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Les Paul hum

Postby _ Six _ » Tue Sep 25, 2012 10:12 pm

It's quite excessive when I take my hands from the strings but calms down a lot when i touch the chassis of the amp or put my hands back on the fretboard. I've checked the earth of the guitar and had a look for dry solder joints. The amp is brand new so I doubt it's that.

The only othe thing I can think of is the mains socket I run my gear off. The house is quite old but the wiring was checked when I moved in 4 years ago. I don't have a mains earth tester here but have just ordered one to test it.

I've heard that Les Pauls can be quite noisy anyway.. I've also read that replacing the internal wiring with better quality stuff can make a difference. My other guitars (single coils) all seem fine.

It might be worth noting that I get some buzz in the amp if I'm touching the strings and touch my MacBookPro too.. It may be a coincidence but I shorted a power adaptor that way a few weeks ago.

Any ideas?
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Re: Les Paul hum

Postby Gary_W » Tue Sep 25, 2012 11:27 pm

When you say you've checked the earth of the guitar, have you got a decent earth from the bridge to the screen on your guiatar cable? It never ceases to amaze me how many guitar seem to have a pants earth in that regard and it drives me potty when I get big noise differneces with hands on strings vs off strings....
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Re: Les Paul hum

Postby ef37a » Wed Sep 26, 2012 4:43 am

Six, it would not matter if the guitar was wired with stuff from the loom of an old Ford Anglia. If the cavity is shielded and earthed it should not hum. Don't go all Russ A on us here!

I am with Garry, the bridge and hence the strings need bonding to the jack earth. I understand this link was left off some guitars out of a well meaning but wrong headed idea of safety? No part of any equipment should ever become live and in the rare event that it does it should blow a fuse or trip an RCB. Do you have an RCB (earth safety trip) in your supply at any point by the way? If not get one, they are not expensive and it looks as tho' a modest test meter should be on the shopping list as well!

Finally, I am concered about your diagnostic technique. "Going round touching things" is REALLY not acceptable!...Err? How do mean the adaptor "shorted out"?!!!

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Re: Les Paul hum

Postby Gary_W » Wed Sep 26, 2012 8:03 am

All wise words!

I've only ever had one LP style guitar (a Burny). I found it a lot harder to screen than my Fender style efforts (far East G&L's). With the Strats I just get completely medieval with the turkey foil so it coats every part of the body cavity. I extend this under the bridge. Then spray glue more turkey foil onto the rear of the scratch plate. That lot, combined with ensuring decent continuity to the bridge in the first place, turns a Strat from a noise monster into something a whole lot nicer. IMO anyway - I know a guy who likes the things to buzz!!

The Burny was a lot tighter inside. Les Pauls are a bit more on the 'wood' side in the wood/air ratio ;). Internally, make sure the switch is wired using screened cable and that the screen is connected electrically back to he jack. Ditto the pickups. But the favourite is still bad continuity to the bridge.
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Re: Les Paul hum

Postby Goddard » Wed Sep 26, 2012 10:24 am

ef37a wrote:I am with Garry, the bridge and hence the strings need bonding to the jack earth. I understand this link was left off some guitars out of a well meaning but wrong headed idea of safety?

Most all Gibson electrics (IME) do not earth the strings/bridge, even models using single coils (although I've seen a few hollowbodies with the tailpieces earthed).

As said, perhaps it was impractical to do so with solid bodies, but even the hollow bodied models, which could be more conveniently connected at bridge studs or tailpieces, do not generally, so it seems to have been a conscious design not to earth the strings, and perhaps there are safety benefits to that insofar as not creating a shorting path to earth when a player comes into contact with a wrongly wired mic (or a mic connected to a wrongly wired desk), etc.

Sure, Fender earth wire to the strings (e.g. at the trem block claw), but they also fit their amps with ground lift switches, so...

If the OP is experiencing hum which diminishes when touching the strings, it might be that his LP was (badly?) re-wired to earth the strings (perhaps by a previous owner?). A continuity check along with a glance at the relevant Gibson schematic is eaily done:

http://www2.gibson.com/Support/Schematics.aspx

Oh, I heartily agree that any continuity check should be done with an Ohm meter, not with one's finger (while hopefully keeping one's other hand in one's pocket...). :smirk:

Or maybe it's a conductive metallic paint finish?
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Re: Les Paul hum

Postby Hugh Robjohns » Wed Sep 26, 2012 10:31 am

Goddard wrote:Sure, Fender earth wire to the strings (e.g. at the trem block claw), but they also fit their amps with ground lift switches, so...


