How did the "magic eye" work on old valve radios ?

[nigelwright7557]

When I was a small child in the early 1960's we had a radiogram with a magic eye for detecting when a radio channel was there.

What was this device ? a valve ?
It had a green light that shone through the display panel.

[Don Hills]

It's a triode valve. The anode (plate) of the tube is coated with green phosphor that glows when hit with electrons. The grid is a single wire suspended between the cathode and the plate. With no potential on the grid, all the electrons from the cathode strike the plate. When a negative voltage is placed on the grid, it deflects the electrons away from the wire, casting a "shadow" on the plate. Higher voltage causes a wider "shadow".

[dcgillespie]

It was in fact an electron "eye" tube, so called because many of these tubes had a appearance of an eye that would open and close with a signal applied to it; how much it opened or closed indicating how much signal was present. They were commonly used as tuning indicators, center of channel indicators, and even record level indicators on early tape recorders. The plate is coated with florescent material in these tubes so that the coating gives off a soft green glow when bombarded by an electron flow. The grid of these tubes would act to cast a shadow on the plate to give an indication of intensity. They are still very cool today!

Dave

[nigelwright7557]

Thank you for your replies.

[g(f(e))]

I have a few of these.

Can anyone suggest a good way to use them in a SE triode design? Even if it is just for looks, I think it would be a good learning project.

Thanks

Gary

[Eli Duttman]

All of the above is correct. The "eye" has to be driven by a "regular" triode. A number of types, like the 6G5 contain both active sections within a single envelope.

FWIW, I've had a hankering to use the 6AF6 twin "eye" as the tuning and signal strength indicators in an all "hollow state" FM tuner project. Bank balance issues have put that idea, along with many others, on an indefinite hold.

[Wavebourn]

In radios AGC voltage (Automatic Gain Control) was used to drive as well Magic Eyes. The better was tuning on the station, the higher was negative AGC voltage. That means, in order to drive Magic Eye you need to rectify output signal from the amp and use a negative polarity of it.

I use 6E1P tubes, they require up to -20V for full picture. I use biased diodes to rectify it, otherwise it starts acting on relatively high volume. Also, some amps like 5-10WPC have not enough of output voltage, so I use additional triodes to amplify signal for the rectifier that drives the magic eye.

I saw sometimes DIYers use signal from anode of output tube; I would not recommend that because even slight non-linear load on rectifier causes audible distortions.

Another trick is to use the same magic eye as AC amplifier to drive itself.

[aardvarkash10]

Originally Posted by g(f(e))

I have a few of these.

Can anyone suggest a good way to use them in a SE triode design? Even if it is just for looks, I think it would be a good learning project.

Thanks

Gary

There are many schematics showing them as indicators in SE amps - in general terms the g1 is taken off the same point as the g1 for the finals

[thoglette]

Originally Posted by Eli Duttman

FWIW, I've had a hankering to use the 6AF6 twin "eye" as the tuning and signal strength indicators in an all "hollow state" FM tuner project.

The dynaco FM-3 used a magic eye and I recall that the schematics (and bunch of other info) was put together by, iirc, gary kaufman in the late 90's

An issue of Sound Practices had a "buddafied" FM-3 and may have schematics, again IIRC

[aardvarkash10]

and i think Steve Bench made an audio amp out of one...