6AF7G

The 6AF7G is an electronic tuning indicator of the type commonly called a magic eye. The RMA registration defines it as Twin Indicator Type: two anodes and one grid, with windings described as continuous and variable-pitch, provide two responses to the control voltage. It displays a control signal rather than serving as an audio power amplifier. The two sensitivities allow different ranges of the same applied grid voltage to be observed.

A precise historical document survives: RMA Release 174, April 17, 1939, registers designation 6AF7G at the request of Compagnie des Lampes, Paris, France. This is a documented registration date, distinct from a commercial launch date not established by the sheet. Preserved photographs show Mazda- and Visseaux-marked specimens; the Visseaux example exposes the amplifier structure below the indicator’s upper portion. These images document particular specimens without assigning identical external construction to every 6AF7G.

The heater requires 6.3 V, 0.3 A, AC or DC. Fluorescent-target voltage is specified from 100 to 250 V, with maximum anode supply 250 V. Three table points use anode and target supplies 100, 200 and 250 V. Series resistors in each anode circuit are respectively 0.5, 1 and 1 MΩ; approximate target currents 0.4, 2.5 and 3 mA. At zero grid voltage, anode currents are P1: 0.15, 0.16, 0.20 mA; P2: 0.12, 0.13, 0.18 mA. Each column belongs to its specified supply and resistors.

Shadow closure demonstrates the sensitivities. For the indicator controlled by P1, the approximate angle falls from 60° at grid zero to 0° at −2 V with 100 V supply, −4.5 V with 200 V and −6 V with 250 V. The P2-controlled indicator instead closes at −5, −15 and −19 V under the same three conditions; its zero-grid angle is also 60°. This explains dual sensitivity without assuming identical indication at every supply voltage.

The base is eight-pin octal. RMA bottom view: 1 no connection, 2 and 7 heater, 3 anode P2, 4 grid, 5 target, 6 anode P1, 8 cathode. Anode numbering here explicitly follows the RMA document. Bulb T-9; stated diameter 1 1/8 inch, 28.575 mm. The example circuit shows two anode load resistors, a 1 MΩ control-input resistor and a 0.1 µF capacitor. It gives 0.5 MΩ as a typical load, while the table specifies the loads of the three points above. Documentation: Radio Manufacturers Association, RMA Release 174, 6AF7G Electron Ray Tube — Twin Indicator Type, April 17, 1939, data submitted by Compagnie des Lampes, one sheet with characteristics, circuit and connections.