怎么把把stringbuilder结果转换成ansi呢

如何把把stringbuilder结果转换成ansi呢？

C# code

  StringBuilder text = new StringBuilder(128);            int tt = PetroChinaHIS10_120520.Performance.PSDNMobile.BriSDKLib.QNV_Tool(PetroChinaHIS10_120520.Performance.PSDNMobile.BriSDKLib.QNV_TOOL_LOCATION, 0, "02885882015", "", text, 128);            string str = "";把stringbuilder结果转换成ansi，不能用unicodestringbuilder 是unicode编码的。。