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Vintage threads from the first ten years

tube socket material question

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10/13/1997 9:03 PM
Blob	tube socket material question Does the material a tube socket is made of make a difference? I've seen molded plastic, ceramic, and some kinda expensive ones made out of something called phenolic. What's phenolic and why is it more expensive or better?

10/14/1997 9:24 AM

The material is significant. Plastics come in two varieties, "thermoplastic" and "thermoset". Thermoplastic does what the name says, gets soft when it's heated. Thermoset is also descriptive; these materials "set" into position with heat, then do not soften with reheating.

Most of the things we think of as "plastic" are actually thermoplastic, and soften with heat. Sockets intended for use with relays are sometimes made from thermoplastics and can soften if you put a hot output tube in them. These are not common, but I have seen it happen.

Thermoset plastics are what are usually used for sockets. The black-brown stuff most are made from is "bakelite" a trade name for a kind of clay-reinforced phenolic. Maybe there's a variety that is purer or more sturdy phenolic that is more resistant - I'm fuzz on that one.

All thermoplastics and thermoset sockets are vulnerable to arcing. When you get enough voltage from pin to pin on an output tube to cause a spark to jump from pin to pin (like when you run the output transformer unloaded) the spark runs along the surface of the socket material and burns a trail on the surface. Since the plastics contain carbon, there is often a carbon residue left on the surface. This residue is partially conductive, and makes that path susceptible to arcing over at lower voltages next time; this can be so bad that it happens with only minor overvoltages, and interferes with normal operation.

Ceramics are not carbon based, do not burn in the normal sense, and don't soften or melt at temperatures achieved in an arc over, so they are
essentially immune to arcing unless covered with dirt and gook that can burn and leave stuff on the surface.

I would class them as poorest - thermoplastic sockets; medium (and most common) - thermoset, which included phenolic; best - ceramic.

Ceramics are the premium solution, IMHO.

11/5/1997 5:41 AM
Mook	Ceramic sockets are more impervious to arching, but after the years, the ceramics chip and crack -- they get brittle. You need to be more careful when inserting and removing tubes in older ceramic sockets. Count on replacing ceramic sockets every 15 years.

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