Opinions.....outside foil

[Enzo]

Fender, peavey, Marshall, etc certainly do not bother to test the caps for foil. And the boutique guys? For them it is more important to do it so they can SAY they did it in their marketing, rather than any meaningful effect.

For scoping this, if you hadn't done it, make sure your probe is on X1 instead of X10, and turn the scope vertical gain all the way up. That is pretty sensitive, in my shop I can usually pick up a local AM radio station signal. Watch the little modulation envelope.

[R.G.]

The music world seems to be a three-way race between the princess and the pea, the blind men and the elephant, and the emperor's new clothes.

[R.G.]

Er, I mean... ... it's always possible to MAKE an outside shield/foil layer. Just paint the outside of the cap with glue and wrap metal foil over it, taking care not to short to **either** lead, then wrap/glue a drain wire over the foil. You can now take the drain wire to any voltage you want to shield the cap.

[R.G.]

But it may be simpler to take two feet of aluminum foil from the roll, and carefully fold it into a close-fitting hat. This will keep the harshness rays from bad sounding music from getting to your cerebral cortex.

Makes elevator music sound better too.

[trem]

Enzo -
No...I Did Not have the probe on x1.....will give that a try. Thank You

R.G. -
I think you forgot:
"Take my wife... Please......."

[R.G.]

I don't get no respect.

[g1]

Just paint the outside of the cap with glue and wrap metal foil over it,

You can also use gum and the foil wrapper it's conveniently supplied in. If you haven't already had to use it as a fuse.

[GainFreak]

Er, I mean... ... it's always possible to MAKE an outside shield/foil layer. Just paint the outside of the cap with glue and wrap metal foil over it, taking care not to short to **either** lead, then wrap/glue a drain wire over the foil. You can now take the drain wire to any voltage you want to shield the cap.

I actually did that once. There was a signal leak from the drive channel. Accidently I found out that when I touch one of the caps the leak decreased to a great extent. I used a copper foil striped from a PCB and soldered it around the cap, then grounded it.

[R.G.]

That's actually a great observation. What a capacitor does in a circuit probably limits whether an outer foil layer is useful or not. A coupling capacitor is working on the theory that the capacitor impedance is low enough that vanishingly little voltage is dropped across it. So the two plates are linked by the smallest of the impedances involved. A filtering cap will have some voltage dropped across it, and might be well served by a shield if the filter impedance is high.

A metallic outside shield that's connected to ground or AC ground and not one of the cap pins becomes an actual shielded path with the addition of the third lead. This probably offers shielding that no outside foil application would give.

I'll have to go think about this for a while.

[GainFreak]

To be more precise the cap in question was the PI input cap (6PS type 22nf OD) and it was already tested for outside foil.

[Malcolm Irving]

On very old non-electrolytic caps the outside foil end was marked by a printed band - so it must have been regarded as important, but perhaps that was just for Radio applications.
I'm guilty of building a small metal box with a change-over switch, two croc-clips to connect the cap, and a jack socket. You plug it into a high gain amp, hold the cap between your fingers and flip the change-over to see which end picks up more hum. It works well, but I have to admit I haven't done any tests to see if orientation of the cap in an amp makes any audible difference. I've noticed that for caps such as Mallory some have a clear difference between inner and outer foils (on my home made box) and some do not show any difference. I wonder if the two foils are sometimes rolled in such a way that each foil is 'outer' for about half the circumference - haven't taken one apart to look (yet)!

[trem]

On very old non-electrolytic caps the outside foil end was marked by a printed band - so it must have been regarded as important, but perhaps that was just for Radio applications.
I'm guilty of building a small metal box with a change-over switch, two croc-clips to connect the cap, and a jack socket. You plug it into a high gain amp, hold the cap between your fingers and flip the change-over to see which end picks up more hum. It works well, but I have to admit I haven't done any tests to see if orientation of the cap in an amp makes any audible difference. I've noticed that for caps such as Mallory some have a clear difference between inner and outer foils (on my home made box) and some do not show any difference. I wonder if the two foils are sometimes rolled in such a way that each foil is 'outer' for about half the circumference - haven't taken one apart to look (yet)!

THAT would be an interesting thread. I would be happy to donate a few caps to anybody that would like to document it with some pictures.

[Mick Bailey]

I've certainly had instances where a capacitor has picked up excessive noise, and has been over-sensitive to hand proximity. The last time this happened I did end up wrapping the (cylindrical) cap with mylar/copper foil and grounding that to get an improvement, though it wasn't as drastic as I hoped. Orienting either way didn't have any effect.

That was in one of my own designs, so really the shortcoming was in that particular layout or implementation. If there was a case for screened caps I guess there would be plenty on the market. A film cap in a metal can with an earth pin, eagerly lapped up by boutique builders. Maybe the military or aerospace industry has these to protect against EM pulses or interference.

Having said that, I used to work on ex-airforce aerial reconnaissance telephoto equipment from the 40s/50s that used grounded cans on paper caps.

[cyclone]

I do it, simply because its an easy thing to do and I can easily see the difference in noise on the scope when testing each cap. Some brands are way noisier than others when reversed on scope. Mallory 150's show very little difference IME. I typically use axial caps, 150's, Mojo's, Cornell Dubilier brands. I've never built an amp with all of them backwards so I don't know if there is any discernable audible effect, but it is quite visible on my 50mhz scope when testing individual caps. I use the Randall Aiken method. I've never built an amp that wasn't dead quiet. Remember to point the outside foil to the closest route to the power supply, whether that be the plate supply or the return. Tone control caps

I test the whole bag when I get them and put a dot on one end with a marker. It may be a waste of time but its only a little time wasted.

[R.G.]

A grounded shield outside a cap is quite a different animal from wondering which of the two leads connects to the outside foil.

A grounded external shield intercepts external noise that would otherwise be coupled in, and shunts it to ground. It also makes the capacitance to ground somewhat larger (if neither of the cap leads is grounded) and essentially constant.

Knowing which of a capacitor's leads connects to the outside foil may or may not be of use. The question is which way do you turn the cap? If a coupling cap goes from a plate to a grid, is it quieter with the outside foil connected to the plate side, or the grid side? Is it quieter if it's in line with the closest mechanical distance as the crow flies (assuming you could get a crow to fly into an amp) at 0 or 180 degrees, or perhaps 90 or 270 degrees to the shortest path? Is it the direction relative to something else that matters, not where the signal flows? If it's oriented with respect to the power supply, that seems to indicate that the orientation in the circuit is a minor issue, and quieting the power supply might produce even better results. Or should it simply point east? west? North? Sou'est?

If there's more noise on the "outside foil" lead of a cap with a 50MHz scope but it's not audible, (1) does it matter? and (2) is the (presumably RF) still there with the chassis/case buttoned up and shielding?

Is there any plausible and measurable effect to work on that might even produce a Theory of Outside Foils?