I first saw these glowing giants, below, as a young ham on a visit to WNEW(AM) 1130 kHz in New York in 1966.
During World War II, these tubes were used to create RF power for induction heating, hardening engine parts for military vehicles. After the war, these proven amplifier tubes were put to widespread use in 50 kW AM transmitters by RCA and Westinghouse.
Derived from the earlier water-cooled 9C22, the first generation of 5671 tubes weighed about 350 pounds. They were redesigned with half as many cooling fins, reducing the weight to 225 pounds by perforating the fins (visible in the photo). The 5671 is removed and replaced using a custom manual forklift.
The photo shows a “hot” spare sitting in a porcelain air cooling chute in an RCA BTA-50F at WKNR(AM), Cleveland, taken during a 2001 visit. The inset image shows the glow in the dark.
By the 1970s AM broadcast transmitters evolved with more efficient designs, eliminating the need for these tubes.
Some of these tubes found a home in museums and private collections. (See Tube Collector magazine, February, 2001, “The 5671” by Ludwell Sibley; visit www.tubecollectors.org.)
Jim Hawkins is a radio enthusiast. Visit his home page hawkins.pair.com for images of broadcast technology, ham radio and more.