6E5

The 6E5 is an electron-ray indicator tube: on the fluorescent target viewed from the top of the bulb, a dark sector opens or closes according to the control voltage. A preliminary RCA publication dated June 1935 presents it as a visual means of accurately tuning a radio receiver, without moving mechanical parts. The document explains that the most precise tuning corresponds to the narrowest obtainable shadow angle. The date shows that the 6E5 was already described technically in 1935; by itself it does not prove when sales or production began.

The tube combines a triode acting as a DC amplifier with a fluorescent-coated target. Electrons from the cathode make the target glow; a ray-control electrode connected to the triode anode shades part of it. In the RCA circuit, a 1 MΩ resistor connects the anode to the target supply. When the triode grid receives a more negative voltage from the automatic volume-control circuit, anode current falls, the control electrode becomes more positive and the shadow narrows. RCA warns that a very strong signal can close the sector completely; in that case it proposes reducing the control voltage with a resistor in the grid circuit.

For the RCA and Sylvania versions with a small six-pin base, the heater is rated at 6.3 V and 300 mA. With 250 V supplied to anode and target and a 1 MΩ series anode resistor, the datasheets give a shadow of about 90° at zero grid voltage and closure towards 0° at around −8 V; target current is on the order of a few milliamperes. These approximate figures vary between sheets and editions: a later RCA sheet, for example, gives about −7.5 V for closure, while the preliminary sheet gives −8 V. The 6G5/6U5 serves a similar purpose but Sylvania describes it with a more gradual cut-off characteristic and a different control range; its response should not simply be attributed to the 6E5.

Mechanical details deserve as much attention as electrical ones. RCA calls for mounting that keeps the top of the bulb visible and suggests a small hood to reduce reflections from outside light. RCA and Sylvania sheets show the small six-pin base, but a 1953 VEB sheet filed under the same 6E5 designation specifies an octal base. Before installing or replacing a specimen, its pin connections and base must therefore be checked against that manufacturer's documentation: the designation alone does not establish mechanical compatibility.