Note
QST, Nov 1939, p. 87.
Radio's Master, copyright 1939, p. H-130.
James Millen left the National Company in May of 1939 to form his own company, the James Millen Manufacturing Company. The Hetrofil, Millen's first ham product, was introduced in October 1939. It was based on a project in the September 1939 issue of QST, p. 11. The unit connected to the audio output of a receiver and was used to reject or suppress an interfering signal or audio beat note. It used no tubes or other active devices.
Audio input and output connection are screw terminals located on the underside of the unit.
Brief review. Radio, Nov 1939, p. 61.