Shielding question.

[Boy Howdy]

A friend who is a tech advising me on a recent Traynor re-build said not to worry about the lack of metal foil over the chassis (you know, the shielding stapled to the inside of the cabinet). On another project (tube preamp) a similar problem/question came up, and a similar answer was offered.

This makes me realize that maybe my assumptions about shielding are in need of an update.

So my question: Is it unnecessary to totally enclose all the electronics within a grounded shield in order to get the quietest (less hum/buzz) signal to noise ratio? And if complete enclosure is unnecessary, what exactly are the requirements?

I'm interested as pertains to guitars, amps and effects here.

Thanks, ya'll.

[defaced]

Test the theory and see what happens.

On my bench, I hear a noticeable difference between having a grounded copper screen over the chassis and when it is removed. This is possibly because of the fluorescent lights in my room, but I've not really tested it. If you want to see if it makes a difference in your application, grab a conductive sheet of something (aluminum foil), ground it to your chassis, and see if you can hear a difference in the noise floor. You will want to isolate one side of the conductive sheet from accidentally touching the components on the inside of your amp and causing a short, I use a piece of paper to do this.

[Enzo]

It is not necesary for the operation of the amp, but it does decrease noise. And considering the extreme expense of slapping some glue and a hunk of tinfoil inside the amp cab, why not do it anyway?

[tubeswell]

Depends how many EMR sources there are in the vicinity of where the amp is and how strong they are. The chassis itself performs a key shielding function (esp if it is steel). However (even in an Aluminium chassis), the signal input end of the amp is most vulnerable to stray EMR, and often if you just shield the cable from the input socket to the first stage(s) grid(s) and then the cable from the volume pot to the second stage's grid, it is enough to avoid the impact of most stray EMR around an amp from causing noticeable interference. As the signal voltages get stronger further on in the amp, they are usually more resilient to the effects of stray EMR (unless you get really strong stray EMR). So AFAICT lining the entire cab with metal foil doesn't do much that the chassis, and individual cable shielding hasn't already achieved. Shielding the guitar body cavity with a grounded shield will also help to cut out a lot of hum.

[GibsonLover]

I think the foil does more to dissipate chassis heat than the benefit you get from electrical shielding. I wouldn't doubt it's a UL requirement for that reason. For magnetic sources, foil will do little, but it may do better with dimmer switches and such, assuming you wouldn't have the same problem anyway bleeding into your pickups. For any reason, it sure couldn't hurt.

[TommyTomcat]

I wouldn't have an amp that didn't have the shielding. I forgot to put some in a deluxe reverb kit I built and was picking up all kinds of noise without the shielding.

[cbarrow7625]

It may be fine on your bench but what about down at your rehearsal space, or your buddy's house, orthe local bar, or the other local bar, or the other local bar, or the other local bar, etc.

You never know what you are going to run into. Shielding of any kind will help reduce noise, RF interference and if it is steel can even help shield out the magnetic field around AC power cables if that is a problem.

As Enzo said, why not just do it? The benefits outweigh the meager cost.

[Enzo]

In a Les Paul type guitar, the control cavity is lined with some shielding, and then the cover plate is covered in shielding, which makes contact with the rest of it when screwed in places. leave off that cover plate and the rear opening allows noise to get in. Not magnetic coupling, just noise.

If you leave one side of an amp chassis open, it allows noise in the room to enter the amp more readily. I'm not concerned that a magnetic field will couple in there, but those flourescent lights a few feet over the amp do concern me.

The foil is for shielding, nothing to do with heat. Peavey for example uses conductive paint rather than foil. The black paint shields but would do nothing to reduce heat.

[Boy Howdy]

Thanks, everybody.

I think my friend may have been unconcerned because he doesn't play out ... and the Traynor has a fair bit of shielded cable in it now.

But I questioned my assumptions (always a good thing, right?) about this subject when I started to think about all the instances of incomplete shielding I've seen, and all the work I could save by doing a similar (poor) job. I always wondered about Strats, for example, that just have that little bit of foil on the pickguard. And there are effects pedals that come in plastic or wood enclosures, etc..

No rest for the tweaker, I guess.

Thanks again.

[tubeswell]

Yep it definitely pays off doing a proper shielding job on yer strat. I did my one according to this:

GuitarNuts.com - Shielding a Strat(tm)

And it worked wonders, esp for stage playing (underneath all those PARs) - quiet as a mouse (But now I am verging on getting of the topic)

I still am of the opinion (from my humble experience) that adding foil to the inside of your amp cab provides minimal extra noise reduction benefit compared to the effect of shielded cables and the chassis and the amps own transformer covers. My tweed style amps don't have any lining on the back panel, yet I don't get any extranaeous noise out of them no matter where I happen to be playing. YMMV

(Altho' I can see why PV needs to use something with their minimalist chassis, which doesn't really enclose their PC boards).

[Enzo]

I don't understand. How does a 5150 chassis enclose the guts any less than a Fender Twin chassis? Your C30 has open ends, yes, but otherwise front, back and bottom are covered like any other amp.

Look at any of the relatively recent Fender SS amps - the ones where the rear panel is tall, but the front panel is normal size, so the chassis is a wedge shape. Open ends on those. And foil down the sides.

[tubeswell]

I don't understand. How does a 5150 chassis enclose the guts any less than a Fender Twin chassis? Your C30 has open ends, yes, but otherwise front, back and bottom are covered like any other amp.

Look at any of the relatively recent Fender SS amps - the ones where the rear panel is tall, but the front panel is normal size, so the chassis is a wedge shape. Open ends on those. And foil down the sides.

Yeah well okay - in those situations it does make sense to have some form of electrical field barrier. But I had a C30 type amp in mind when I said what I said before (- I guess I should have made that more explicit).

[Chuck H]

I am doing a project right now that has me in posession of two identical amps. One is shielded with foil where the chassis meets the cabinet and one is not. They are both quiet, unless... If there are any of the common offenders in the room like flourescent lights or dimmer switches, and in rooms that have a bazillion wall plates for all manner of media, the amp without shielding is definitely picking up more external noise than the shielded amp. As was noted before, it may be questionable how much advantage this method has over shielded cables inside the chassis. These amps have no shielded cable. But... Shielded cable does impart a capacitance on the element being shielded. This will effect the tone for better or worse. And since I have the benefit of two identical amps on hand with this one difference I would advocate shielding regardless of the use of shielded cable.

FWIW I just bought a 20'x8" roll of 3oz. copper "flashing" from ACE Hardware for US $38.00. Since it will probably be a year before I use it all it seems like a good enough deal just to have it on hand for all kinds of projects. This stuff would make an ideal shield where a chassis meets the cabinet.

Chuck

[jakehop]

FWIW: Most Carlsbro amplifiers come without this foil. I installed it, as I converted one to a SuperLead (Drake iron + Mustard caps) with some modifications, and it's practically noiseless when the guitar is turned down!

Jake