7A6

The 7A6 is an indirectly heated loctal twin diode intended for detection, automatic volume control and small-current rectification. Its two sections have separate anodes and cathodes, allowing different functions in one receiver. Sylvania and Tung-Sol compare its electrical characteristics with the 6H6, but with heater current reduced to 150 mA; this similarity does not imply direct socket interchangeability.

An RMA document dated June 10, 1938 announces commercial release of the 7A6 by Philco Radio & Television Corporation alongside other types. The attached Philco sheet, dated June 8, 1938, already describes the twin diode with a Loktal base. This provides specific industrial documentation of its appearance without dating it from specimen photographs alone.

The envelope is T-9 glass and the eight-contact loctal base uses diagram 7AJ in later sheets. Heater pins are 1 and 8; diode 2 has cathode 2 and anode 3, diode 1 anode 6 and cathode 7. Pin 5 connects the internal shield separating the sections; pin 4 is unconnected. RCA permits any mounting position. Operating heater data are 6.3 V at 0.15 A, AC or DC; RCA distinguishes historical nominal values of 7 V and 0.160 A. Sylvania explains that nominal 7 V refers to operation on a 130 V line and permits series or parallel heaters.

RCA’s June 20, 1947 sheet gives design-center limits per diode of 150 V RMS AC anode voltage, 45 mA peak anode current and 8 mA DC output. Peak heater-to-cathode voltage is limited to 330 V in either polarity. Tung-Sol instead lists 48 mA steady-state peak anode current, also specifying 420 V peak inverse voltage and an 11 V drop at 16 mA DC per anode. These are different quantities and conditions: the drop at 16 mA does not replace RCA’s DC output limit.

Beyond detection and volume control, Tung-Sol documents half-wave rectification and half-wave or full-wave voltage doublers, all for low current drain. Its doublers at 117 V RMS have an 8 mA maximum output and minimum effective supply impedances per anode of 30 Ω or 15 Ω according to configuration. Filter capacitors above 40 µF may require additional impedance. Capacitances also need context: Tung-Sol gives 0.05 pF maximum between anodes, RCA 0.1 pF, with an external shield connected to the cathodes. Such details show why the label twin rectifier alone does not describe the 7A6’s full radio-circuit role.