Novice grounding question...

[Alex_Law]

I'm thinking this will be an easy one:

My combo exhibits quite a bit of hum when the guitar is plugged in and I'm not touching the strings; the hum all but disappears when I'm touching the strings or indeed any metal on the guitar. The combo doesn't hum at all when the guitar isn't plugged in to it.

Is it safe to say that this is a shielding issue with the guitar and that the amp has nothing to do with it, or is it (also) an grounding issue with the amp? BTW, single coil pickups, and yeah - I know they're supposed to hum some.

I merely ask because my plexi reissue doesn't seem to have the symptoms nearly so noticably, and they both get plugged into the same outlet. The amps are sitting a few feet away from each other though, at different angles.

Alex

[Plexi]

Alex: yep single coils will cause more hum/noise may change by the position you hold your guitar and hear a difference,or switch to the out of phase position. Cables too, and all that.

One thing after looking that might help. In Gabis drawing for the heyboer PT, he has the Hot black/white 120v and the black/common shown in the drawing. They should be switched around. That might help a grounding noise like what your describing. His drawing shows the black/white going to the common hookup,and the black/common going to the hot hookup.
reversed. If you hadn't posted,i'd probably never went back and looked..lol
You might look at this , and see how it compares to our hookups,
its one of the reissiue.
http://www.drtube.com/schematics/marshall/9005.gif

Richie

[Alex_Law]

I'll try putting it in a different room to see if that makes any change. My house is old and wired completely at random, or following a subversive scheme in order to break as many regulatory codes as possible, to set a record perhaps? Either way, not ideal.

My wiring's OK for the PT stuff you mentioned; I was going by Graydon's written instructions for that part of the hookup. Probably worth fixing that drawing, though!

Thanks for the feedback.

This amp sounds so wonderful the more it "breaks in" - I'm so glad I went with this project.

Alex

[cosmicgroovemachine]

First off, any guitar without EMG pickups (or similar active pickups that require no ground) will hum when you cease to touch a metal part of the guitar. Shielding will help but not eliminate the problem. when you flip your guitar over and look in the trem cavity you will notice a wire running to the spring claw. This is your string ground, I know it may sound silly but your body become part of the ground.
Maybe it is less noticable with your plexi cause of better sheilding and or head room. is the back panel on your 18 watt shielded??? that should help.

[loverocker]

I've often wondered about this touch-strings-stops-hum thing. I'd have thought that is adds a loop - which is bad for hum, no? And sometimes I've got thick rubber-soled Doc Martins on, and it still cuts the hum down when I touch the guitar. What sort of ground are DMs?

Wondering... ... If you jump in the air while holding the strings, is the path to earth cut, and so does the hum come back?

[EBE]

I put bill lawrence L-280s in both my strats. Very quiet and they sound pretty authentic.

[Alex_Law]

Wondering... ... If you jump in the air while holding the strings, is the path to earth cut, and so does the hum come back?

OK, who doesn't feel the need to try this now?
[/b]

[RicF]

One thing after looking that might help. In Gabis drawing for the heyboer PT, he has the Hot black/white 120v and the black/common shown in the drawing. They should be switched around. That might help a grounding noise like what your describing. His drawing shows the black/white going to the common hookup,and the black/common going to the hot hookup.
reversed.

Crap. I knew I should have asked about that. I wired it up as per Gabi's layout, and have been wondering about reducing hum, so I'll switch it around and see if that helps...

[Gabi]

Good eyes Richie!

I am glad you trapped that wiring... I did this correction in my layout. Please check if it looks good:

http://18watt.com/storage/18watt%20Layout%20Gabi.pdf

thanks,
gabi.

[Plexi]

Yep Gabi that fixed it.. in the old days a person would just flip the plug in the socket, that was the ground switch. But now days,plugs have one big blade prong,and the other is small,and you can't flip them..lol.
Some amy say it don't matter on the new plugs because they have a ground..but one never knows if your in an old house or bar,and they didn't hookup the ground..so it could help in maybe. i still see people use an adapter,and modify it so they can still flip it over in the AC socket..

Still its just good to see its correct.

Richie

[zaphod_phil]

Wondering... ... If you jump in the air while holding the strings, is the path to earth cut, and so does the hum come back?

OK, who doesn't feel the need to try this now?

What do you mean? All guitarists should do this regularly when performing. Hey, sometimes I even pogo while playing bass!

[zaphod_phil]

I'm thinking this will be an easy one: My combo exhibits quite a bit of hum when the guitar is plugged in and I'm not touching the strings; the hum all but disappears when I'm touching the strings or indeed any metal on the guitar. The combo doesn't hum at all when the guitar isn't plugged in to it.

Usually, this is a symptom of the ground connection between the bridge and the guitar's internal signal ground back to the amp having been broken, or having gone high impedance. You should first open your guitar and make sure you have a good soldered ground connection between the bridge and the electronics. I don't think this is in anyway related to the single coils.

Next, if that 'hum' is buzzy sounding, this tends to indicate RF noise traveling along the shield of your guitar cable. In that case make sure your 18W input jacks are the kind that are insulated from the chassis. Then use Randall Aikens trick, where you connect a 0.01uF cap from the ring/outer side connection of the jack plug to a near point on the chassis with very short leads. That will soak away the high frequency noise to ground, without causing any ground loops at the same time. It may be that your Plexi already has something equivalent to this scheme already.