The 01A valve is a general-purpose directly heated triode and one of the early pioneers of the American broadcasting boom. Production began in 1922 and continued throughout the 1920s and 1930s. The 01A's popularity stemmed from the fact it could be used as both a detector and an amplifier, and significantly the power requirement was greatly reduced from it's predecessor the UV-201.
RCA Radiotron/Cunningham 01A Datasheet from 1933
The UV-201 tungsten filament required 5V at 1A current but the new 01A's current requirement was reduced to 0.25A by virtue of a small proportion of thorium oxide being added to the tungsten filament material. The resulting atomic surface layer made electron escape easier allowing useful emission at lower temperatures. The thoriated tungsten filaments really glow, unlike the pure tungsten versions, and add a certain visual charm to proceedings.
Super Airline 01A - thoriated tungsten glow
The first 01A valves had brass UV bases with short pins and a balloon bulb with tip on the top. By 1924 the bases were now moulded bakelite and the tips were gone and in 1925, the longer pin UX bases were introduced. Later versions of the 01A had shoulder-type ST or dome bulbs.
Gettering, the reflective inner surface on the valves, was typically silver from the process of flashing material containing magnesium. Occasional production runs used phosphorous - this had the effect of producing a gold or rainbow coating inside the valve, purely for decorative purposes.
Super Silvertone 01A, gold gettering, ST bulb - 1939
At launch, the 01A was considered suitable for loudspeaker use, but over time this advice changed to use with a power valve, improved loudspeakers and expectations of greater volume exposed the limits of what had once been impressive. In Sylvania's data sheet, they note 'very satisfactory reproduction can be obtained providing good audio transformers are employed'. It's modest gain and current limit this valve to early stage amplification. As a preamplifier it's perfect.
Cunningham brand CX-301A, silver gettering, globe bulb
Over time the 01A was superceded practically by improved and dedicated valves, convertors, detectors, amplifiers etc. Type 26 and 27 could be run off AC mains supply, type 30 battery valves had a more efficient filaments and 112, 71A, 45 valves had higher output for amplification.
Silvertone 201-A, silver gettering, globe bulb
At Davison Audio we'd argue that for low gain, early stage amplifier duty, the quality of audio reproduction the 01A is capable of has perhaps never been bettered. A true classic!
The 01A Preamplifier