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Black Stax jacks

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Wondering what type of plastic Stax uses for the (black) jacks on their amps. Or if anyone knows of a black plastic or material that is machinable and plays nice with high voltage.

I don't know what Stax uses, thought it might be bakelite as it is hard and brittle but it fails the "409" test for bakelite and a sliver that I cut from one floats as well (bakelite is heavier than water). You can sometimes identify plastics by the way they burn. Couldn't ignite the (tiny) slivers that I had. Possibly a phenolic smell after heating, might be a lower density resin related to bakelite.

DIY jacks are usually made from white teflon (or delrin). Black coloured materials typically have carbon black in them which is conductive (bad for HV). A few people have made black jacks which are usually described as "black teflon" (could also be black delrin). I believe @kevin gilmore mentioned a while back that he tested black delrin and it began conducting at 200 volts. Natural, white teflon and delrin are both rated around 500V/mil, presumably less for the black versions. The only datasheets that I could find for black delrin left the voltage rating spec blank..

Digging around a bit, I found three machinable plastics that might work. Ultem, Noryl and Stycast 2651-40/11. Ultem (polyetherimide /PEI) might be a good choice. It's similar to PEEK, is used to manufacture high voltage circuit breakers and is said to be machinable. There are separate data sheets for the black and natural material which is good. However the voltage rating given is identical on both datasheets which is a little suspect. Anyway the black Ultem is said to have a dielectric strength of 830V/mil which is promising if that is accurate for the black version. Appears to cost two or three times what teflon does, but that would only impact the final part cost by a few dollars. Machining costs dominate and would ostensibly be similar.

Any thoughts on Ultem, alternate plastics or the best way to test (hipot)? 





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