Good eye tubes such as 6E5 and 6U5 are getting harder to find. Many of the old ones have lost most of their green glow. You can increase the green by a slight modification of the circuit. These tubes were originally specified for operation at about 250 volts. By increasing the voltage on the target, pin 4, a dim eye tubes may again be useable.
To increase the brightness pin 4 should be connected to a higher voltage in the radio. If the radio doesn't have a higher voltage, the addition of a diode, capacitor and resistor should add around 100 volts. Most radios use an 80 for a rectifier. This tube has a high voltage drop and in addition, the rest of the radio circuit loads the voltage down .
Connect a 100 ohm resistor, for protection, to one of the 80 tube plates. The other end of the resistor to the anode of a diode (1000piv). The cathode of the diode should go to the positive end of a 10 mfd 450 volt cap. The negative end of the cap goes to ground in the radio. Now remove the wire which connects pin 4 of the eye to B+ and connect pin 4 to the junction of the diode and 10 mfd cap.
The actual voltage on the eye will be slightly less than 1.4 times the AC plate voltage on the 80. The higher the voltage on the eye tube, the larger the AVC voltage needed to close the eye. The 6E5 tube is more sensitive than the 6U5 or 6G5 and will have more movement when tuning in a station.
If you notice no movement of the eye when tuning in a strong station, either the AVC voltage is missing or a small 1 meg resistor built into the 6E5/6U5 socket is open. This resistor is between pins 2 and 4 and needs to be replaced if it is open.
A voltage doubler can be used on a radio that doesn't have a high enough AC voltage such as AC/DC radios. The doubler can be made up of two diodes and two capacitors. Again, a resistor should be used for protection.
The 1629 tube can be used to replace 6U5, 6E5, 6G5, etc. tuning eyes if you make an adapter and increase the filament voltage to 12 volts. At first it seems like another filament transformer must be added but this is not necessary. The 1629 as well as most tubes will operate on AC or DC for the filament voltage. For AC radios you can make a simple voltage doubler that will convert the 6.3 volt filament of the original tuning eye tube to over 12 volts DC for the 1629. To make the doubler two 470 mfd capacitors and two 1N4001 or similar diodes are required. See the diagram below:
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