A 'ground lift' switch shouldn't affect the safety earth bonding of the amp. It should merely increase the impedance between the audio circuitry ground and the mains safety ground -- by a hundred ohms or so. Enough to discourage ground loops but not enough to affect safety.

If it is isolating the mains safety earth in any way it is unsafe!

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Re: Les Paul hum

Postby Random Guitarist » Wed Sep 26, 2012 11:55 am

Goddard wrote:

Most all Gibson electrics (IME) do not earth the strings/bridge, even models using single coils (although I've seen a few hollowbodies with the tailpieces earthed).


I woulldn't agree with that 100%.
My experience is the opposite, there's usually a ground wire from the control cavity to one of the tailpiece studs. The bare end of the wire is in the hole when the stud is inserted and makes a physical contact. Not as good as a soldered connection but it is there.

The exception is some 80's Gibsons where they started putting a sort of complete shielding in the control cavities, but I believe the ground wire to the stud has been reintroduced. My two recent Gibsons certainly have them.

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Re: Les Paul hum

Postby zenguitar » Wed Sep 26, 2012 1:19 pm

I would agree with Rhino Time here. With the exception of a small number of instruments a long time ago every Gibson I've worked on has had a ground link to the strings. And that includes a period working as the guitar tech for a Gibson Main Dealer.

As RT points out, the connection is made by a short wire to one of the bridge/tailpiece studs. And like a non-trem Strat and a Tele, it is a mechanical connection between the hardware of the bridge and a bare wire. No solder, no screw clamp, just trapped between the wood of the body and the metal of the bridge. On the Strat and Tele style bridge plates it's easy, you just drill a hole from the control cavity and it exits on the front of the instrument. The bare end of the screen wire is trapped between the body and the bridge plate which neatly hides the hole.

On a stud mounted bridge there's no plate to hide the hole, so you have to be creative. A hole is drilled from the control cavity to exit in one of the holes where the bridge stud bushings are fitted. Typically, an uninsulated length of solid core wire is threaded through until the end is poking into the bushing hole. Then the bushing is pressed into place. If everything goes right, the bushing traps the wire between itself and the wood of the body. But there is a lot of potential to go wrong. The hole for the link wire might appear too deep in the bushing hole so that the bushing doesn't reach or grip it. The link wire might slip out a little while the bushing is being pressed in so that there is no end to trap. Uninformed fingers sometimes can't resist on pulling at the wire to work out where it goes. It's often forgotten when a learner rewires a guitar. Changing the bushings when fitting a new bridge can let if fall out, especially when the person doing the work doesn't realise that there should be a wire there ;) And very rarely the bushing holes can get bigger as the wood seasons and dries out, either the bushing gets loose or there's no pressure on the wire and it either falls out or makes a bad connection.

SO...

step one. Check that there is actually a wire going from the back of one of the pots to a hole in the side of the cavity and heading in the general direction of the bridge studs. If it is there, good so far. If it is not connected to the pot, but is still in place, it needs to be soldered to the back of the pot. Then give it a gentle pull, if you can feel resistance then it appears to be trapped securely between the bushing and the body. Ideally, double check with a meter that you have continuity. Touching the wire with one probe and the stud with the other must have an electrical connection.

If the wire is loose, missing, or doesn't have electrical continuity then you have a problem. The bushing has to be carefully pulled out, a new wire fitted, and the bushing pressed back in place. It's doable, but not the easiest of jobs without some specialist bushing pullers.

Andy :beamup:
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Re: Les Paul hum

Postby ef37a » Wed Sep 26, 2012 4:06 pm

Thanks Andy.
Re bush pulling!
I have had success (in a non guitar field) using a large socket, ID bigger than the bushing, some threaded rod (often called studding) and a nut and oversize washers or a metal plate suitably drilled and covering the socket.

Naturally one would need to protect the body of a guitar.

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Re: Les Paul hum

Postby _ Six _ » Wed Sep 26, 2012 8:36 pm

I wasn't testing it by touching things!

I've checked the soldering again and it was a dry solder joint on the neck volume pot.
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Re: Les Paul hum

Postby Goddard » Wed Sep 26, 2012 11:11 pm

I stand corrected. Grounding the tp on Gibsons seems to be the rule after all rather than the exception. Always happy to learn something, even if it's that I was wrong. Thanks Andy!
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Re: Les Paul hum

Postby Goddard » Wed Sep 26, 2012 11:31 pm

Hugh Robjohns wrote:
Goddard wrote:Sure, Fender earth wire to the strings (e.g. at the trem block claw), but they also fit their amps with ground lift switches, so...

A 'ground lift' switch shouldn't affect the safety earth bonding of the amp. It should merely increase the impedance between the audio circuitry ground and the mains safety ground -- by a hundred ohms or so. Enough to discourage ground loops but not enough to affect safety.

