6AD6G

The 6AD6G is an electronic tuning indicator with a circular fluorescent target and two independently controlled shadow sectors. It contains an indirectly heated cathode, two ray-control electrodes and the luminous target, without an integrated amplifier triode: drive comes from an external circuit. National Union’s sheet linked to RMA release 138 of 26 May 1938 already documents this structure. The shadows can share one control signal or respond differently according to their electrode connections.

The heater requires 6.3 V AC or DC at 0.15 A. The tubular T-9 bulb uses an octal-family base with seven physical pins, 7AG; contact 6 is absent. Pin 1 is unconnected, 2 and 7 heater, 3 and 4 shadow controls, 5 target and 8 cathode. Any mounting position is allowed. RCA specifies maximum overall length 2⅞ inches and maximum diameter 1⁵⁄₁₆ inches; these are component envelopes, not dimensions of the visible display alone.

Tables describe each shadow at two target voltages. At 100 V, control about +45 V closes the shadow to 0°, 0 V corresponds to 90°, and about −23 V opens it to 135°. At target 150 V the same angles require about +75, +8 and −50 V. RCA, Tung-Sol and National Union give target currents respectively 1.5, 1 and 0.8 mA for the first case, 3, 2 and 1.2 mA for the second; RCA warns of wide variation. More negative control therefore widens the dark sector. Voltages refer to the cathode and are not precision calibration data.

Tung-Sol specifies target operation between 100 and 150 V, maximum ray-control supply 150 V and maximum heater-cathode potential 100 V. RCA’s tentative data of 1 May 1941 confirm target maximum 150 V and recommend minimum heater-cathode potential when not directly connected. The 6AF6G appears beside the 6AD6G in Sylvania’s manual but has its own voltages and angles; similar designation does not justify transferring its conditions automatically.

The application distinction is separation of display and amplification. Parallel controls produce symmetrical shadows; separate drives let one close at small AVC voltage and the other at a larger voltage. Sylvania cites a 6J5G for weaker signals and a triode-connected 6K7G for stronger ones. Tung-Sol instead illustrates a 6AE6G circuit with two 1 MΩ resistors and 250 V general supply, target marked about 125 V. National Union also shows control linked to the AVC-regulated plate current of the IF amplifier. Thus the two shadows offer two readings of tuning whose sensitivity depends on the whole circuit.