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Tim Stanley's responses to pickup cavity shielding questions.
Also see my grounding FAQ since shielding and grounding are related issues.
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IMHO, before you buy new pickups, do what the manufacturer failed to
do properly - i.e., shield the cavity. That is the basis of many
pickup problems - after that - decide if new pickups are in order.
Vintage guitar collectors presumably prefer their vintage noise and
should not follow the following advice as it will result in a quieter,
but less valuable, vintage guitar.
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Material
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Stewart-MacDonalds (Athens, OH) sells mail order shielding paint.
That is what I always use. One small can is enough for > 5 guitars at
3 coats per guitar.
Other people like to shield with foil/sheet metal. Seems hard to work
with to me. But, no doubt it provides a lower resistance path to
ground for that pesky noise. And it may be easier to attach the final
ground wire to metal, than to the painted surface. Just make sure all
of the pieces of tape/foil are electrically attached/soldered
together, and then finally to the common ground point.
Get a small (3/8" maybe) cheap tough paint brush from a hardware
store. You will throw it away when you are done and you don't need
a quality brush for this application.
Get in a patient neat mood. This paint is a bit nasty (ventilation
needed) and awkward to apply (light tar, well, it certainly ain't
latex). If it gets on something it shouldn't be on - remove it right
away. Really.
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Disassemble
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Unscrew all the pots and switches from the guitar and pickguard before
you paint. Don't be lazy - completely disassemble everything so you
do a complete job. Consider well that you are doing what the
manufacturer was too cheap to do properly - this is no time to be
lazy. Usually, the potentiometer case/switch case/jack case is metal,
and screwing it down to your soon-to-be shielded pickguard or guitar
will shield it as well. Very nice, indeed.
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Guitar
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Paint the interior of every cavity that you can reach. Paint the
outside of the timber (under the pickguard) out and (*very*) slightly
around the pickguard screwholes. Of course, take care that you don't
go too far so that the shielding paint will remain under the pickguard
and not ruin your axe's finish. At the same time, be sure to extend
the lip out and around as many screwholes as possible. Then when you
screw the soon-to-be shielded pickguard to the guitar, a connection is
made between the shield on the pickguard and the cavity.
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Pickguard
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I paint the entire back of the pickguard, although it may not be
really necessary on some guitars if the exposed (non-shielded) wires
are only in the cavity. Do it anyway. Then, when the pickguard is
screwed down to the guitar, it makes a connection with the shielded
portion of the timber over the entire perimeter of the cavity. Now,
the entire cavity interior is completely shielded.
This is not a ground loop, rather, you have created a ground plane
that surrounds all of your electronics. You do have only one
connection, between the pickguard and the timber, it is just a large
perimeter connection.
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3 coats
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Also, I use 3 coats for coverage and to reduce the sheet resistance of
the paint shield. The paint is a bit thick - after it dries, you can
visibly see the direction of your brush strokes. So, e.g., first coat
is left to right; second coat is up and down, and third coat is
diagonal. Thusly, at the end, everything is evenly covered despite
your brush strokes.
The paint seems impossible to remove from the brush, wet or dry. Wrap
the brush in foil between coats and it will be just fine for a few days.
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Ground
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The paint shield has to be connected to the ground of the system. It
is likely that the back of one potentiometer in the system has been
used as a common ground point. As such, if the conductive
potentiometer case is mechanically attached to the paint shield, then
the paint shield is connected into the system.
However, you might want to take a short pan head screw, and a bare
wire, and mechanically attach the wire to the paint shield with the
screw. Then, solder this wire to the back of the potentiometer with
all of the other ground connections.
In either case, use an ohm-meter to verify that the shield (whether it
is paint, foil, tape) is well connected to the system ground.
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Ground loops
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One practical matter to remember, even for a college educated EE. All
grounds are not alike. Be *very* careful not to create a ground loop.
Follow these simple rules:
+ All grounds should come together at one and only one point
(usually the back of a pot case).
+ Don't use the shield as ground, rather, use it to conduct
noise to ground. The sheet resistance of the paint is at least
a few ohms - not a good ground conductor, but low enough to
conduct the noise to ground. Do you see the difference?
+ Thusly, there will *never* be two paths to ground.
OK - all fine and well. After reassembly, you will see significant
improvement. Works great for me. Fact is, single coils always pickup
more noise than humbuckers.
