I'm looking at replacing my ST70 tube sockets from the originals (I think my bias pots show that the amp was built in 1963). Differing tube sockets are plated with tin, cadmium, and gold and I hear there are silver plated sockets out there too.

I'm really just looking for a good socket for my application. I know gold and silver have better conductivity, but I was wondering where or why tin and cadmium are also used.

Thanks,
Bugs

Tin and Cadmium are both used as platings to reduce corrosion on the spring-steel based metals used in tube sockets. Tin works in the same way it does on tin cans, by excluding oxygen from making contact with the steel. Cadmium, being a more chemically "active" metal, behaves like zinc in that it not only excludes oxygen but acts as a sacrificial layer, oxidizing to protect the steel should corrosion actually begin.

Both are also easier to solder than steel.

Cadmium has been found to be toxic. It's a carcinogen, and workers are cautioned about handling it. In fact the control levers underneath some fine old turntables were plated with cadmium for corrosion prevention; those working on them should take precautions such as using gloves and dust masks. I learned this only after about forty years of owning and using such a turntable. There's no exposure if you just use such a component, but if you service it, you can easily come into contact with the cadmium plating, which is soft and can come off as a powder if it is abraded.

Incidentally the dull silvery plating on the Dynaco Mark II is reportedly cadmium; for some reason it seems harder and less easily abraded than that on the turntables I spoke of.