capacitors - Metalized Polyester Film vs. Polyester Film

[amonte]

I'm in the process of placing an order for some parts on Mouser, including some capacitors.  I'm finding that the brand I'm searching (Xicon) has both of these types of capacitors available in the sizes I'm interested in, with a noticeable (if small) difference in price, with the metal caps being slightly less.  I also noticed that the polyester film caps are available in different "tolerances", with the smaller tolerances costing more than larger tolerances.

My questions are:

1) In general stompbox design, is one of these types (Metalized Polyester Film or Polyester Film) more favorable than another?

2) If Poly Film is more favorable, in general what tolerance is considered acceptable, or "close enough"?

3) For voltage purposes, what do you guys usually go with?  Is there a general rule of thumb?  I realize that size is an important factor, but do you go for the smallest voltage available or do you generally stick with one voltage (ie, 250V)?

[Transmogrifox]

I use both caps indiscriminately.  I don't think there's a physical difference, as polyester is the dielectric material, not the conductor.  Size may differ, but with both I have disemboweled all that can be found is two srips of aluminum foil sandwiched in "saran-wrap" rolled up like a cinnamon roll with metal leads sticking out each side.

Voltage rating for stomp boxes is a moot point, as a trend the lower voltage caps tend to have smaller physical size (if space is an issue).  You won't go anywhere near the rated voltage unless you're building a tube circuit.

Tolerance:  I usually go for 10%.  Most stompbox effects circuits are generally not very sensitive to component variations of this degree.  I have made capacitors by glueing foil to cardboard (very high tolerances--like the difference between .01uF to .1uF)

[mlabbee]

Here's a page with an interesting analysis of various kinds of capacitors:

http://members.aol.com/sbench102/caps.html

Basically, it suggests that the paper/oil caps are the most linear, with the plastic film caps coming next.  I suspect there's not much difference between the one's you're looking at, although tighter tolerances witll probably give you more predictable behavior from build to build.

As for voltage ratings, I don't think it makes much difference for non-polarized caps, so just make sure you get at least high enough for the circuits you plan on building (add in a padding to be sure).

Electrolytics depend on the the electricity flowing through them to keep the electrolytic layer intact (this is why electrolytics that sit unused for a long time go bad).  So keeping the value relatively close to the voltage you'll be using will ensure the longest life (i.e., don't use 100V caps for a 9 volt circuit).

[RedHouse]

Polyester Film and Metalized Polyester Film should be the same construction.

The Mylar (Polyester) has a conductive metalic coating deposited on one surface.

The other Polyester type would be Film and Foil where actual foil is sanwiched between the Mylar material.

[Tom Sawyer]

The caps metalized that i have seen are usually smaller than no-metalized. I guess the size is very important in stompboxes.
Im not sure but the metalized support more tension than no metal.

[MartyMart]

I use MMK "poly film" and also AVX, they seem like good quality caps, I  tend to go for the 63volt versions as they are smaller than 100v or 250v, otherwise values above .22uf can get mighty BIG !!

Marty.

[Satch12879]

DigiKey stocks Panasonic Poly Film capacitors, both polyester and polystyrene if memory serves.