Capacitor outer foil test device

[pdf64]

My guess is that the construction of the particular cap type you have been testing is such that it does not have an outer foil.
Even with those that do, the difference can be negligible, or more related to how tightly the cap is being held.
Try some different cap ranges and values.

[FUCHSAUDIO]

An old departed mentor taught me a cool NYC studio trick: Take an old tape head (from a tape recorder) and put it on a piece of coax. Connect a cap across a generator, and then use the head to "sniff" the field around the cap with a scope or AC voltmeter. So, if the outer foil is grounded, you would have lower signal picked up, since the outer layer forms a shield. I've never heard any noticeable sonic difference in the circuit when carefully picking cap orientation fwiw. This can also be used to detect transformer magnetic fields and/or wires radiating signal as well. I assume the frequency wants to be high, maybe 5 or 10-K, which could make things easier to detect.

I did have a few early amps that were unstable at high gain (oscillated) which I used copper foil wrapped around a cap and grounded, to remedy. Worked like a charm.

[pompeiisneaks]

In my testing of differing caps, some have negligible noise on either side, making detecting outer foil quite difficult.

The larger the capacitance, the more it seemed to pick up the noise.

OTOH, I've stopped doing this on builds due to it being a significant time sink, with, to my ears, imperceptible difference to the sound of the amp. Others disagree with this

~Phil