Introduced in 1939. The masterpiece of pre-WWII Hallicrafters. 15 tubes, P-P audio, calibrated bandspread, Lamb Noise Silencer, Amplified AVC, on and on. The styling was beautiful and the performance incredible. Many SX-28s are still being used today for SWLing and in vintage AM ham stations because of their fabulous audio quality.
Most hams find the sensitivity, selectivity and stability quite acceptable for communications today.
The RBB covers .54 to 4.0mc and features double preselection on all bands, meters galore, super smooth tuning with accurate dial readout and a squelch control. Uses 11 tubes with double preselection, crystal filter and the catacomb band switching. At the end of WWII, most manufacturers had been building for the war effort since 1942. They didn't have anything new or ready for production for the 1946 model year. National offered the HRO-5TA1 in 1946 as a new model.