The phrase refers to vacuum tubes, specifically the HTA 100 model, produced during their initial manufacturing period. These tubes were crucial components in early electronic devices, particularly audio amplifiers, radios, and test equipment. Their construction and characteristics defined the performance capabilities of the equipment in which they were used.
These original tubes are significant due to their unique sonic qualities, build quality, and historical importance. They often exhibit different performance characteristics compared to later production runs or reissues. The scarcity and desirability of these tubes among vintage audio enthusiasts and collectors stem from their perceived superior sound and historical value.