Finding outside foil on new CDE and IC caps

[$10_LR-5555A]

I am trying to find the outside foil of a few different CDE and IC caps but no matter how I test them I can't find a difference either way... I have tried using an oscilloscope (HP 130C) and connecting to an amplifier to check for sound differences. No luck.. When connecting NOS Sprague 715P 0.002uf 1600DC cap to the oscilloscope or an amp I see an obvious difference on each side. When testing other NOS caps like 220P and red dipped ones marked with EM they also are different for each side.

Why can't I find the outside foil of these new CDE and IC caps? Are they made differently? The new caps I tried so far are:
CDE 715p 2700pf 1.8kvdc
CDE 715p 4700pf 1.8kvdc
IC PPA 0.047uf 2kvdc

This was my first time ordering new film caps so I am a bit confused.. I was just going to put them in without testing for the outside foil but I thought this would be a good exercise for trying to figure out how to use an oscilloscope. Now I am just curious about these caps.

 

[Buck Rogers]

Easy, You can use a guitar amp or a phono input.

Connecting the outer foil to the hot input will have much louder hum when you touch it compared to when the inner foil is connected.

Connect one lead of cap to hot plus input, touch outside of cap set volume, Connect other lead touch note volume.

Only connect one lead at a time. Louder one is outside foil.

Hope that helps.

Buck

 

[$10_LR-5555A]

Easy, You can use a guitar amp or a phono input.

Connecting the outer foil to the hot input will have much louder hum when you touch it compared to when the inner foil is connected.

Connect one lead of cap to hot plus input, touch outside of cap set volume, Connect other lead touch note volume.

Only connect one lead at a time. Louder one is outside foil.

Hope that helps.

Buck

I usually use the aux input on a headphone amp but I just tested with the phono and aux inputs on another amp and there is still no noticeable difference. I downloaded a dB meter app on my phone and with the phono input both sides of the particular 2700pf cap I picked up are at 73dB not touching the volume between. Testing with aux input was lower at 56dB on both sides. Ambient is 43dB.

 

[Buck Rogers]

Too bad, I use a tube guitar amp and it's an easy 100% test.

You only connect one end, Other end is floating.

Could try a 470K to ground on the other connection...

Buck

 

[TriodeLuvr]

Many film capacitors are not made with a single outside foil. Construction of the Illinois Capacitor no. 473PPA202K (.047uF 2KV Polypropylene) is described in the data sheet as "Double Metallized film with internal series connections." If you Google that phrase and look at images, you'll see these are layered, rather than wound.

Jack

 

[Buck Rogers]

The square ones have an outside on both ends... So no real outside.

 

[$10_LR-5555A]

Many film capacitors are not made with a single outside foil. Construction of the Illinois Capacitor no. 473PPA202K (.047uF 2KV Polypropylene) is described in the data sheet as "Double Metallized film with internal series connections." If you Google that phrase and look at images, you'll see these are layered, rather than wound.

Jack

The square ones have an outside on both ends... So no real outside.

That is interesting... So these new 715p orange drop caps possibly don't have an outside foil like the old ones do? That is what I was thinking but wasn't sure. If that's the case then it doesn't matter how I position the cap right?

I did test with the 470k resistor and it sounded the same. Also the scope showed 4cm on both sides with and without the resistor. I know the scope needs recapped and possibly other stuff replaced specifically on the horizontal side but other caps still showed significantly different.

I am still new to electronics so anytime I see something like this I try to stop and look into it and figure out why before continuing. I couldn't find any information on these caps possibly not having an outside foil so that's why I posted. I did learn the other day what happens when you install an electrolytic backwards though. Fuse blows and then some tin foil and bringing up on the variac it leaks and then blows the cap lol.

 

[TriodeLuvr]

The necessity of having an outer foil that can act like a shield depends on the application. For instance, a grid coupling cap is expected to provide a low impedance path between the driving circuit and the grid at all frequencies of interest. For hi-fi, that would be at least 20Hz to 20kHz, and this range includes the undesired mains frequencies of 50H/60Hz, as well as the doubled mains frequencies of 100Hz/120Hz that result from full wave rectification. In this case, unwanted pickup of those frequencies won't be materially affected by the orientation of the capacitor, because pickup by the outer foil will be conducted to the opposite lead with little attenuation. On the other hand, a capacitor used for high frequency equalization might be a much smaller value and might exhibit considerable attenuation at these undesired frequencies. This means that connecting the outer foil to a low impedance point can minimize the conduction of those frequencies into a more sensitive node of the circuit.

Jack

 

[gadget73]

The old 715p types I have don't really register as having an outside foil either. Just how its constructed. The tubular IC MPW types sure do though.

 

[$10_LR-5555A]

The necessity of having an outer foil that can act like a shield depends on the application. For instance, a grid coupling cap is expected to provide a low impedance path between the driving circuit and the grid at all frequencies of interest. For hi-fi, that would be at least 20Hz to 20kHz, and this range includes the undesired mains frequencies of 50H/60Hz, as well as the doubled mains frequencies of 100Hz/120Hz that result from full wave rectification. In this case, unwanted pickup of those frequencies won't be materially affected by the orientation of the capacitor, because pickup by the outer foil will be conducted to the opposite lead with little attenuation. On the other hand, a capacitor used for high frequency equalization might be a much smaller value and might exhibit considerable attenuation at these undesired frequencies. This means that connecting the outer foil to a low impedance point can minimize the conduction of those frequencies into a more sensitive node of the circuit.

Jack

Thanks for the explanation. I still find the whole outside foil thing confusing but now that you mentioned the mains frequencies... yesterday I did notice if the probe for my scope was within 1 foot or so of the battery backup I have under the desk it showed on the scope. I assume that is somehow related to why the orientation matters? If it is then it is a good visual to see on the scope to understand better.

The old 715p types I have don't really register as having an outside foil either. Just how its constructed. The tubular IC MPW types sure do though.

The old 715p caps I have dated 82 show about a 0.5-1 cm peak to peak difference at 2mv/cm. I agree not really much of a difference but the new ones didn't show a difference at all. Maybe just age and manufacturing differences?


I think I will just put the new caps in without worrying about orientation and just have the writing facing up. Just trying to do the best job I can with what I know and the tools I have. When changing the caps I usually do a few at a time then test in case I mess up or there is a difference.

Working on a Lafayette LA-55 that had a blown main cap and other electrolytics not anywhere near their capacitance. It sounds good as is and only very little noise at 50% volume with no music playing. There is currently noblecon and tdk caps in it. Couldn't find much info on them so I assumed they might need replaced after all this time as well. Figured worst case is if they test good and I don't like the new caps I'll have to put them back in.

 

[gadget73]

The old 715p caps I have dated 82 show about a 0.5-1 cm peak to peak difference at 2mv/cm. I agree not really much of a difference but the new ones didn't show a difference at all. Maybe just age and manufacturing differences?

possible, I think mine are mid 90s vintage.

 

[WntrMute2]

Here is a thread I started over at DIY Audio.
Easily showed a difference on everything I tried except some Kemet stacked film caps.
Determining Outer Film
One caution is that manufacturers printing alternates each cap. So If you are placing them printing side up you may well be alternating outer foils if you have a random sample of caps.

 

[BrassTeacher]

Alternative when you cannot find the outside foil, or if it is stacked:

Wrap the cap in copper foil or tape, solder a small wire to it, then ground that wire to signal ground