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Do you know the best way to shield the coupling cap ?

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The 300B amp I am building need a coupling cap of 0.33uF. When I place it closed to the 1st stage, it osclilate badly.

Now I have to completely take it off from the chasis to eliminate the osc. I have been told to shield the coupling cap with "Conductor sheet wrapping around the cap" and then ground the conductor sheet to the ground.

My question:

(1) What is the theory for using only conductor (aluminium, copper etc) to shield the coupling cap. My previous experience in reducing the hum interference from the Power tranny to Output tranny was to use a Z11 silion steel sheet (same effect as Mu metal) to wrap around the power tranny and it worked.

But only aluminium can do the job for the coupling cap? Is it a different secneraio ?

(2) I have just been told that the way to WARP around the coupling cap should not be wrap the cap closely, instead of wrapping the cap closely by the conductor sheet it should be isolated from surrounding component. The reason heard is:: The conductor sheet closely touch around the coupling cap WOULD affect the behaviour of how this coupling cap work the way it should be....

Can you share you idea, since I do not know what is the best way to solve this problem as my coupling cap right now is placed outside of chasis.

Regards

Eric

Follow Ups:

RE: Do you know the best way to shield the coupling cap ? - Swanson 09:10:41 07/01/12 (1)
The only place I have ever seen a shielded coupling
cap in an audio application was in the old German
table and console radios made by companies such as
Grundig,Blaupunkt and Metz.Usually they had copper
foil wrapped tightly around the cap and the foil
was grounded.Anyone who has ever owned one of these
radios knows they sound awesome but I would certainly
not call the sound audiophile accurate.
Would wrapping your coupling caps tightly with
copper foil and grounding it affect the sonics of your
amp?Who knows.You will just have to try it out and see
for yourself.You might just discover something interesting.
Regards,
Swanson
- RE: Errrrrr........ - Neff 18:07:07 07/02/12 (0)
  I advised Eric a few days ago on how to shield a capacitor. As per Thomas reply above, the stage gain is too high. I also advised Eric a few days ago to experiment by removing the cathode caps to reduce gain & likey improve the sonics.
  
  I have been involved with shielding from audio to RF frequencies since the 1960s. Nothing like being a ham radio operator keeping RF out of consumer products too or applying my knowledge with resolution on projects with General Motors, General Electric, Hewlett Packard & NASA to name a few. Actually quite simple.
RE: Do you know the best way to shield the coupling cap ? - Thomas Mayer 09:06:31 07/01/12 (1)
Hi!

It seems that there is some unwanted coupling from the driver to the input stage of your amp. I never had to shield a coupling cap. I'd rather suggest to find the reason for that coupling instead of trying to solve it with patchwork. Cascaded ECC99 stages have a lot of gain. Way toomuch to drive a 300B to my taste. A lot of gain makes the input stage much more sensitive to pick up unwanted signals

Best regards

Thomas
- RE: Do you know the best way to shield the coupling cap ? - Eric Chan 09:14:56 07/02/12 (0)
  I seem to have identified the coupling root cause (coupling cap), but the space left inside the chasis is limited so that I have to shield the cap.

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