You lost me. Why is it pointless to shield that which offers no shielding ?Funkalicousgroove wrote:If you are using an aluminum chassis this is a pointless exercise since Aluminum is impervious to magnetism, and offers no RF shielding.
You would almost need to have your aluminum coated with alodyne for the shielding to have any effect.
Shielding for inside of head cab
Moderators: pompeiisneaks, Colossal
Re: Shielding for inside of head cab
Re: Shielding for inside of head cab
Dense here...are you guys talking about the plate/sheet or screen to cover the open side of the chassis, or KOC's notion of running sheilding everywhere you can inside a head?
Re: Shielding for inside of head cab
I was talking strictly under the chassis.
There's two kinds of shielding that's important to amp builders.
Magnetic shielding helps keep stray magnetic fields out, like those produced by a motor or a power transformer. Only iron-based alloys help here, because other electrical conductors do not conduct magnetism well.
Mild steel of the type generally used for amp chassis does not help a lot on this score, but it works better then nothing. Special iron-based alloys like the materials used in transformer cores or the mu metal used to shield microphone input transformers works much better, but are heavy, weak structurally, and expensive.
Electronic shielding helps to keep out the rest of the electromagnetic spectrum of noise, including AC hum floating around in the air. Any electrical conductor (steel, aluminum, copper, ...) works well for this purpose. The copper shield in your guitar lead is the example we all are very familiar with. Try using an unshielded speaker cable and compare the difference in hum!
Another experiment, remove the tube shield from V1 in a higher gain amp, and put your finger close to it. You'll hear the hum. Put the shield back on, and the hum is greatly reduced with the same finger in the same position.
This shielding is the reason it is good to put some sort of metal under the chassis, and it can be made of any good electrical conductor, as long as it is physically well connected to the chassis.
--mark
There's two kinds of shielding that's important to amp builders.
Magnetic shielding helps keep stray magnetic fields out, like those produced by a motor or a power transformer. Only iron-based alloys help here, because other electrical conductors do not conduct magnetism well.
Mild steel of the type generally used for amp chassis does not help a lot on this score, but it works better then nothing. Special iron-based alloys like the materials used in transformer cores or the mu metal used to shield microphone input transformers works much better, but are heavy, weak structurally, and expensive.
Electronic shielding helps to keep out the rest of the electromagnetic spectrum of noise, including AC hum floating around in the air. Any electrical conductor (steel, aluminum, copper, ...) works well for this purpose. The copper shield in your guitar lead is the example we all are very familiar with. Try using an unshielded speaker cable and compare the difference in hum!
Another experiment, remove the tube shield from V1 in a higher gain amp, and put your finger close to it. You'll hear the hum. Put the shield back on, and the hum is greatly reduced with the same finger in the same position.
This shielding is the reason it is good to put some sort of metal under the chassis, and it can be made of any good electrical conductor, as long as it is physically well connected to the chassis.
--mark
Re: Shielding for inside of head cab
Seems like lead foil is the way to go. First it would aid in electronic shielding. Second, it would keep Superman's prying eyes out of your amp guts!!!! 
Re: Shielding for inside of head cab
Regular Heavy Duty aluminum foil from your kitchen -- just handle it carefully, smear a light coat of glue on the surface, and press it down with a rag between your hand and the foil. You really only need to shield the underside of the chassis. Cuts with a scissor, easy to get, and cheap.
Re: Shielding for inside of head cab
In my case, I needed something that grounded to the chassis. Since steel and aluminum can have a reaction, I used aluminum. It killed the noise that I had.Funkalicousgroove wrote:If you are using an aluminum chassis this is a pointless exercise since Aluminum is impervious to magnetism, and offers no RF shielding.
You would almost need to have your aluminum coated with alodyne for the shielding to have any effect.
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statorvane
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Re: Shielding for inside of head cab
theundeadelvis wrote...
I think for an amp, to ground the RF, conductivity is the key.
Well, the only magnetic paint I've dealt with exhibited a significantly higher resistance than say foil. I recently installed a Sustainiac in my son's Ibanez, and the instructions specified not to shield the neck pickup cavity (where the driver unit is located) to prevent squeal. Well his cavity was shielded with paint. I measured the resistance - pretty high. From the neck cavity to the bridge I got something like 1.3K ohms. I did not remove the paint and it works great - no squeal - controllable endless sustain. I don't know how well it would have worked it had a better, lower resistance shielding. Maybe the intent is to keep ferrous metals away from the driver.I've always been curious if magnetic paint would work.
I think for an amp, to ground the RF, conductivity is the key.
Re: Shielding for inside of head cab
+1 Use 3M Super 77 and glue on heavy duty BBQ tin foil. Not the toughest stuff, but easy to work with and does sheild away nasty AC buzz.Phil_S wrote:Regular Heavy Duty aluminum foil from your kitchen -- just handle it carefully, smear a light coat of glue on the surface, and press it down with a rag between your hand and the foil. You really only need to shield the underside of the chassis. Cuts with a scissor, easy to get, and cheap.