Tripathi, 22, arrived at Wantage Road on a short-term deal at the start of this season while 26-year-old Harrison, who can also keepwicket, joined the staff last year after impressing on trial.
McCullum, passed fit to play and keepwicket despite a hip injury, was trapped lbw by Shaun Tait off the second ball of the innings, while Guptill and Fulton fell soon after each other as Bracken and Mitchell Johnson turned the screw with a succession of maiden overs.