NMAC IN CLASS B AIRSPACE BTWN A B737-400 DSNDING THROUGH 6500 FT AND A C182 IN LEVEL FLT WITH NO XPONDER ON. ATC DID NOT HAVE PRIMARY OR SECONDARY TARGET ON THE C182 SO THEREFORE; MADE NO TA. RPTING CAPT INCREASED HIS RATE OF DSCNT TO HELP PREVENT A POTENTIAL COLLISION.

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WHILE BEING VECTORED FOR APCH ON HDG 100 DEGS; AND DSNDING TO 6000 FT; WHEN LEAVING ABOUT 6600 FT; WE SAW A C182 10-11 O'CLOCK OUR POS WITH NO PREVIOUS TCASII ALERT AND/OR ATC TFC INFO. THIS CESSNA WAS ON A 210 DEG HDG AND ESTIMATED ON A LEVEL FLT PATH AT 6500 FT IN THAT MOMENT. WE ESTIMATE THAT WE CROSSED THE OTHER PLANE AT 6400-6350 FT DSNDING AFTER DISENGAGING THE AUTOPLT TO INCREASE OUR RATE OF DSCNT WHILE MAINTAINING CONTINUOUS VISUAL CONTACT; AT THE HORIZ AND VERT ESTIMATED DISTANCES PREVIOUSLY MENTIONED. AFTER INFORMING ATC ON FREQ 128.40 MHZ (PHILADELPHIA APCH) ABOUT THIS EVENT; THE CTLR RPTED TO US THAT THIS TFC WAS NOT ON THE RADAR SCREEN; NOT EVEN THE SECONDARY ECHO. THIS COULD MEAN THAT THE CESSNA DIDN'T EVEN HAVE A XPONDER ON WHILE CRUISING (PROBABLY DUE TO AN INFLT XPONDER FAILURE) AND ALSO NEVER REQUESTED ATC CLRNC TO FLY IN CLASS B AIRSPACE. ANOTHER PROB HERE; BESIDES THE PROX OF THE FLT PATHS; IS THAT NOT EVEN THE CTLR WOULD HAVE NOTICED THIS TFC HAD WE NOT RPTED IT; AND THIS COULD MEAN THAT THIS KIND OF UNAUTH FLT COULD BE A 'COMMON' PRACTICE FOR THIS AND MAYBE OTHER PLTS.