Brush Soundmirror BK401

Though the Brush BK-401 (Soundmirror) is the second magnetic tape recorder produced by the Brush Development Company, it is the first consumer home use machine ever produced in the United States. This very expensive machine had three motors, two for the reels and one for the capstan. It was the later makes and models of recorders that were produced with one motor and a number of belts to mechanically tie everything together. Later on, upscale three motor consumer machines made a comeback in the late 60s through to the late 80s when they stopped being produced.

Soundmirror reel to reel recorder used ¼ inch tape and was full track mono. Brush also sold tape called “Magic Ribbon” or Magnetic Ribbon at approximately $2.50 per 1225’ 7" EIA hub metal reel. Unlike later tape recorders which used tape based on acetate or Mylar, the tape used on this machine was metal oxide coating on paper tape. The tape speed was only 7 ½ IPS which gave a total record time of 32 minutes per reel.

This machine had a quirk in that the take-up reel wound in the opposite direction from the supply reel with the oxide coating facing outward. The tape had to be rewound before the next playing. It is presumed they designed it this way so as to indicate the reel had been played and needed rewinding.

Another annoying quirk is the operator must unthread the tape from around the playing path and thread it through a rewind path before rewinding.

A third quirk is there is no rubber pressure roller on the capstan. Instead, the capstan itself has a friction sleeve made of cork where the tape wraps around it three quarters of a turn.

Recordings may be made either from a crystal microphone, which came with the recorder, or a phonograph ceramic pickup or the detector stage of a radio. Difficulties with tape head slit tilt alignment were avoided by using the same head for both recording and playback. A separate head, spaced about 1 inch upstream from the recording head is used for erasing. During recording or playback the tape is held against the recording and erasing heads by felt pressure pads, which are withdrawn when the "stop" switch is depressed. The recording level can be monitored during recording by means of a green magic eye tube.

In late 1946 when the BK-401 was introduced, it was priced at $299.00 which would be 33 hundred dollars in 2011 dollars.

According to Bulletin No 12 from the Magnetic Products Division of the 3M Company, both the Brush - BK 401 and BK 403 get very hot. “After prolonged operation, the surface of the motorboard on these recorders became quite hot. Because of the heat resistance of the metal reel, its use was recommended.

Along with thee motors, the BK-401 had ten tubes including the eye tube.

Frequency Response: 100 - 5,000 Hz + 6db
Dynamic Range: 35db
Distortion Factor: 5%