I'm surprised that the Packard-Bell PB250 wasn't considered. It could
run from a standard US lighting circuit. Circa (IIRC) 1959. Not a drum
machine, but used a recirculating magnetostrictive memory.
It also seems that there may have been a few military computers that
would satisfy your criteria.
As Will has stated "first" is a very dangerous word and "minicomputer"
is, in fact, a term of art. Like pornography, it's what it is because
someone says so.
--Chuck