I have an ASCII-extended txt file and i need to convert it to a byte array.
Then don't go via text at all. If you just want to grab the bytes within a file, that's really easy:
byte[] data = File.ReadAllBytes(filePath);
Any time you find yourself decoding bytes to get to text, and then encoding that text with the same encoding to get at the bytes, you should be asking yourself whether you really need that text conversion in the first place.