Magic Eye Stuff

Animated .GIF courtesy of Ake Holmes

Magic tubes came into use in the mid-30’s as a tuning aid, in high end radio receivers. With the manufacturing infrastructure at that time, they where more cost effective than an electromechanical meter movement. They stayed in use well into the 70s. They where still being built into some solid state receivers, well after cheap meter movements came into use . But they eventually fell out of favor.

They are relatively easy to use. The shadow (or shadows) vary with the input voltage. Most come with a built in amplifying triode, since the signal they intended to measure varied at most 20V, but the shadow itself usually needs a voltage varying close to 100V.

Here is a project I made using this tube family.

The Magic Eye Winker:(Click of photo to view video)

Here is the schematic for the winker above (Click on drawing to enlarge):

This circuit can be used with a wide variety of magic eye tubes. There are a couple of caveats. The eye type needs to have an internal triode. The ones listed on the table do. And these triodes must have adequate gain to keep the phase shift oscillator going. Tubes like the 6G5/6U5 have remote cutoff characteristics, which doesn’t work,  and the EM84 just doesn’t have enough gain.  Most of the tubes shown, have 300mA filaments, and the 6.8uf cap provides that current, if run from 120V, at 60Hz.

The Japanese 6ME5, and 1629 (VT138) have 150mA filaments, so the capacitor must be reduced to 3.3uf, to limit the current to that value.  Also, if either of these tubes are used, then the rectifier must also have a 150mA filament. Use a 12AL5 for 150mA, and a 6AL5 for 300mA.

Using capacitors for current limiting will work quite reliably. The tube filaments have good thermal inertia, and handle the initial charge up surge quite easily. I ran a much more demanding test on the filaments 1000 times, and the tubes showed no ill effects.

Here’s a version that will work for non-amplified eye tubes, like the 6AF6. Its made to work by adding a amplifying tube. In this case a, 150mA filament , 6BH6 sharp cutoff pentode. Its hooked up in triode mode though. The tube needs to have sharp cutoff characteristics. Remote cutoff won’t work.

I haven’t found any references to this circuit. If you find any on the web, they originally came from me. I have no delusions of being some great design guru, who just invented this circuit. I’m pretty sure this kind of circuit was made as a toy, in a few engineering labs in the industry, during the tube era (1935 -1970).  But being a toy, they never bother to publish it. Or even buried any info on it, for fear that their superiors would take some punitive action.

Click Here for 6AF6 Winker Video

Power supply section omitted, since its temporarily assembled on a protoboard.

Magic Eye – article at Wikipedia

The former Soviet Union made many tubes, including eye tubes, that were similar to western types. Here is the pinout of a Soviet 6E5S (or 6E5C):



Unlike the U.S. 6E5, the Russian one has an octal base. The U.S. version has the older large 6-pin format.