Shielding effect of chassis

[Ripthorn]

So I am about to put my little amp all together in the chassis, but I have one remaining question. I made my own chassis and it has no underside (all the trannies mounted on top side). I was wondering if this will allow any hum from the trannies to get to my circuit or if I will be okay. I figure I can test it later on, but I want to try to get some feel for what kind of work might be involved there. Thanks.

[Cephus]

So I am about to put my little amp all together in the chassis, but I have one remaining question. I made my own chassis and it has no underside (all the trannies mounted on top side). I was wondering if this will allow any hum from the trannies to get to my circuit or if I will be okay. I figure I can test it later on, but I want to try to get some feel for what kind of work might be involved there. Thanks.

The following is what I have learned through my own experience and through others' help.

Is your chassis steel or aluminum? Though I've never experienced a problem, magnetic interaction can supposedly take place on a steel chassis since steel is magnetically conductive. Aluminum does not have this property. I've used steel successively for amps without any problem, however.

Ideally, power and output transformers should be aligned 90 degrees to each other to prevent potential coupling, or at least spaced as far apart as practical. Also avoid placing your transformers too close to the preamp stage.

Cephus

[Sonny ReVerb]

I take a piece of aluminum roof flashing and attach it to the cab with contact adhesive underneath where the chassis will be mounted. It makes a cheap lid that seems to do the job.

[Structo]

Also if you use a choke and also on the output transformer, I have seen guys route the wires from those two over the top of the chassis to where they need to enter the area where they are connected to the circuit.

In other words, the holes for the wires don't necessarily have to be right below the choke or OT.

They can be drilled above where the connection is and run on top of the chassis.
This supposedly helps prevent any induction of noise or hum into the signal.

[solderstain]

Ideally, power and output transformers ... spaced as far apart as practical. Also avoid placing your transformers too close to the preamp stage.

The concept of placing the PT as far away from the preamp as possible is clear. But what harm does a surface-mounted PT, on the outside of the chassis do if it's near the preamp? There are any number of amps that do that - mount the PT and the OT on opposite ends of the chassis. I can't imagine that an OT would produce enough of a field to interact with the circuitry on the opposite side of a metal chassis. There are plenty of people positioning the OT near the preamp. Do you mean just the PT?

[Kregg]

The way I see it, it isn't that big a deal. And it adds to the tube mystic. The only reason I see for using more aluminum (or copper) is not to shield, but, to dissipate heat away from valuable electronic components, as well as, combustible surfaces.

[Phil_S]

I'm even cheaper than Sonny. I use heavy duty aluminum foil, the kind you find in the kitchen. When there is nothing else to attach it to, I smear glue on cardboard (the corrugated type from a box) and put the foil on that. I often use a piece of scrap #2 pine board. If you are putting it in a cab, you can attach it to the cabinet interior.

What's needed here is a shield from RF. Without it, you might get lucky, but you can also pickup radio, florescent, and other interference. I found that placing the amp on top of a speaker cab requires a shield, otherwise you get trash coming into the amp from the speaker magnet.

BTW, you are less likely to cut your hands on kitchen foil than flashing, thought flashing is certainly nice stuff. I've got a roll I save for special occasions.

[Andy Le Blanc]

Aluminum tape, you can put a small transmitter on a cigar box, with aluminum tape inside.

[Structo]

I used some Stew Mac 2" copper shielding tape on my D'Lite head cab for shielding the open side of the chassis.
I just stuck it to the top of the cab.
Used a few staples to make sure it doesn't come loose.

I dabbed some solder around on the tape overlaps to ensure zero ohms.

But, I couldn't really tell a difference before or after doing it.

[Andy Le Blanc]

I was in an Ampeg recently, the internal shield made removal of the chassis a real joy.
Most hum I get is from the instrument pick-up coupling with the PT. Even with
very primitive chassis, I've seen some marshals than have nothing on the
ends and no other protection, with very few issue. In a Rivera, I've seen
very strategically placed tape under the verb tank and above the chassis.
Depends upon the circuit and application.

[Ripthorn]

Thanks for all the replies. I do have the transformers mounted 90 degrees, spaced far apart, not over the preamp, wires running on the outside, choke mounted 45 to the PT, etc. I just wanted to know about having the open side of the chassis. Aluminum foil was my first thought, and I am glad to see that it has been used. It might be funny for people to see aluminum foil inside a nice mahogany cab, but I don't mind that much.

Again, thanks everyone for the replies, they have been very informative and helpful. I think i will go with foil or something.

[Phil_S]

...Aluminum foil was my first thought, and I am glad to see that it has been used. It might be funny for people to see aluminum foil inside a nice mahogany cab, but I don't mind that much...

And there is no functional difference between the shiny side and the dull side. Also, what I do is goop the cardboard or wood with the glue, spread it with a scrap of cardboard used like a squeegee, and then take a rag to rub the foil in place. If you want a neat application, lay it out flat and cut it with a razor knife or tear it carefully along a metal straight edge. You can get it to look nice enough that no one will think twice about it.

[Structo]

I had limited mounting options on this Hammond organ amp conversion.

It doesn't hum.

[IMG:800:600]http://i23.photobucket.com/albums/b392/ ... G_0020.jpg[/img]
[IMG:800:600]http://i23.photobucket.com/albums/b392/ ... G_0092.jpg[/img]

[Phil_S]

Tom,
Nice shoehorn job there!
Phil

[M Fowler]

Tom,

I like it and makes me want to get my organ amp going. From what I have heard the organs were overpowered and make great guitar amps is that right?

Marrk