Cap Orientation Test

http://www.webervst.com/wwwboard/amps/messages/21211.html

Capacitor outside foil orientation test results – PLEASE READ THIS!!!

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Posted by Randall Aiken on November 02, 2000 at 21:10:01:

Just for grins, I set my signal generator up for it’s max 28V p-p output
signal (to simulate a reasonable output signal at the plate of a tube), and
connected the ground of the signal generator and the scope ground to one end
of one of my 0.1uF film/foil caps, and held the signal generator “hot” lead
next to the cap. I then set my scope to the 20mV/div setting, and put the
scope probe tip on the other unconnected cap terminal. I measured the amount
of signal present, then I turned the cap around, connecting the signal
generator and scope grounds to the other side of the cap, and the scope
probe tip to the side that was previously connected to the grounds and
re-measured. I was shocked to find that in the first position, there was no
discernable signal, but in the second, there was a 5mV p-p signal!

This test would indicate that the outside foil lead is the one that results
in no induced signal when the scope and generator grounds are connected to
it, because it is acting as a shield against the generator signal.

To put this into perspective, consider that most tube stages have a gain of
around 50. A 5mV signal induced into the grid of that stage would result in
a 0.25V signal at the plate. Amplify that a few more times and you get a
great big signal at the output. Of course, in most amps, there isn’t a big
signal sitting on top of the cap, but there may be one running near it. The
induced signal strength falls of with the square of the distance, so it
decreases rapidly, but it doesn’t take much to induce a hum or oscillation.

After playing around for awhile, I found an even easier method to identify
the outside foil – simply hook the scope probe up across the cap, set it to
trigger on the AC line, adjust to the 20mV per division setting (or the
lowest setting you have), and grab the cap tightly between your fingers.
Measure the amplitude of the induced 60Hz AC signal, then reverse the leads
and grab the cap again (better yet, keep holding the cap the same way and
reverse the leads). You will find a very large difference in the induced
60Hz signal from your fingers. The position that gives the lowest signal
level is the one you want. In that position, the scope ground lead is
connected to the outside foil.

I also found that larger value caps have a smaller induced signal, which is
due to their lower impedance at 60Hz, so this test is hard to see on film
caps above 1uF or so, unless you have a very sensitive scope.

This test has made a believer out of me, I’m going to test and connect the
outside foil of all my caps to the lowest impedance point from now on. This
is usually the plate terminal on coupling caps, or the grounded end on shunt
caps.

Randall Aiken

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