When were polystyrene capacitors introduced?

[DaveD]

I assumed that the original capacitors in my KLH Seventeen pair would be electrolytics. The speaker designer and technician who I have working on them thought so, too, because of their age. But, he said that the 8 microfarad ones are polystyrene, yellow, made in Mexico. He did not comment on the dual-section one attached to the tweeter switch. I have just learned about this component on this forum, and will have to ask him if he took the stuffing out to see it, and if he can find a replacement of the same type.

This pair of speakers is from 1974. I wonder if the original design and voicing of the speaker had to have been done with electrolytics, because they were the only ones made in 1967, or thereabouts. The polystyrene ones could have been substituted by Singer. Or, were polystyrenes available to our man Henry at the time of his work?

[DaveD]

Well, I have found a wonderful post on the AR forum titled "Early Capacitor Manufacturing History"

http://www.arsenal.net/dc/dcboard.php?az=s...g_id=4281&page=

Based on the information presented, it looks like the capacitors available in 1967 would have been the wax-paper type, with very low ESR. This is opposed to the much higher ESR of electrolytics, which were introduced around 1970. It is concieveable that old stock PCB oil caps, also stated as having low ESR, were used in voicing the speakers.

Now I am wondering if the ESR of the polystyrene caps in my 1974 Seventeens would be similarly low compared to that of the wax-paper type. Since polystyrene is not covered in this history, they may have been brand new in 1974. And, how does the ESR of polystyrene compare to that of polypropolene ones?

This brings up the possibility that the Seventeens were voiced with low ESR wax-paper types, later forced to use high ESR electrolytics in their middle period (hurting their sound?), and then finishing with low ESR polystyrene ones in the final few years (if, indeed, the polystyrene ones are low ESR).

Any learned observations will be appreciated.