6N5

The 6N5 is an electronic tuning indicator, commonly called a magic eye, combining an amplifier triode and circular fluorescent screen. A dark sector changes width with voltage applied to the triode grid. A receiver can take the control voltage from its AVC circuit: a narrowing shadow makes approaching correct tuning visible. Tung-Sol describes the internal connection between amplifier and ray-control electrode. The triode controls the luminous indication; the documented purpose is visual feedback for radio tuning.

A precise historical reference is RMA Release 76, dated June 12, 1936, sponsored by National Union Radio Corp. as tentative 6N5 data. It lists an ST-12 bulb and small six-pin base. RCA and Tung-Sol sheets in 1940 instead document the double-branded 6AB5/6N5; Tung-Sol states that this version replaces both types. Sylvania describes the 6N5 as similar to the 6AB5, differing in bulb and plate-current cutoff characteristic. These are documented stages of the family, not grounds to turn a sheet date into a universal first-production year.

Nominal heating is 6.3 V AC or DC, 150 mA. Tung-Sol recommends the type where low heater drain is needed; Sylvania compares its 150 mA with the 300 mA of 6E5 and 6U5/6G5 types. Indicator action is similar, but electrical characteristics differ. The 6R base uses pins 1 and 6 for the heater, 2 for triode anode, 3 grid, 4 fluorescent target and 5 cathode. Any mounting position is allowed. RCA recommends keeping heater-cathode potential difference low when they are not directly connected.

Shadow behavior also depends on the circuit. RCA’s table, with anode and target supplies at 135 V and a 0.25 MΩ series anode resistor, gives approximately 0 V grid voltage for 90° shadow and −10 V for closure to 0°. With 1 MΩ in series, closure instead needs about −15.5 V. At zero grid voltage, the two conditions give triode anode currents 0.5 and 0.13 mA, and target currents about 2 and 1.9 mA, with wide variation stated. The 1936 tentative data and Sylvania sheet give about −12 V for closure with 0.25 MΩ. These belong to their respective specifications, not one guaranteed universal threshold.

RCA and Tung-Sol limit anode supply to 180 V and specify a 100–180 V target range. Curves relate grid voltage, triode current and shadow angle, helping distinguish indication problems from changes in control signal. The RCA combined type has a T-9 bulb, maximum diameter 1 3/16 inches, about 30.2 mm, and overall length up to 4 3/16 inches, about 106.4 mm. Replacement assessment requires sensitivity, supply, dimensions and heater data as well as base; similar luminous appearance does not make all magic eyes automatically interchangeable.