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When I plug in my guitar cable by itself in the amp (without the guitar attached) it makes a bad noise. I changed the cables and the noise is still there in the amp. Also, when I plugged just the jack without cable, it makes less noise. I tried different jacks to ensure that the fault is coming from the amp. Also it produces more noise when I touch the metal corner! Any idea?
Note: the amp does not make noise if it is ON without cables or jacks plugged in.
This is normal.
Figure 1. The photos of the HF-S sockets show the normally closed contacts of the jack plugs. Image source: Canford.
The sockets are wired so that when the jack plug is unplugged the tip contact is shorted out to ground.
When you plug in an unterminated plug you pick up electric fields from mains wiring (and anything else), amplify it and create noise. That's why guitarists plug the lead into the guitar first and then into the amp.
This is normal behaviour for a guitar amp, which is why there is an accepted cabling/switching/volume order when setting up.
If you have no other earthing/grounding issues, then plugging just a guitar lead in without attached guitar just turns the whole thing into a big crude arial. It will predominantly hum at about G♯ in EU or B♭ in the US [ 50Hz vs 60Hz mains].
You can make it far worse by adding yourself to this arial by touching the tip of the jack, or slightly improve it by touching the shield or body. Anything else you touch will add to the amount of mains hum being picked up by your new crude arial*. Plugging in a cable-less jack just makes less of an arial.
Plugging a guitar in completes the circuit so you [in theory] no longer just have a big, dumb arial; unless your guitar is poorly shielded. The usual trick is to plug the cable into the guitar first, then into the amp. It's also wise to turn the amp down first, or you get bangs & thumps as different parts of the plug contact all the wrong places before it finally settles into its 'click' locator.
Guitar amps are not actually the finest pieces of delicate electronics, unlike hifi or scientific equipment. They are designed to colour sound in weird & wonderful ways. Making them silent in operation is not high on the priority list, so long as they are reasonably quiet once fully connected. You might have noticed they can still make a lot of background noise anyway, once you're up at gig volume with a distorted sound.
*There's a whole slew of 'fixes' for hum, interference, ground loops/ground lifts, induced RFI… a lot of which borders on voodoo, so I'm not going to go anywhere near it.
A lot of these people here are experts in electronics… I'm just a guitarist of 50 years' experience, who learned a lot of this through simple empiricism. [I also these days use a totally hum-free guitar, based on transducers not magnetic pickups.. but that's a whole other story.]
The amp likely has a switched input jack. If there is no plug inserted, the signal input will be connected to signal ground to prevent any signals from being anplified.
When you have a plug inserted, but with no signal connected, the input signal will be floating and because it has very high impedance and very high gain, the input will receive and amplify any kind of stray signals or noise it happens to pick up.
What everyone is saying here is correct. You're breaking the ground with the switching jacks, and your body becomes a dipole when you touch the metal near the amplifier. The fact that the metal corner is not grounded, but just screwed into the wood is important. An antenna is created by any two charged elements separated by a distance.
That said, if you have amplifier noise when the guitar is plugged in you may be able to eliminate it with a Grounded Adapter 3-Prong to 2-Prong Outlet Converter for the amplifier. Bend the ground prong so it is not touching the outlet screw. The house ground is for fault protection in the event of a A/C short in the amplifier, but is a source of noise. Pretty much everything in your house is dumping noise into your house ground.
If you're interested in the engineering details of ground this talk is fantastic.
