Choosing caps

[RC cola]

When I look at catalogs for electrolytic capacitors I get rather overwhelmed by all the different specs until I just give up. Whats the basic spec to look for with replacing the cathode caps and the filter caps? I have a Gibson Scout with a gawd awful hum with scratchy pots. Other than that it sounds really good. All these caps are 20 uF at assorted voltages.

[Enzo]

Well, basically you start with value and voltage.  Unless you have a good reqason otherwise, stick with the value intended.  You can always use a higher voltage cap than you need, but the higher the voltage, the physically larger they tend to be, so there could be issues with space.   If you are looking for a 500v 22uf cap, if you are looking at a place like Mouser, the selection filter should seriously reduce the number of possibilites with that voltage alone.   If you are shopping somewhere like Antique Electronics, they may actually have a better 500v cap selection than Mouser, because it is a sort of specialty item.  SO if you really needed a 22uf at 450v, a 500v one would work fine too.

They come in shapes.  Axial leads means one wire out each end.  These caps would be laying down on the circuit board, like parts on an eyelet board in an old Fender.  The other style is radial leads.  That means both wires come out the same end.  These caps are intended to be mounted on a circuit board standing on end.  In a pinch we can usually fudge the "wrong" style into an amp, but it is better to use what was intended.

Sometimes size matters.  Sometimes it matters in caps too.  If I have a radial cap standing on a circuit board, I better make sure it isn't so tall it won;t let the lid fit back on the amp.  So we sometimes need to watch dimensions.  Mostly that isn't so much of an issue.

[RC cola]

I was too vague with my question. Those basics I know about it was other specs listed I'm not sure of. For instance I see low ESR, high ripple, low ripple, suitable for high frequency applications, high speed switching power supplies, general purpose and so on. Is there some intended use for a cap that could make it not suitable to be used for basic filter and cathode bypass situations?

[J M Fahey]

Start with "general purpose" specified, you should have no problems.
The extra data becomes indispensable in SMPS, switching in general, high frequency work and such, but for plain old classic circuits ... not that much.

[RC cola]

I ordered some caps and what I got does not matchthe data sheet description. I have never seen a cap looking like what was sent.

I ordered some axial 22uf 25 volt Vishay with a description typical for an electrolytic, metal can with plastic overwrap, series 138 AML type. This description is on the data sheet and not just going by a pic for the part listing from the supplier.

What I got looks like the case is a hard plastic, no overwrap with the value 22uf 25 V printed directly on the case, a black band on one end, the other end has a slight bevel around the outer edge and I see what looks like 138 KO printed on it. Its an odd tannish gray color. Its an axial type.

I searched the Vishay site for a Series 138 K but nothing like that shows up.

Anyone familiar with this part?

[joecool85]

Can you post a picture of the cap you got?  Also, where did you order it from?

[J M Fahey]

Never seen one like that either, but it should work.
Vishay is (I think) the largest capacitor manufacturer in the World, and they may have changed production methods, so why not?

[RC cola]

I got them from Mouser. I called last night and the teck said it was a polarized cap, black band is the negative end. Its a longlife according to him.

[joecool85]

I got them from Mouser. I called last night and the teck said it was a polarized cap, black band is the negative end. Its a longlife according to him.

What's the mouser part number?

[RC cola]

594-2222-138-36229

Thats the part number.

[Enzo]

The part is what you want.

Here is the data sheet:
http://www.vishay.com/docs/28332/138aml.pdf

Here is the Mouser entry:
http://www.mouser.com/ProductDetail/Vishay/MAL213836229E3/?qs=cAZGeXF9iMp%2fuSAYTbSq8w%3d%3d

You can see in the data sheet and the Mouser photo, one end has a constriction in the body.  That is the positive end, regardless of the color of the skin.  The data sheet shows a common labelling of a lighter body with a dark stripe, and the stripe is full of negative signs.  The stripe is really a series of arrows pointing towards the negative end.  But there are various other paint schemes in use on caps.

These are aluminum e-caps, so the body is most certainly aluminum.  The plastic coating is probably shrunk on even if it doesn;t look like it.  I haven't seen a molded plastic one since those awful white Mallorys from 30 years ago.   I suppose it could be dipped, but I doubt it. 

Shrink plastic sleeving can be preprinted, then fed around the caps at high speed on the fly, and shrunk into place.  Printing directly on the part requires more complicated handling in the factory.

Overwrapped does not mean some sort of baggie packaging.


Low ESR, high ripple, high temperature, etc etc. are all more capable caps than the run of the mill.  The extra capability doesn't make it work different, it means it will work in more demanding places.  Like an extra heavy duty extension cord.  Your table lamp just needs a puny little extension cord, but using a big 10ga cord won;t affect the table lamp.  It just means if it had to, that heavy cord could run some 10hp motor, while the little cord could not.  Same with your caps.  The ones that will work boiling hot will also work at the temperatures in your gear.

Little cathode bypass caps are about the least stressed parts in an amp.

[RC cola]

Look at the data sheet, second page under the selection chart. The left side graphic is what I got. Its a solid material with no heat shrink overwrap like aluminum can types are done.

I will try to get a pic of one posted soon.

[Enzo]

I stand corrected, I haven't gotten any of that form yet.

But what the sheet is telling you is that the 138 series can come in at least those three forms.  You got that form on the left instead of the one on the right.  Electrically it shouldn;t matter.

[RC cola]

I missed that graphic when I was looking to see if I had some how put the wrong part number in when I placed the order. The tech at Mouser pointed it out to me when I called. He even seemed puzzled when I first described the cap to him.

[RC cola]

This is the cap I was speaking of.