G'luck,
T
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Related questions and comments
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It doesn't matter
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| Also, it seems that shielding the pickup cavity may have pretty irrelevant
| results because the pickups sit so far out of the guitar (at least
| according to Foley's book) -- any opinions on this?
I disagree. I have Foley's book. Foley is correct that the part of
the pickup sticking out of the guitar is not shielded and will not be
helped. But part of it is below you pickguard - the back of which
will be shielded - and will be helped. Also E and B fields might get
sufficiently weakened by the shielded *plane* of the pickguard in
which the pickup sits. (and I can't recall my physics well enough to
give details - heck, I can't remember what to do with my thumbs when I
am playing guitar, nevermind describing the direction of E/M
fields...). There is no question that the pickup's orientation with
respect to the electric and magnetic fields affects the amount of
noise picked up. I think Foley's point, of course, is that the pickup
has an *enormous* amount of wire in it, and any non-shielded part may
come to dominate overall shielding impact.
BUT - IMHExperience - especially with single coil Strats - shielding
makes a real difference. Audible. This is not a "silver wire" sort
of improvement, if you have been reading that thread. This is not an
alder vs. ash Strat sort of thing. It is positively real and I bet it
could be quantified with normal lab-bench instruments by first year
electrical engineering students.
In fact, FYI - Gibson (at least used to) shield their cavities with a
thin brass plate. On an old SG I own, with two humbuckers connected
with manufacture-supplied very high quality shielded cable, the brass
plate serves only to shield the cavity wiring. Therefore, ergo etc.,
it must be of some value, else they would have saved their pennies and
left it out.
I have paint shielded two Strats and one Gibson SG. I have paint
shielded two additional loaded pickguards that I swap back and forth
on one Strat. All with one can of paint and there is more left. 3
liberal coats each. It matters.
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Shielded cable
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| is it the wiring, which has been redone several times with new shielded
| wires and new solder. Amp, cable, etc. have been the same.
OK - just be *sure* that the shielding in the shielded wire is not
causing a ground loop somewhere. In nearly all instances, the shield
of the cable should be connected at *one*and*only*one* end. Otherwise
it is a ground loop. It may or may not matter. Since that sort of
answer sucks, my philosophy is to never have one.
I use shielded cable from the input jack to the first connection in
the cavity. I hook up the hot and the shield on both ends. This wire
carries hot and ground from the cable into the guitar. Ground is
hooked up at both ends.
The wires to many pickups is made of shielded cable, e.g., the PAFs on
my old Gibson SG - always hook the shield to the system ground point.
I do not use shielded wire on point to point internal connections for
several reasons:
1) I find its size awkward to work with in tight cavities and on
complex wirings.
2) I find the extra ground connections for each shield to make
for very sloppy wiring jobs prone to human errors. I am human.
Intermittent wiring errors cause me to lose my temper and that
takes all the fun out of playing guitar.
3) I shield the cavity of my guitar with shielding paint, and as such,
do not require additional shielding on each and every wire.
That's it. The rest of my internal wiring is done with regular,
appropriate small gauge wire. I have done the Dan Armstrong scheme
twice and that is the most intricate and complex (and most cool)
wire-intensive wiring design I know. In both instances, I also added
push-pull phase switches, too. I depend on shielding paint to shield
the wires inside of the cavity from external noise. I have had no
problems with this approach. I avoid shielded cable except where I
can not shield cavities. It is too hard to use in small spaces and
short lengths.
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Pickup Covers
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Some netters have gone to the trouble of shielding the inside of
pickup covers and feel that it helps alot. A reasonable thing to do,
after all, consider the metal cover on the Gibson PAF humbuckers. I
use a (really) old (really) noisy Dimarzio Super Distortion Single
coil in the bridge position of a strat. The plastic pickup cover
slides on and off very simply. I painted the inside of the cover, and
made sure that this cover shield got connected to the rest of the
cavity shield. I found no audible difference with this improvement
and A/B tests were pretty easy to conduct. So, my experience
shielding covers, with *this* pickup on *this* guitar differs from
other netters. Consider that I had already fully shielded the cavity
when I did this test. Perhaps I already removed *most* of the noise,
and the incremental improvement was too small to audibly discern.
