Hum level decreases when touching chassis or strings

[Leevi]

I think this is quite well known occurence and it is probably related
to bad grounding either in guitar or amplifier. However I would like to ask
if there are some easy methods to minimize it?
/Leevi 

[HotBluePlates]

I don't know the answer for this, so I'd be interested in seeing what folks have to say.

It seems to be related to the capacitance of the body, and I know I've rigged a ugly but working solution to situation that worked for a particular fiddle player who played through an amp. But that was no more than giving him essentially a "guitar string to touch" on his fiddle to kill the buzz. That particular case won't be of help here, because it was effectively no different that a guitarist touching the strings to stop buzzing.

[JayB]

I think this is quite well known occurence and it is probably related
to bad grounding either in guitar or amplifier. However I would like to ask
if there are some easy methods to minimize it?
/Leevi 

I'd double check the earth ground in the wall outlet your using. It should be minimal but you will have some. Your becoming the shield when you touch anything. I have one guitar, no matter what I do, hums all the time. If I position my hand in front of the input jack of the guitar, it's dead quiet.

[plexi50]

I have been working and playing around for the past 2 weeks with ground ideas on a particular amp and isolating all grounds from a common ground this amp had. Bad layout of this amp but so far i have gotten the hum down to very near silent. Silent until i am at level 10 on the pre and post gain and even then a dramatic improvement

At this point i believe that if my guitar were shelided under the pick guard including small coax from volume pot to input i could have a more precise result

Also as was mentioned by JayB i know that they dont build houses worth a **** anymore and i know most contractors skimp on everything including using a nice 8'+ long copper ground rod pounded into the ground (Here In Florida the Water Table) that your electrical junction box is suppose to be wired to for a good earth ground

The last house i lived in i was outside one day looking at the rod in the ground connected to the Junction box. It was loose and only like 4' long if that. It wasnt copper either which would be the best

All things said i think we all need to recheck out house grounding and do it right ourselves

Then you could be certain to rule out your earth ground as being a problem as far as the house supply goes

[jojokeo]

I think this is quite well known occurence and it is probably related
to bad grounding either in guitar or amplifier. However I would like to ask
if there are some easy methods to minimize it?
/Leevi 

It's your ground not good on your guitar likely. I recently had this condition w/ my Les Paul '58 reissue and even after replacing the pickups w/ Seymour Duncan's I had more humm than w/ my strat single coils! I'd touch the chassis w/ my toe while playing or the ground sleve w/ my finger and seem to notice a difference. AFter checking the strings, bridge tail piece to the jack's ground/plate there was an open circuit. Also you can open the tone/vol compartment and check the ground to there also. But, ultimately your ground has to be good all of the way to the guitar's input jack as this is the point that connects the guitar's ground directly through the chord and to the amp.

I had no ground wire at all. It was never installed by Gibson which is kind of crazy to believe. I routed/drill a new channel to the bridge post assy. internally from the compartment, unscrewed the tail piece screw that holds the tailpiece, drilled a small hole into the end, fluxed and soldered a small braided ground wire, and hooked it up. Result, no more noise and quiet as a church mouse.

I had another issue once on an amp that may or may not be of help to you(?) - I changed the input jack from a switchcraft type to a non-grounding cliff jack and it helped a lot on that amp.

[JayB]

Also as was mentioned by JayB i know that they dont build houses worth a **** anymore and i know most contractors skimp on everything including using a nice 8'+ long copper ground rod pounded into the ground (Here In Florida the Water Table) that your electrical junction box is suppose to be wired to for a good earth ground

The last house i lived in i was outside one day looking at the rod in the ground connected to the Junction box. It was loose and only like 4' long if that. It wasnt copper either which would be the best

All things said i think we all need to recheck out house grounding and do it right ourselves

Then you could be certain to rule out your earth ground as being a problem as far as the house supply goes

Half of my house has a bad earth ground. I redid the copper wire from fuse box to ground. Still thinking how to do the rest of the house which means I'll have to rewire that side of the house.

[jojokeo]

Also as was mentioned by JayB i know that they dont build houses worth a **** anymore and i know most contractors skimp on everything including using a nice 8'+ long copper ground rod pounded into the ground (Here In Florida the Water Table) that your electrical junction box is suppose to be wired to for a good earth ground

The last house i lived in i was outside one day looking at the rod in the ground connected to the Junction box. It was loose and only like 4' long if that. It wasnt copper either which would be the best

All things said i think we all need to recheck out house grounding and do it right ourselves

Then you could be certain to rule out your earth ground as being a problem as far as the house supply goes

If that isn't the truth!!! I took advantage of some house re-building about 10 years ago and while relocating electrical for pool pump, light, solar panel, & salt system equipment and garage rewiring upgrades, I installed two ground rods and bunch of other GFCI stuff. Then I realized that the year my house was built it had only two prong outlets through out. I was lucky that the wiring used had a ground wire included but back in the day, they simply cut it and hooked up the outlets for the two prongers. I had to cut above each box to access the ground wires, install all new boxes w/ new grounded recepticles, etc... to finally have a good ground on everything. What a PITA...

