The 0A2 is a cold-cathode, gas-filled voltage regulator tube — it ionizes rather than using a heated filament, holding a steady 150V reference across a power supply's B+ line.
Tube type: Cold-cathode gas voltage regulator
Base: 7-pin miniature
Heater: None (cold-cathode, gas-ionization type)
Typical use: Voltage reference/regulation in vintage radio, instrument, and amplifier power supplies
Part of the 0-series cold-cathode regulator family alongside 0B2, 0C3, and 0D3.
