NU-42 is a National Union, 6 pin, 42 output used in radios during the 1930's. It's same as 6F6G except for the base. You can find additional information here:
http://www.duncanamps.co.uk/cgi-bin/tdsl3.exe/partsearch?TSD=42&SID=0002D738FDB23505
Norm
:I NEED TO KNOW WHAT KIND OF TUBE THAT I HAVE IN MY POSSIESON,THE TUBE IS IN NEW SHAPE AND PROBLUY DOES WORK THE TUBE IS A NATIONAL UNION NU-42,WOULD LIKE
:SOME INFO ON IT,THANK YOU