How to define characters to use when creating a salt

Question

I need to generate a salt, but I need all characters to be part of a list of numbers and letters.

string usedChars  = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789";

Gero the salt with this function:

public static byte[] GenerateSalt(int SaltByteLength)
    {
        System.Text.Encoding enc = System.Text.Encoding.ASCII;
        RNGCryptoServiceProvider rncCsp = new RNGCryptoServiceProvider();
        byte[] salt = new byte[SaltByteLength];
        rncCsp.GetBytes(salt);

        return salt;
    }

Function can be changed to specify characters to be used?