If it is isolating the mains safety earth in any way it is unsafe!

hugh

Hugh, what you suggest makes perfect sense, but see S2 in this schematic:

http://support.fender.com/schematics/gu ... ematic.pdf
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Re: Les Paul hum

Postby zenguitar » Thu Sep 27, 2012 12:19 am

ef37a wrote:Thanks Andy.
Re bush pulling!
I have had success (in a non guitar field) using a large socket, ID bigger than the bushing, some threaded rod (often called studding) and a nut and oversize washers or a metal plate suitably drilled and covering the socket.

Naturally one would need to protect the body of a guitar.

Dave.

Exactly Dave, that's how I do it ;) Some rings of cork make nice finish protection, and a sharp scalpel blade run carefully around the outer edge of the bushing will break any overspray from the finish and help prevent it chipping out.

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Re: Les Paul hum

Postby ef37a » Thu Sep 27, 2012 4:44 am

zenguitar wrote:
ef37a wrote:Thanks Andy.
Re bush pulling!
I have had success (in a non guitar field) using a large socket, ID bigger than the bushing, some threaded rod (often called studding) and a nut and oversize washers or a metal plate suitably drilled and covering the socket.

Naturally one would need to protect the body of a guitar.

Dave.

Exactly Dave, that's how I do it ;) Some rings of cork make nice finish protection, and a sharp scalpel blade run carefully around the outer edge of the bushing will break any overspray from the finish and help prevent it chipping out.

Andy :beamup:
Ah! When you said "special bush pulling tool" I thought you meant some expensive and esoteric device peculiar to gitists!

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Re: Les Paul hum

Postby zenguitar » Thu Sep 27, 2012 12:27 pm

ef37a wrote:Ah! When you said "special bush pulling tool" I thought you meant some expensive and esoteric device peculiar to gitists!

Dave.

I can offer expensive and esoteric peculiar to luthiers if you prefer ;)

Schatten knob and bushing puller.

Always a useful link because it shows how it works, making it easier to improvise one yourself. If you were to order it on it's own it would work out at £40-45 delivered once you add shipping, duty and VAT. If you were to add it to a bigger order, it would add about £35 to the cost.

If I had the spare cash and was putting together an order from StewMac I would definitely consider it over making my own or improvising with sockets. In one package it has Allen bolts and washers for pretty much every thread known on a bushing used on guitars, and the Pyrex glass gives you a very clear view of how cleanly the bushing is pulling out potentially avoiding a finish repair.

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Re: Les Paul hum

Postby ef37a » Thu Sep 27, 2012 4:18 pm

Hi Andy,
Once again an SoS link did not work but I tracked the puller down anyway.

Re the transparent "socket" I am sure you could find a little engineering shop that could turn you up a couple out of acrylic or polycarb tube. (bit 'o'horsetrading? Everyone has a kid or a grandkid with a geetar!)

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Re: Les Paul hum

Postby dmills » Thu Sep 27, 2012 9:25 pm

Goddard wrote:
Hugh, what you suggest makes perfect sense, but see S2 in this schematic:
http://support.fender.com/schematics/guitar_amplifiers/59_Bassman_schematic.pdf

A weird thing to do, but given the chassis ground is not disconnected by that switch, not actually dangerous if that cap is suitably rated (Class Y).
I am not really sure what it is supposed to accomplish, but this is not a classic lethal 'earth lift switch', I would probably pull it anyway, especially if the cap was not really suitable, but it just looks like a very poor substitute for an inter winding screen to me.

Regards, Dan.
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Re: Les Paul hum

Postby ef37a » Thu Sep 27, 2012 9:59 pm

Hi Dan,
I think this was done to combat weird mains supplies about the States?

I believe there was often no earth (even if it looked as tho there was!)and the ACTUAL wiring of neutral and live was inderterminate. One switch position would be found to give less hum.
We used to find this with venues sporting just 2 pin 2,5 or 15A sockets, one way round hummed less but I carried 20mtrs of earth cable with a big croc on the end and used to go off in search of a radiator or the bogs!

With that amp tho, worst case scenario of no earth and the cap switched to live? Painful on 115V, possibly lethal on 240?

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Re: Les Paul hum

Postby zenguitar » Fri Sep 28, 2012 12:54 am

ef37a wrote:Hi Andy,
Once again an SoS link did not work but I tracked the puller down anyway.

On this occasion, at least, I think the problem is down to the StewMac site, I copied the link after using the option to convert to sterling and I think that's what messed it up.

And yes, you are right about the acrylic and a local workshop :) We really are on the same wavelength when it comes down to finding a way to 'leverage' a little help ;)

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