Electron-Ray Indicator Tube

a visual display that is used for the accurate tuning of a vacuum-tube radio receiver to the wavelength of the station being received, for the adjustment of the recording level in a vacuum-tube tape recorder, or for zero adjustment in electronic measuring equipment. An electron-ray indicator tube is a composite vacuum tube that has a display device and an amplifier tube in its bulb. The amplifier tube is usually a triode.

The display device contains the following components: a low-voltage luminescent screen with a phosphor deposited either on a metal substrate or on a transparent conducting film that coats the glass of the bulb, electrodes for shaping electrons emitted from a cathode into a beam, and deflection electrodes. The cathode is housed together with the amplifier tube.

A signal to be indicated is first rectified and is then fed to the control grid of the amplifier tube. The current in the anode circuit depends on the level of the signal to be indicated. In turn, the anode current determines the ratio of the anode potential, deflection-electrode potential, and screen potential. The deflection electrodes are connected to the anode inside the bulb; the screen is connected to the anode by a resistor with a resistance of 1–2 megohms. The deflection electrodes deflect the electron beam so that, when the beam impinges on the screen, it produces two bands of light on the screen. The gap between the two bright bands is dark. An electron-ray indicator tube usually operates in such a way that the two bright bands draw closest together when the signal is maximum.