Bottlehead Forum
General Category => Technical topics => Topic started by: dbishopbliss on December 03, 2011, 07:53:08 AM
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I've been hanging out in the guitar forums lately and I came across a post where someone was asking how to detect the outer foil of a cap (http://www.thegearpage.net/board/showthread.php?t=569125). I always thought that non-polarized caps weren't really directional and never gave much thought to it. However, this video (http://www.youtube.com/watch?v=VP6ZIHXq6Yg) has made me realize that there may be benefits. Subsequently, another poster added this comment:
The proper way to connect the outside foil is to the low impedance side of the circuit, which, in the case of coupling caps, will normally be the plate of the previous stage. If it is a bypass cap to ground, connect the outside foil to the grounded side. If it is a bypass cap from a signal to B+, connect the outside foil to B+. The outside foil will act as a shield against electric field coupling into the capacitor, so you want it to have the lowest impedance return path to ground.
Has anyone here considered this? I've just never heard it mentioned before except when asking about the leads of auricaps in the Foreplay.
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I have heard about this before but hadn't given it too much thought. I remember reading that some of the polys are marked. I just bought a couple of 100uf Solens for my Crack and I didn't see anything that would look like obvious markings to me. My guess it that you would have to email the manufacturer to know for certain.
Thinking out loud made me go get the Solens and examine them again. One lead is longer but there is no indication of what that means. Just downloaded the PDF from the Solens site and that didn't help either. I will send an email and see what they say.
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For some reason I only hear this when someone is using the Auricaps. Maybe because the manufacturer clearly states how to wire the caps.
Here is the link:
http://www.audience-av.com/capacitors/a_applications.php
I don't have a guitar amp to test with and the audio was such that I didn't understand his explanation anyway. But I do have a scope and I really followed that one.
Edit:
So I don't have to wait for the video to load again the oscilloscope method is to clip the ground and signal leads of one channel to the cap leads. Put an AC cord that is being used against the body of the cap. Read the scope, then swap the scope probe leads. Whichever orientation has the lower impressed voltage on it the ground lead is on the outer foil.
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Even though the guitar amp method is a little tough to follow, you can easily hear that there is more noise when the cap is oriented one way versus other. Next time I build an amp (actually I'm restoring a guitar amp now), I will be marking the outer foil end on my caps and installing them as described. It would be interesting to compare the two in an audio circuit.
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Usually, the shorter lead is the foil side, which you also want to try to get as close physically to the lowest impedance point. Generallly the foil is also related to the direction of the writing on the cap, but it's early, and I'm somewhat confused on what the rule about this is, so I won't even guess as to which end the writing goes to.
HTH,
Jim
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The writing is known to go in both directions, sometimes even on different models from the same company!
You can test it. Put some AC across the capacitor, and wrap a bit of foil or copper tape around the cap. Then measure voltage from each lead to the foil - no voltage = outer foil, some voltage = inner foil.
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No wonder it confuses me :-). How can you remember a rule when there is no rule?
-- Jim
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Some manufactures will mark a transformer's (or inductors) DOT with whether it should go to AC ground or AC hot. I haven
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. . . You can test it. Put some AC across the capacitor, and wrap a bit of foil or copper tape around the cap. Then measure voltage from each lead to the foil - no voltage = outer foil, some voltage = inner foil.
I love it. I'm a pragmatic person. So the oscilloscope and ac across the cap with foil on the outside makes a lot of sense to me. This is what I will be doing in the future.
Thanks to David and Paul for the good "how to."
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Could always just hack one open.
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I got an answer from Solen about how they mark their caps. According to Chris, the longer lead is connected to the outer foil and should be connected to the negative side or ground.
Keep in mind that this may not be a universal standard across manufacturers.
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Ok, so the bottom line here is... there is no standard, or if there is, compliance is voluntary :-).
-- Jim
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You can test it. Put some AC across the capacitor, and wrap a bit of foil or copper tape around the cap. Then measure voltage from each lead to the foil - no voltage = outer foil, some voltage = inner foil.
Sorry to resurrect an old thread, but I'm wondering about the results I got: I put 15VAC across the cap with foil wrapped around it, and tested the VAC from the foil to each lead. On one side, I got ~0.7VAC, and on the other ~2.5VAC. Is the side with the lower voltage the outer foil?
Thanks!
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This was an interesting vid
http://youtu.be/BnR_DLd1PDI
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I seen that video posted on DIYAudio the other day and built one just like it. It doesn't do anything you cant do manually with a scope, except for one scenario. Sometimes you will find a capacitor where the scope trace signal difference is very small between ends, to the point you cant tell the difference by the time you disconnect the leads and flip the capacitor around. With this tool it switches ends every few seconds which makes it much easier to detect the difference.
Its actually so easy to use i went through my entire cap stash and marked every single one of them in no time. The end of the capacitors with the outer foil was completely random for just about every cap, orange drops, red drops, misc polyfilm, known audio brands, and Russian surplus caps. The only exception where caps that had the outer foil marked, vintage caps, and Auricaps, all of which were marked correctly as you would expect.
Parts cost is ~$3-4, enclosure was $6 i think from Mouser, its handy, and it works.
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This was an interesting vid
I don't think he has enough test gear to be trustworthy...
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I've known guys with that much test gear and more. I've had five times that, mostly inherited, in the past. It took years to get rid of it all. It's a particularly odd obsession. Usually half of it is obsolete and doesn't work right and they really only use about 1/10 of it.
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Paul,
Did you count the oscilloscopes? I see 11.
The one he uses is the same Tektronix that I used in school. It is tubed.
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I don't think he has enough test gear to be trustworthy...
I just happened to see his vid on his lab about an hour before I checked this post. He has a few rooms going.
http://youtu.be/7A_NqNyvaBM
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I'm impressed that all the stuff works. I still suspect he never uses some of it.
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The vintage bug bites softly at first.
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Paul,
Did you count the oscilloscopes? I see 11.
Yeah, you really need that 12th scope to get the job done!
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This might be that 12 th scope. Looks like a beauty too. Over 100 vacuum tubes.
http://youtu.be/BvbNBZX6kNE
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I know them well. I have many, many hours using them for measurements of all kinds.
They also came with a small roll of sliver solder (the "L" is not pronounced) inside them. Techs would pop them open and take the solder.
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I've always pronounced it without the "L" myself. I just thought that was how it was pronounced, until I began to notice that Canadians and some other English speaking folk pronounce the"L".
Now I cannot pronounce the word at all.
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I should have said here we don't pronounce the L. Well some guys do and they also pronounce the L in salmon also.
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I pronounce it like the "American English" sample here http://dictionary.cambridge.org/pronunciation/british/silver (http://dictionary.cambridge.org/pronunciation/british/silver) which clearly pronounces the L to my ears. I'm confused?
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Solder, not silver. ;)
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Ooh right. Still cant hear it without an L, even when i say it with an american accent :)
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It's kind of like the way you put an invisible "I" in aluminum.
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lol
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I did measure a number of cap and used a wall wart w/ 19VAC. I attached the red lead to the center and the black to the outer portion of the plug. Then attached the leads to the cap and measured the voltage from the foil to each lead. The red lead had consistently lower readings. If I swapped sides with the clips, the side that previously had the higher voltage (black lead) now had the lower voltage w/ the red lead. Anyone else notice this?
Am I missing something?
Thanks.