This tube was rebranded as United "6SQ7". United was an old US maker of transmitting and industrial types. They NEVER made receiving tubes. Someone else owns the brand name and has used it to rebrand many types. Most older metal tubes have the type number stamped into skirt, and this tube is no exception. however, one can see from the photo that "6SZ7" is stamped on this tube (I colored it in for visibility). The 6SZ7 is the 150mA filament version of the 6SQ7, which has a 300mA filament. So, the 6SZ7 will sub for a 6SQ7 in some cases. However, the vast majority of sets that use a 6SQ7 use it as part of a series string of tubes, and in this use, the 6SZ7 will not work.