LGT DEPARTS WITHOUT CLRNC FOR TKOF.

Narrative

STX TWR CTLR CLRED US 'ON TO POS AND HOLD.' I REPEATED; 'ROGER; ON TO HOLD; RWY 9.' AFTER TAXIING ONTO HOLD THE CTLR BEGAN TO CHANGE OUR DEP CLRNC AND STATED; 'ACR; AFTER DEP TURN TO...HDG AND MAINTAIN ....' THE CAPT; AFTER HEARING 'AFTER DEP...;' PUSHED THE THROTTLES UP TO THE TAKE OFF EPR SETTING AND BEGAN THE TKOF ROLL. I HEARD NO CLRNC TO TKOF. THE ACFT BEGAN TKOF ROLL AND I READ BACK THE CLRNC; FINISHING WITH 'UNDERSTAND CLRED FOR TKOF.' I RECEIVED NO REPLY FROM TWR. THE AIRSPD WAS ACCELERATING THROUGH APPROX 120 KTS (VREF APPROX 140 KTS) SO I ELECTED NOT TO SAY ANYTHING. AFTER BECOMING AIRBORNE THE CAPT SCOLDED ME FOR READING BACK THE CLRNC DURING A CRITICAL PHASE OF FLT. I REPLIED THAT I DIDN'T THINK WE WERE CLRED FOR TKOF. THE CAPT TURNED TO THE FE AND ASKED; 'WERE WE CLRED FOR TKOF?' HE REPLIED; 'ALL I HEARD WAS THE DEP CLRNC.' MY BIG MISTAKE WAS NOT PULLING THE THROTTLES BACK AND TACTFULLY TELLING THE CAPT WHAT I THOUGHT I DIDN'T HEAR. INSTEAD; I DOUBTED MYSELF AND READ BACK THE CLRNC HOPING TO GET THE TKOF CLRNC AFTER THE READBACK. THE FE IS VERY PROFESSIONAL AND WAS ALWAYS IN THE LOOP. HE SHOULD HAVE DONE THE SAME THING BEING MY BACK-UP. THE TWR COULD HAVE PREFACED THE CLRNC WITH; 'ACR; CHANGE IN CLRNC; AFTER DEP MAINTAIN....'