This VLF receiver, designed by the famous National Company, is an RBL-4. It looks very much like the National NC-100 and the RAO receiver. It is a TRF design with 2 RF amplifier stages, followed by a regenerative detector, a limiter, and two stages of audio amplification. The 2 RF amps and the detector are 6SK7s, the first audio amp is a 6SG7, the limiter is a 6H6, and the output audio amp tube is a 6K6. It uses a 5U4 rectifier tube. The receiver covers 15 Khz through 600 Khz and has very good audio filtering. It is very sensitive, stable, and selective, and is fun to operate. The military lable below tells the details.