Voltage converter for using 220V turntable in US?

A bigger worry is the line frequency. Most of the world uses 50 hz power, the US uses 60 Hz. Motors whose speeds are set based on the line frequency won't run at the right speed when moved to a country using a different line frequency.

Thanks,

So, to safely play a turntable with 220/250 Volt and 50Hz in US, what kinds of provision have to be made?

you're better off getting a whole new motor. voltage is easier to adjust than the frequency, which as noted above will affect playing speed, which is sort of a big deal when you're talking about a turntable. depending on the TT you're talking about there might be ways to mechanically adjust the motor or deck to deal with the 50hz/60hz difference.