[bluesbear]

Unplug the guitar. If the hum goes away, it's the guitar.... unless it's single coils but then touching the strings won't help much!
Dave

[Leevi]

Unplug the guitar. If the hum goes away, it's the guitar.... unless it's single coils but then touching the strings won't help much!

Unplug the guitar from the amp?  I think the hum disappears since the input jack connects the input to the ground.
If I unplug the cable from the guitar I think there will be more hum since the circuit is then open.

Thanks for the answers guys. It seems there is not an easy solution for the problem

/Leevi

[jojokeo]

Sorry maybe I wasn't exactly clear  - take your VOM and check the resistance btwn the input jack's ground and any other metal parts like your strings, brigde, etc. it's quick & telling. You don't want any resistance seen.

[PRR]

If you have to touch metal to kill hum/buzz, your amp is probably not grounded to the house electrical system.

It is usually NOT important (for hum/buzz) that the house is grounded to dirt. That IS important for safety if you work on dirt or concrete or near pipes. However for hum/buzz, you only need your amp chassis to be AT the general stray electric field inside the room, and proper chassis-wall grounding usually makes that happen.

Problem if you have a ground-neutral fault (open or short), but that's uncommon in simple residential work. (It got very annoying at work, where different ends of the same concert hall got different electric feeds.)
You do want a good dirt-rod to minimize outdoor or cellar shocks, and to divert lighting and primary surges coming in from the street. I used to lose a couple modems every summer until I rationalized my dirt-grounding.

Eight-foot dirt-rod? I have solid rock 4 feet down. Well, I could bust it up with a pick-axe, a couple hours per foot..... but I doubt a hole in the rock is a good ground unless very large and filled with concrete. I got two rods in the dirt slant-wise, and found another rod (never connected), plus a rod at the utility pole 50 feet out from the house.

[Tiny_Daddy]

When I bought this house there was no ground. The electrical was connected to an abandoned section of old gas pipe. The utility ground at the pole was broken at the top and at the bottom of the pole. A lightning strike to the pole awoke me to the problem and destroyed my answering machine and cordless phone by arcing to the phone line. A call to the power utility got the ground repaired at the top of the pole but not the break at the bottom which I fixed after they left. Check your house.

[plexi50]

Ya know what. Just for my sanity which is already in doubt im going to have some one grab a hold of the ground rod tommorrow outside of the house and see if the amp reacts at all. As well they may have used a smaller gauge ground wire in the walls to the outlets. Ive seen that too

[bluesbear]

Just to make myself clear, I meant unplug the guitar from the amp. I assumed you'd know not to leave the cord plugged in... and it seems I was correct.
Dave

[Leevi]

PRR

If you have to touch metal to kill hum/buzz, your amp is probably not grounded to the house electrical system.

That is not the problem. Normally when I start to solder amp this is the first thing I do.

One reason could be the following: I was testing the amp in a separate garage building which is recently build and
where the grounding is taken from the main building. I have to test the amp in the main building as well. But I would
say this a problem more or less almost in all amps even in the commercial ones.

/Leevi

[The_Gaz]

Bluesbear's method of finding the source works to a certain extent, but only from the first gain stage and on. A shorting input jack effectively shorts the first grid to ground, so you can't rule out the possibility that the buzz might be getting in the wiring from the input jack to the first grid. The simple solution is to build a 'dummy plug' by taking a 1/4" jack (and only the jack), and soldering the tip to sleeve. This way you can lift the tip from the tip shunt without pickup an extraneous noise from outside the amplifier.

[Leevi]

I think the hum disappears since the input jack connects the input to the ground.

A shorting input jack effectively shorts the first grid to ground, so you can't rule out the possibility that the buzz might be getting in the wiring from the input jack to the first grid.

Yes, I think it can be tested like you proposed.

/Leevi

[Merlin]

This effect is normally due to your body acting as an antenna which picks up radio noise and so sets up an electrostatic field between your body and the pickups (through the back of the guitar, which is right up against your body of course). If you move away from the guitar the field strength decays and the hum dies down. You probably notice the same effect when moving your hands near an open (live) amp chassis.

When you touch the strings (or chassis), which are grounded, your body is brought to the same potential as ground and the electric field vanishes. The only way to fix it is to fully shield the interior of your guitar, or keep touching the strings!

[Leevi]

I think you are fully right Merlin. I believe in that theory which I even tested
and noticed in practise. The similar behavior can be illustrated if you move the guitar from
one position to another or even rotate it. In some positions you'll get more hum
than in others.

You probably notice the same effect when moving your hands near an open (live) amp chassis.

This is also true.

Thanks
Leevi