B737-500 CREW HAD THE PAX OXYGEN MASKS DEPLOY BECAUSE OF MISSET PRESSURIZATION CTLS.

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CLRED FROM 10000 FT TO 16000 FT. WHILE CLBING THROUGH APPROX 12000 FT; WE HEARD A HORN. BOTH PLTS IMMEDIATELY IDENTED HORN AS TKOF WARNING HORN. UNFORTUNATELY; I CONTINUED CLBING WHILE WE TRIED TO IDENT THE SOURCE OF THE ALARM. I CONCLUDED THE GND/AIR COMPRESSOR SWITCH HAD MALFUNCTIONED; HENCE; THE TKOF WARNING HORN. THE FO WAS SCANNING THE OVERHEAD AND I SAW HIS HAND REACH FOR THE CABIN ALT HORN CUTOUT. THAT WAS WHEN I REALIZED THAT CABIN ALT HORN WAS SOUNDING (BOTH SYS USE THE SAME INTERMITTENT HORN). THAT MEANT THAT CABIN ALT WAS ABOVE 10000 FT. I IMMEDIATELY LEVELED OFF; BUT IT WAS TOO LATE. THE #1 FLT ATTENDANT ENTERED THE COCKPIT AND RPTED THAT THE MASKS HAD DROPPED. I PUT ON MY MASK; DECLARED AN EMER AND DOVE BACK TO 10000 FT. CABIN CTL WAS RECOVERED AND WE RETURNED TO DEP ARPT. I REGRET COSTING THE COMPANY 35 OXYGEN CANISTERS. WE KNOW THE CABIN ALT HORN EXISTS; BUT WE SO RARELY HEAR IT IN TRAINING; THAT IT CAME BACK TO BITE ME. THE TKOF WARNING HORN IS ALWAYS HEARD SOMETIME DURING TRAINING. IT WAS THE FIRST TIME I HAD HEARD IT INFLT AND THE FIRST TIME SINCE TRAINING. 3 ACFT TYPES AGO; HAD I IMMEDIATELY IDENTED IT AS THE CABIN HORN; I WOULD HAVE LEVELED OFF MUCH EARLIER. THE MANUFACTURERS SHOULD ALSO TRY TO DIFFERENTIATE THE SOUND. THE PROB WAS TENTATIVELY TRACED TO A STICKING OUTFLOW VALVE. HOWEVER; THAT MAY NOT HAVE BEEN THE ONLY PROB. I HAD NOT CLOSELY MONITORED THE NEW FO'S USE OF THE PRESSURIZATION PANEL (HIS THIRD FLT). I SHOULD HAVE MORE CLOSELY SUPERVISED HIM ON THE AFTER START CHK AND THE BEFORE TKOF CHK. EVEN THOUGH WE DID NOT FEEL PRESSURE CHANGES IN OUR EARS AS WE CLBED; THERE IS STILL A QUESTION OF WHETHER THE PACKS WERE ON FOR TKOF.