Signal Capacitor "polarity"

        I recently read an article in Vacuum Tube Valley (http://www.vacuumtube.com/) magazine, issue # 11, that dealt with this subject… I was skeptical about such a claim at first. I’ve always known that electrolytic caps had to be ‘polarity oriented’, but never gave signal caps a second thought when installing new ones in amps, stereos, TVs, etc. Well the article explained some experiments with several different types of caps; Illinois Cap MWR - metallized polyester, Nichicon Metallized, 716P SBE Orange Drops, Hovland Musicap - polypropylene film and foil, and 2 others that I wasn’t familiar with.

         I had to know what the idea behind this article was so I called Charlie Kittleson at VTV and asked if this really was a ‘concern’ about ‘signal transfer’, and if it actually affected tone in any way. He said it had a lot to do with tone, and asked me how many Fender amps I had. I’ve got about 7, and he asked me if I had time to try an experiment with one of the amps, and to check all the signal caps and their ‘polarity’.

      The signal caps should be wired so that the inner foil points toward the output and outer foil points toward the input. The way this is figured out is this: the outer foil was sometimes identified with a printed band on older caps and normally is on the left side of the capacitor when reading the printed information on the cap. If you wire the outer foil to the output side, the sound will be muffled and rolled off. If in doubt, try reversing the leads. Also, avoid using the same brand and type of capacitor throughout the amplifier. Mix and match capacitor brands and types (oil, film, paper, etc.) like you would add spice or seasoning in your favorite dish.

          ....the reason why the inner foil is connected to the grid of the output tube:
1. Impedance and reactance are different.
2. This in turn, has an effect on the frequency response going through the cap.
3. In general, most inner foils are on the right side of a cap when reading it. When caps are unmarked, you can run a high frequency pulse through it and measure the response on a scope. The best response is indicated by hooking up the positive probe of the scope to the inner foil.

             Well, I ‘popped the hood’ on my BFSR and checked all the caps - guess what? 3 of the signal caps were ‘backwards’… I reversed them after listening to the amp for about 15 minutes, and played through it again. The highs were crisper, the mids came alive, and the amp sounded better!!! I couldn’t believe my ears. If you, too, are skeptical try it on your amp; it DOES make a difference!

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For more on this, check out Randall Aiken's page:  http://www.aikenamps.com/OutsideFoil.htm

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         I’ve learned a lot from Weber Amp BB (http://www.webervst.com/vstbbs/bbs.html), and a bunch from the first 14 issues of VTV (http://www.vacuumtube.com/) also. There are articles like this ‘cap polarity’, history and a featured story on particular tubes in each issue, and reviews in what’s going on in the ‘Tube community’. Each tube featured has a sonic evaluation in both stereos and guitar amps, and how they fair in both application. I did a review on the Allen Guitar Amp Kit ( http://www.allenamps.com ) , it is out in Vacuum Tube Valley issue 12, and I contributed to a 6L6 shootout in issue 13, and a 12AX7 shootout in issue 14.

-Ron (Uncle Spot)

                                                                    Last updated: 1 Nov 2000

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