Electron-ray Tuning Indicator Tube
(Tuning/Magic Eye)

Function:

If a superhetrodyne receiver is not tuned properly to a station, distortion may result due to side-band cutting of the signal. Some sets use an electron-ray (tuning/magic eye) tube to aid in tuning the set exactly to the station. The eye tube gives a visual indication of when the receiver is properly tuned to the signal being received.

Theory of Operation:

Figure 1 is a diagram which shows the internal construction of the electron-ray tube. This is a cathode-ray tube which shows a deflection on a target anode depending upon the voltage applied to the triode grid. The tube is a combination of an ordinary triode section with two special electrodes, the target anode, and the deflector electrode.

Electron-ray Tube Internal Construction
Figure 1

The target anode has a coating of fluorescent material that glows when struck by electrons. The target anode is connected to B+ of the receiver and attracts electrons from the cathode. The deflector electrode is a thin vertical vane between the cathode and the target and is connected to the plate of the triode section. This vane shades a small portion of the target and prevents some of the electrons from striking that part of the target leaving a nonflourescent or shadow area. The width of the shadow depends upon the relative voltages between the deflector and the target. The more negative the voltage on the deflector with respect to the target, the more electrons are deflected from the area of the target that is behind the deflector and the larger the shadow. A strong negative voltage on the deflector will produce a large shadow (see Figure 2A below). When the deflector is at the same voltage potential as the target, few electrons are deflected and the shadow is small (see Figure 2B below). For intermediate values of voltages on the deflector, the angle of the shadow is somewhere in between the two extremes. The dark spot in the center is caused by the cathode light shield.

Figure 2
Electron-ray tube

Refer to the schematic in Figure 2 above.
The target anode (pin 4) is connected to the B+ of the receiver, and the plate of the triode (pin 2) is connected to B+ through R-128, a 1 megohm resistor. Notice that the deflector is tied internally to the plate of the triode so it will always be at the same voltage potential as the plate. The grid (pin 3) of the triode section is connected to the AVC voltage bus of the receiver. The amount of AVC voltage developed is dependent upon the strength of the signal being received. When no signal is being received, the AVC voltage is zero. This permits a relatively high flow of plate current in the triode which causes a high voltage drop across R-128. This results in a much less positive voltage on the plate of the triode and the deflector anode. Since the target anode is connected directly to the B+ supply, it is much more positive than the deflector therefore electrons are deflected at a wide angle, causing a wide shadow on the target. When the receiver is tuned to a station, the AVC voltage developed causes the grid to become more negative which causes the plate current to decrease, reducing the voltage drop across R-128. This makes the triode plate and the deflector more positive with respect to the target anode, thus the deflection of electrons is less and the shadow angle narrows. A strong signal can produce enough AVC voltage to completely cut off the current flow in the triode so that there would be no voltage drop across R-128, and the plate and deflector would be at the same voltage potential as the target. This would produce very little or no shadow on the target.

Troubleshooting:

The tuning eye is not a complicated circuit to troubleshoot. If there is no glow on the target, a new tube is needed. If the tube glows but the deflection does not change when a signal is received, and the AVC circuit is operating normally, R-128 is likely open. This resistor is often mounted inside the socket that the tuning eye tube plugs into, and to change this resistor, the tube socket must be opened.

Replacing Tuning Eye Tubes:

Some tuning eye tubes, such as the 6U5, are becoming scarce and costly. A much less expensive 1629 eye tube can be used with an adapter. The 1629 uses a 12 volt filament so provisions must be made to increase the 6 volts of the filament voltage of the radio to 12 volts. This is accomplished by a voltage doubler circuit that is built into the adapter. The adapter also converts from a 6 pin socket to the 8 pin octal required by the 1629. No circuit changes need be made to the receiver when using the adapter. The cost for a 1629 is around $5 dollars, and the cost of the adapter is $15.95. The 1629 and adapter can be purchased from Antique Electronics Supply.

The CHRS Website has a tips page on tuning eye tubes.

Electron-ray Tube Specifications
Type Typical Plate and
Target Voltage		Series Triode
Plate Resistor		Triode-Grid Voltage
6E52501 megohm -8.0 for shadow
angle of 0 degrees
0 for shadow
angle of 90 degrees
6G5/6H52501 megohm -22 for shadow
angle of 0 degrees
0 for shadow
angle of 90 degrees
6U52501 megohm -22 for shadow
angle of 0 degrees
0 for shadow
angle of 90 degrees
16292501 megohm -8.0 for shadow
angle of 0 degrees
0 for shadow
angle of 90 degrees