EM1

The EM1 is an optical tuning indicator for radio receivers, recognizable by the luminous cross viewed from the end of its bulb. A Philips article about the earlier name 4678 explicitly notes its new designation, EM1. In Philips sheets the indicator is not a mechanical meter: a conical surface coated with fluorescent material glows where electrons strike it, while four small radial deflection plates create four shadows of variable width. To the listener, the width of the bright sectors is the visible cue for finding the best tuning.

The lower section contains a triode whose grid receives the receiver's variable automatic-gain-control voltage. Its anode is internally connected to the four deflection plates and is fed through an external 2 MΩ resistor. A change in grid voltage changes the triode current and plate potential, and thus the deflection of electrons travelling toward the screen. According to the Philips sheet, in a normal circuit the desired station is found when the bright sectors are widest. The response depends on the receiver's control circuit, not on the tube alone.

The cathode is indirectly heated: its heater takes 6.3 V and 0.20 A and can operate on AC or DC, in a series or parallel arrangement according to the circuit. The Philips base is type P with side contacts. The sheet gives 200 or 250 V for the supply to the screen and the anode resistor. It also warns that in AC/DC sets the DC voltage on the fluorescent screen must not fall below 200 V; use in such receivers therefore depends on the mains voltage and any voltage doubler.

The Philips table gives a concrete example of the indicator's sensitivity. At 200 V, changing grid bias from 0 to −4 V changes the angle of a bright sector, measured at the screen edge, from about 20° to 90°; at 250 V the stated range is about 16° to 90° for 0 and −5 V. These are test points for the documented circuit with a 2 MΩ anode resistor, not a universal signal-strength scale. A Dutch Philips publication also shows a connection taking control voltage from the detector diode, allowing weak signals that have not yet activated delayed automatic gain control to be tuned visually.

The Philips article on the 4678 explains the cloverleaf appearance: four fluorescent patches separated by shadows widen as the control voltage becomes more negative. It also describes mounting the tube behind a window in the front panel and using a dark surround to make the cross easier to see. That document records the design concept and earlier designation; the EM1 operating figures used here come from sheets dedicated to the EM1.