A Cornish Franklin?

We have no way of knowing quite what the accent would have sounded like for American-born colonials in that era. Historical linguists' best guess is that there would have been a range of highly-localized, regionally bastardized British accents. Hence, you get variations on vaguely Cornish- or Irish-sounding accents with a slightly modern-American, proto-Southern (a la Jefferson) or proto-New England (a la Adams) twang.


"Why is it that every time I need to get somewhere I get waylaid by jackassery?"

Jefferson had a familar accent from one of the English regions, but I can't quite put my finger on it. Maybe a soft West Country accent - no doubt they went by where his mother was born (EDIT: actually, she was born in London, so that can't be right).

I liked that they mostly had British-sounding accents, as colonials would have done at the time. John Dickinson sounded like a Hobbit at times though - still a great performance.

Washington was of Scots descent, maybe the Irish lilt you're hearing is actually Scottish.

Come to think of it, Adams was by far the most American-sounding of them.
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I know I'm late in the game but want to add that the original "British" accent is much closer to the present-day American accent. The British upper-class developed a non-rhotic pronunciation of words around the Industrial Revolution to distinguish themselves from the lower classes, which means that they don't pronounce the 'r' sound. Most English have adopted this pattern of speech now, but you will notice that the Irish, Scottish, and Northern English have retained their rhotic speech patterns as have most of Americans (except Boston/New York ironically). So, really Colonial American accents would have sounded more like Irish/Scottish accents as displayed in the miniseries. Of course our accents have continued to evolve but I think that's the reason for the various accents in the show.