Technically the 'tt' sound is not a diphthong because it contains no vowels. Also, pronunciation within the Americas, and even within the USA varies greatly, so in Boston you might get 'Beh-tea' and in San Francisco you would hear 'Beddy'.
Generally North Americans make the 't' sound more like a 'd' - as even English people do with words like 'Saturday'. It is not a good idea to try and imitate other accents, but be aware that if you ask for a 'ladder' in a British accent, an American might assume you want a 'letter'. (This actually happened to me in a hardware store in Canada.)