On January 1, 1929 new US communications regulations took effect that supported the outcome of the 1927 International Radio Conference. Hams now had only a small percentage of the HF spectrum. In order to avoid interference their signals had to be harmonic free and clean with little FMing, drift or AC note. The ARRL estimated that only about 10% of the existing stations could meet these new standards. Many pages of QST were devoted to helping hams get ready for Jan 1, 1929.
I've assembled and used a station that reflects the state of the art during this era. The transmitter is a two tube TNT (tuned plate - not tuned grid) while the receiver is a National SW3 three tube regen.