Perhaps others shielded the pickup covers prior to shielding the
cavity, and so achieved large improvements since there was so much
room for improvement (this is what I suspect leads to the difference
in perception). Perhaps my environment is too quiet to really stress
the extra cover shielding. Perhaps my cover shield connection to the
cavity shield is inadequate. Yet again, YMMV.
Finally, some netters feel that a using a shielded pickup cover vs.
unshielded pickup cover makes the pickup sound different. Probably
does. Again, we can get in E and B field issues and I currently lack
the inclination to get out my physics books to refresh my
understanding. I personally can not tell a difference with my limited
experience. But let's face it, some people remove those metal PAF
covers so I assume they have an _actual_ reason. I mean a reason
besides "Joe Smoke of my fave band Thunderous Lightning removed his
PAF covers just before he was electrocuted due to a faulty string
ground caused by a roadie who got bad shielding information from some
pinhead on the Internet and this is my tribute to him", or "a Guitar
Player article told me to and I do everything they say".
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Aluminum Foil
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| to have to do it myself. What I want to know is, could I do it with
| the aluminium foil kicking about the kitchen, plus some glue? I intend
| to do the pickup cavs, the trem cav, and the control cav over again,
| plus the insides of the pickup surrounds for that wee bit extra!
| What do you think??
I have attempted this before. In principle, you can, in practice, you may
have troubles because:
1) the foil is too flimsy, it rips and tears too easy, even doubled up,
2) it is hard to reliably attach it to the guitar (I tried thumbtacks),
3) you certainly can not solder to it.
4) I could not to a neat job. And messy jobs end up with shorts and other
problems that must be avoided.
But, your milage may vary.
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Misc
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| From what I understand, I just stick in the tape and solder it all
| together? Seems like the solder connections could be iffy. After that I
| would then solder a wire from one of the tape segments to ground? I have
Yup - to one and only one point. A common ground lug - much like the
ones on the back of the one of the pots in most implementations. In
fact, simply connecting the shield to that point will do the trick.
| While paint is more expensive, if it makes life simpler I may go with the
| copper shielding paint. Any opinions on that?
It is nickel actually, methinks. Hard to say. Probably not copper
though. I have no complaints with the paint, I have never used
tape/foil. People who have claim it is no problem.
G'luck,
T
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Digest of pickup cavity shielding articles
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Newsgroups: rec.music.makers.guitar
From: joebac@cup.hp.com (Joe Bac)
Subject: Shielded Strat = less humm
Date: Mon, 28 Nov 1994 19:41:10 GMT
Organization: Hewlett-Packard
Hey folks - over the thanksgiving holiday I decided to take the time to shield
my main gigging strat. For the most part, I can deal with the humm/buzz when
I'm not in any high gain mode, but there's those times when the next tune in
a set requires a high gain right from the start. And there's that annoying
buzz that drives everyone nuts before you get the tune counted off. Well
doing lots of shielding helps tremondously. Here's what I did...
First I painted all the pickup and electronics cavities with shielding paint.
I put two thick coats on. Then I got some rolls of that copper shielding
tape. For each pickup - I first wrapped black electrical tape around the
bobbin. Then a strip of copper tape over it. Make sure it does not touch
the hot lead wire from the pickup. Then take a loose leaf paper hole puncher
to a strip of copper tape that is traced to the top of a pickup. Then punch
out holes for the magnets. Place it over the top of the pickup and make
contact with the copper tape that surrounds the sides. Also place a strip of
copper tape on the back side of the pickup (no holes needed on the back side).
Then make small solder connections among the three pieces of tape. You can
connect to the black wire terminal of the back side of the pickup for
grounding. Repeat this for the other two pickups. Now if the back side of the
pickguard is not covered with shielding foil, then take strips of the copper
tape, in rows, to the back of the pickguard. You should, but don't have to,
connect each strip with a little dab of solder. Reinstall all your electronics
to the pickguard. If you did everything right, there should be a common
ground to the whole pickguard assembly.
Now, if there is no ground lug in the guitars cavity (which is now painted),
pick a spot and make one. A ground wire should run from the whammy bar
spring retainer to this ground lug. The guitar jack ground wire goes to
the lug. Of course this lug is screwed in the painted cavity area. Then
a ground wire runs from the lug to your favorite ground spot on the pickguard
assembly (this is usually the casing of the neck pickup's tone control.
For extra effort - you can replace the hot wires of the pickups with shielded
wires. Also do this with the hot wire from the jack to the volume pot.
