produce as much Eddy currents. Right?

That's correct. As similar as yellow brass and nickel silver are, which is essentially white brass, brass seems to be more conductive.

Seth Lover wanted to use stainless steel for covers and baseplates.

The cover on the filtertron has less surface area, so it disrupts the eddy currents.

Thinner is also better.

[Joe Gwinn]

Joe, in your Maxwell-Wein impedance bridge, what is Vdet?
(I'm guessing a DMM)

Yes.

Lx wound be the coil(s) right?

Lx and Rx are respectively the inductance and AC resistance of the pickup under test.

Could you fill in the values for what we here should consider, or could use as a standard test (or tests) across the board? and would you mind walking me through (or link me) using the Maxwell-Wein test jig with pickups to pot some standard results that could be used to compare one pickup to another?

The starting values are given as the example values on the schematic.

As for leading you through, I can do that. But first build the setup and play with it a bit. It isn't hard, although it becomes slow to find the null when the Q is very low (like 0.5). One uses the DMM to measure the actual values of the resistors and capacitors, as the marked values are not very accurate.

What kind of oscillator do you have? This matters a lot.

[RedHouse]

...But first build the setup and play with it a bit.......What kind of oscillator do you have?...

Ok I will have to do that, I haven't yet.

My Function Generator is a really old HP 3311A, it's ancient, but works really good so I can't bring myself to toss it for a newer one. My thoughts were to actually make a littel self contained unit, perhaps with an XR2206 and DPM, etched board and all, what is critical mass about the osc?.

[Joe Gwinn]

My Function Generator is a really old HP 3311A, it's ancient, but works really good so I can't bring myself to toss it for a newer one.

The 3311A can be used for initial playing, but will prove frustration when trying to make measurement because the harmonic distortion is too high.

My thoughts were to actually make a little self contained unit, perhaps with an XR2206 and DPM, etched board and all. What is critical about the osc?.

What is critical is low harmonic distortion. The 3311A is 3%, which is simply hopeless, and the XR2206 is 0.5% which is still not nearly good enough. If you will build an oscillator, I would suggest a Wien Bridge oscillator of some kind.

http://en.wikipedia.org/wiki/Wien_bridge_oscillator

The book by Jim Williams cited in the wiki article is very good, and has a chapter on the design of an audio oscillator generating essentially perfect sinewaves, yielding a circuit one can build.

Also, go to http://www.linear.com and search for "wien bridge". Be sure to spell "wien" correctly - as wein won't quite work.

[RedHouse]

The 3311A can be used for initial playing, but will prove frustration when trying to make measurement because the harmonic distortion is too high.

What is critical is low harmonic distortion. The 3311A is 3%, which is simply hopeless, and the XR2206 is 0.5% which is still not nearly good enough. If you will build an oscillator, I would suggest a Wien Bridge oscillator of some kind.

http://en.wikipedia.org/wiki/Wien_bridge_oscillator

The book by Jim Williams cited in the wiki article is very good, and has a chapter on the design of an audio oscillator generating essentially perfect sinewaves, yielding a circuit one can build.

Also, go to http://www.linear.com and search for "wien bridge". Be sure to spell "wien" correctly - as wein won't quite work.

Ok will do.

I remember building the 'ol Wein Bridge back in the 80's, something like using a lamp as the element, will check your links but also I'll just have to dig out my old papers on it, dang you're high maintenance...

BTW, it's <3% at maximum amplitude (which is like 10 volts P-P) but very small sine waves (under 500mV) it is quite good, don't know the exact dist spec though.