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Outside foil on Capacitor


Where To Connect the Outside Foil on Capacitors

Some non-electrolytic capacitors have a banded end, occasionally labeled "outside foil". These capacitors are typically made by taking a long narrow strip of insulating material and placing a strip of metal foil on both sides of it. The two pieces of foil become the plates of the capacitor, and the insulator is the dielectric. This long strip is then wound into a cylindrical shape, leads are attached to the two foils, and the entire assembly is then potted in some type of material designed to keep moisture out of the capacitor and to keep the capacitor mechanically stable. Since the capacitor is wound into a cylindrical shape, one of the foil sides is on the outside, and the other is on the inside. The outside foil terminal connection is then marked with a band to indicate the outer foil position.
Why is the outside foil marked?

Why do the capacitor manufacturers go to the trouble of marking the outside foil with a band? Aren't electrolytic capacitors the only ones where polarity matters? While it it true that polarity on a non-electrolytic capacitor doesn't matter, signal-wise, the outer foil is marked because it can be used as a shield against electric field coupling into the capacitor. In order to take advantage of the shielding properties of the outside foil, the capacitor must be connected in the circuit in a particular orientation.
Where to connect the outside foil?

The proper way to connect the outside foil is to the low impedance side of the circuit, which, in the case of coupling caps, will normally be the plate of the previous stage. If it is a bypass cap to ground, connect the outside foil to the grounded side. If it is a bypass cap from a signal to B+, connect the outside foil to B+. The outside foil will act as a shield against electric field coupling into the capacitor, so you want it to have the lowest impedance return path to ground.

For Mundorf Capacitor
the shorter lead also indicates the capacitor's outer foil.
The side with the short lead is the outer foil side of the capacitor and should be connected to the plate side of the 12AT7s, pins 1 and 6.point 33, 35, 39, 41

 photo MCap_Evo_Details_zpswtyy6oi4.jpeg

See http://www.aikenamps.com/index.php/where-to-connect-the-outside-foil-on-capacitors
Thank you Enrique for this important info..

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Edits: 12/27/16 12/27/16 12/27/16 12/27/16

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Topic - Outside foil on Capacitor - vkung 12:07:23 12/27/16 (1)


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