Why does an amp make a 60hz sound when I touch the input cable?

[Austin]

The nuvistor is completely enclosed by metal and doesn't that serve to help protect against radio waves like a mini faraday cage? Some people say you should earth a faraday cage and some people don't think it matters, I have had hard time actually finding much info about it. The neutral wire in houshold wiring should be at earth potential and doesn't every house have a copper rod in the ground that the pipes and wiring all connect to? So in that sense, (especially using a three wire plug)that would connect the chassis and that metal shell to earth no matter what you call ground in your circuit.

[Enzo]

At the risk of dwelling on it, yes your house wiring has its common earthed, so earth and ground are essentially the same thing there. But what if you build a battery operated radio? Or TV? It will have no earth connection of any kind. Yet sensitive parts of it are shielded.

That nuvistor is a metal thing, but it plugs into a socket with all the leads exposed to the electrical environment, they need shielding even if the tube itself does not. At least potentially.


If you have an experiment to perform using a van as a faraday cage, go out in the van and try the experiment. If the van does work as a shield, great, if not, clamp a wire from its frame to a water pipe and see if that makes any difference.

[Steve Conner]

The principle of the Faraday cage is that any electrical malarkey going on inside it has no effect on the outside, and vice versa. That holds true whether it's grounded or not. For example if an aircraft is hit by lightning at 10,000 feet, the metal fuselage still protects the passengers.

Of course any practical shielding enclosure tends to have wires penetrating it, in which case it's no longer a true Faraday cage and the analysis gets much more complicated. But it generally turns out that grounding the enclosure is necessary for safety anyway.

[Austin]

The principle of the Faraday cage is that any electrical malarkey going on inside it has no effect on the outside, and vice versa. That holds true whether it's grounded or not. For example if an aircraft is hit by lightning at 10,000 feet, the metal fuselage still protects the passengers.

Of course any practical shielding enclosure tends to have wires penetrating it, in which case it's no longer a true Faraday cage and the analysis gets much more complicated. But it generally turns out that grounding the enclosure is necessary for safety anyway.

Thanks for the info. Isn't lightning a form of dc? I was thinking faraday cage for rf frequency ac but I see your point. If an airplane was a real faraday cage then your cell phone wouldn't work in there, as the 12cm wavelength can find its way in and out. But I wonder if I tape up a little box made out of metal window screen and put a cell phone in there if it will work if it is not earthed. Window screen is probably less than a mm for the holes and that would be about 300 ghz according to Wavelength <-> Frequency Calculator Converter (JavaScript) - RF Cafe

I don't know exactly why I find this so interesting.

[Enzo]

In the time it has taken to try to come up with an answer, the experiment could be already tried.

[rjb]

In the time it has taken to try to come up with an answer, the experiment could be already tried.

But... how do you get the lightning to strike the van?

[frus]

there are also some interesting uses of faraday shield on wikipedia:


A booster bag (shopping bag lined with aluminium foil) acts as a Faraday cage. It is often used by shoplifters to steal RFID-tagged items.[4]
Faraday cage - Wikipedia, the free encyclopedia

[printer2]

So we were checking out some of the building systems at work. We were in a complete copper shielded room, all the joints of the sheets of copper were soldered together. The door was open but we were fairly deep inside it. I thought what the hell, tried my phone and called our PM coordinator. The reception was not great nut I got him. He was not impressed as he was 10 feet from me.

[Austin]

So we were checking out some of the building systems at work. We were in a complete copper shielded room, all the joints of the sheets of copper were soldered together. The door was open but we were fairly deep inside it. I thought what the hell, tried my phone and called our PM coordinator. The reception was not great nut I got him. He was not impressed as he was 10 feet from me.

It bounced around in there until it got out the door that was left open I imagine. Very cool. Does your phone get any reception when the door is closed? Is the copper shielding earthed somehow? Is it copper sheet?

[sgelectric]

This is a 1982 Keyboard magazine interview of electronic music pioneer Wendy/Walter Carlos done by Robert Moog.
In its discussion of her new studio, she speaks of the Faraday shielding in the walls/ceiling/floor and its impact on the perceived S/N ratio of the studio's equipment.
Interesting.

http://www.wendycarlos.com/other/PDF...erview-82*.pdf

SG

[printer2]

It bounced around in there until it got out the door that was left open I imagine. Very cool. Does your phone get any reception when the door is closed? Is the copper shielding earthed somehow? Is it copper sheet?

I did not get to try it with the door closed. It was coffee time.

It is a heavy copper sheet fully grounded, conductive door seals, I really should get back to check it out before everything is finished. Had some pictures of it but not really sure where they are now.

[Enzo]

Well, in defense of you cell phone, that 10 foot call did not go direct. You phone connected with a cell tower nearby, which then forwarded your call to a cell transmitter, which connected to his phone.

[Austin]

I wonder if there was anymore of a delay or echo from the time lapse of thousands of times the signal could have bounced around in that room before finding the front door. Kinda like a aluminum lined cardboard thing that comes on a "Hot Pocket" pizza snack that you use over the hot pocket to toast the bread when you microwave it.

[Enzo]

Time lapse? Electromagnetic waves propagate at the speed of light. They will go all the way around the earth seven and a half times in a second. A few feet would be darn hard to measure.

Your cell phone does not send out a directional tight beam that would bounce around looking for an exit like a bird in the house. The phone emits radio waves in all directions, some of which would go right out the door. MUch like the stereo in the other room comes right through the door into the kitchen rather than bouncing around a few seconds before excaping.

Here is a classic, Grace Hopper, one sharp lady. She talks about nanoseconds:
Grace Hopper - Nanoseconds - YouTube

[Tom Phillips]

Great illustration. A reminder of why wire length and equal length of each bit path on the bus became important when computers started to get fast,