If all goes well, there will be far less buzz when the strat is being used
in a high gain situation.
One final note - TAKE YOUR TIME, BE PATIENT. AND MAKE SURE NO SHIELDING
MATERIALS TOUCH THE HOT SIDE OF ANY OF THE PICKUPS. Test this with an ohm
meter before you plug in.
It works - good luck!
--
Joseph G. Bac Phone: 415 691-5417 Fax: 415 691-5030
Hewlett-Packard Co. ARPA: joebac@cup.HP.COM
100 Mayfield Ave. MS 36LF UUCP: hplabs!hpiatmh!joebac
Mountain View, CA 94043
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Date: Tue, 29 Nov 1994 13:49:48 GMT
Message-Id: <17759.9411291349@ewe.dcs.ed.ac.uk>
To: tjs@eecs.umich.edu
Subject: Re: screening. latest..
From: Chris Herron <ncah@dcs.ed.ac.uk>
I spoke to Kent Armstrong today, he designs pickups here in Britain,
plus is the distributor for WD. products + parts. He supplies
self-adhesive copper foil in sheets of 8ins by 12ins. (by the way, he
also does aluminium solder - he suggested I use it with Alu. foil and
a pritt stick, but recommended the copper stuff). I think you might be
able to get copper foil from WD in the states, and it could be quicker
than painting on conductive paint, plus it solders ok.
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From joebac@cup.hp.com Mon Dec 5 10:15:50 1994
Newsgroups: rec.music.makers.guitar
From: joebac@cup.hp.com (Joe Bac)
Subject: Re: Shielded Strat = less humm
Date: Fri, 2 Dec 1994 19:23:01 GMT
Nntp-Posting-Host: zx-11.cup.hp.com
Organization: Hewlett-Packard
X-Newsreader: TIN [version 1.2 PL2]
Hey folks - After reading some of the follow-up posts about overkill etc.,
in how I described the shielding - let me explain why I did what I did.
I've gotten sick and tired of the noise problem on stage. So I asked the
guitar tech who I go to about what to do. The guy I go to was/is the
tech for the Doobie Bros and was on the road with them during all those
glory days. Now I was only thinking of painting the cavities - when I
mentioned this to Mark, he got this crazed look in his eyes and said, "Joe,
you what to shield the fuck out of it". Then he proceded to pull out the
materials and told me what to do - so that's what I did and it works. Some
people believe that shielding the pickups will effect the tone. As far as
I can tell, my tone is the same. So I guess what I'm saying is I'm glad
to have a guy like Mark around and I'm glad I took his advice.
--
Joseph G. Bac Phone: 415 691-5417 Fax: 415 691-5030
Hewlett-Packard Co. ARPA: joebac@cup.HP.COM
100 Mayfield Ave. MS 36LF UUCP: hplabs!hpiatmh!joebac
Mountain View, CA 94043
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From nyeda@cnsvax.uwec.edu Wed Jan 18 11:21:31 1995
Path: zip.eecs.umich.edu!newsxfer.itd.umich.edu!gatech!howland.reston.ans.net!news.moneng.mei.com!uwm.edu!news.doit.wisc.edu!koala.uwec.edu!uwec.edu!nyeda
From: nyeda@cnsvax.uwec.edu
Newsgroups: rec.music.makers.guitar
Subject: Re: Damn Traffic Light!
Date: 17 Jan 95 22:30:44 -0600
Organization: University of Wisconsin Eau Claire
Lines: 13
Message-ID: <1995Jan17.223044.1@cnsvax.uwec.edu>
References: <D2Iou0.HpF@cup.hp.com>
NNTP-Posting-Host: kimera.uwec.edu
joebac@cup.hp.com (Joe Bac) writes:
>My rig has to get placed right in a window. So now it dawns on me to
>look where the traffic signal box is located and the sucker is right
>outside where my amp sits [...] I'm thinking of making some sort of
>shield that I can place behind my amp in the window.
A big piece of window screen or hardware cloth should work. It will
work best grounded to something like a pipe with a length of wire but
this may not be necessary.
David A. Nye MD (nyeda@uwec.edu) * Midelfort Clinic, Eau Claire, WI
For all but the elite, work holds less promise, less purpose, less
security and less dignity than a generation ago -- Peter T. Kilborn