Originally Posted by
mech986
You'll have to ask Greg Timbers as he helped design it and would know. However, I think we can deduce this much from your link - the caps themselves are foil wrapped which probably is the metallic side of the dialectric. There must be a non-conductive material so its either air, paper, or plastic (of some type). Given that the speaker was designed in the mid 70's, the prevailing non-electrolytic caps were mylar, metallized mylar, paper, and then a whole host of specialized materials like polycarbonate, teflon, polystyrene, and the like. Here's the drawbacks as of 1973:
polypropylene, metallized PP - very expensive as volume production not yet available, low esr, good volumetric efficiency, hard to get in smaller production quantities
Teflon - super expensive, large size, military only back then
Polystyrene - low voltage and current capability, poor volumetric efficiency (big size, low uf values), got expensive, burned easily.
Polycarbonate - low volume made, good size and efficiency, but not much available
Paper - high voltage capable, poor volumetric efficiency, tend to get leaky, rarely used in speaker crossovers
Mylar - low cost, fair volumetric efficiency, good voltage tolerances
Metallized mylar or film and foil construction - moderate cost, good to very good volumetric efficiency for a non-polar cap, decent ESR (when people thought of those things), many manufacturers.
Non-polar electrolytic - very high volumetric efficiency, sometimes decent sometimes poor tolerances in value, low cost, decent reliability, high ESR (generally) - could have been used
My sense is that they were metallized mylar caps, without the usual outer coating and end epoxy seals to keep cost down. As these were crossovers made point to point without Printed circuit boards, the ability of gluing or fixing the components in a potting material like most JBL consumer and pro crossovers of that time made the sand/cardboard tube/wax seal an easy way to deal with the cap without melting it.
Tons of caps were used, so saving money while getting the best performance relative to the cost was probably a high concern.