Note
Service magazine, Nov 1937, p. 665.
Service magazine, Jan 1938, p. 37. (a few details)
Radio's Master Encyclopedia, copyright 1937, second edition, p. F-1.
Radio Craft, Apr 1938, p. 707.
Radio News, May 1938, p. 67.
Linear from 15 Hz–20 kHz.
1690, 2 inch, $47.00
1691, 3 inch, $62.50
References
Brief description. Radio News, Sep 1937, p. 186.
Brief description. Service magazine, Feb 1938, p. 32.