This has given me the best results, although if I don't want to use the "Baldies," I can connect up a Hi-Z magnetic cone speaker like a Radiola 100A which then eliminates the need for an audio output transformer and provides ample volume (the Hi-Z speaker solenoid coils connect between AF plate and B - just like an audio output transformer.) The RHM functions quite well with 75 year old components - every part was the best that was available at the time.Today, the RHM performance seems antiquated and crude but in 1932 it was "state-of -the-art" and the fact that the receiver is still operating and is still fairly accurate in its dial readout is testament to National's build quality and Herbert Hoover Jr. This same design team again worked together in 1934, producing the famous HRO receiver.RCA supplied some airports with this 16 tube superheterodyne receiver beginning around 1937.
The Comet Pro came out in 1931 and, from 1932 up to about early 1934, only National and Hammarlund were offering commercially-built, shortwave superhets.
The RIO didn't use plug-in coils but had two switched tuning ranges that covered frequencies below 500kc.
I have owned the RHM receiver shown above since 1990.
The AVR-11 receivers provided frequency coverage from 140kc up to 23mc which was more or less standard for the largest "All-wave" receivers.
The airports favored the longer wavelengths used in air navigation, weather reports and for some airport communications in the 1930s.