NMAC. B737-200 CAPT HELPED THE FO; WHO WAS FLYING; TURN TO AVOID A PIPER CHEROKEE 6 DURING CLBOUT FROM JNU.

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DEPARTED JNU ON THE CUSHI 1 DEP WHICH TAKES US STRAIGHT OUT THE LOC. AFTER CLEAN-UP AND CHKLIST COMPLETION; I SETTLED INTO THE CLB; ALERT AND HEADS UP LEAVING AN EXTREMELY BUSY ARPT. ABOUT 7-8 MI OUT; CLBING THROUGH 3000 FT MSL; I LOOKED AHEAD AND OBSERVED A SMALL ACFT; DEAD AHEAD; SAME DIRECTION; AND AN OBVIOUS POTENTIAL CONFLICT. OUR CLOSURE RATE WAS FAST; AND THE FO; THE PF; NEVER FIXED ON HIM UNTIL I BEGAN INITIATING AN EVASIVE MANEUVER. WE JOINTLY EXECUTED A VERY SMOOTH SLIGHT L TURN AND SLIGHT PITCH UP AND WATCHED THE ACFT PASS JUST BELOW AND TO THE R OF OUR FLT PATH. BECAUSE OF SNOW COVERED MOUNTAIN BACK DROP; IT WAS EXTREMELY DIFFICULT TO SPOT THIS ACFT. BECAUSE THIS OPERATOR CHOOSES TO OPERATE THEIR ACFT WITH NO XPONDER; WE DIDN'T HAVE THE LUXURY OF A TCASII RA. WHY THIS PARTICULAR PLT WAS OPERATING DEAD CTR ON THE JNU LOC (OUR WELL KNOWN DEP PATH) I HAVE NO IDEA. HE DIDN'T DEPART QUITE CLOSE ENOUGH FOR US TO BE LOOKING FOR HIM. THERE WAS NO HEADS UP FROM JNU TWR; NO RADAR COVERAGE THAT LOW; AND NO TCASII. IN MY OPINION IT WAS JUST PLAIN LUCK THAT DAY THAT AVOIDED DISASTER. IT WOULD HAVE BEEN AN EASY TARGET TO MISS. I WAS WELL RESTED AND FORTUNATELY NOT AT THE END OF A 6 SEGMENT; 12 HR DAY. THE FAA OFFICIALS WHO INTERVIEWED US UPON ARR IN ANC THANKED ME WHEN THEY LEFT FOR BEING ALERT ENOUGH TO SEE AND AVOID. AFTER 23 YRS OF FLYING; THIS EVENT HAS HAD AN IMPACT ON MY OUTLOOK IN TERMS OF PLT FATIGUE AND LONG DUTY DAYS. BASICALLY; IT WAS MY ALERTNESS AND NOTHING ELSE WHICH AVOIDED A MIDAIR DISASTER.