The line cord was connected to a convient place for the manufacturer. In an AC/DC radio one side is B- and goes all over the radio.
You may better off connecting one side of the line to negative of electrolytic caps. Even though B- goes all over a radio sometimes using a different point for connection will cause a little hum.
Norm
:Currently restoring this radio and noted that negative side of the line cord was soldered to pin 5 of 12SK7. What determined placement of such a connection such as line cord in radio production? Could it not be connected at negative side of the electrolytics?