> The eye is especially useful because the AGC action tends to increase the audio volume of a mistuned station, so the volume varies relatively little as the tuning knob is turned. The tuning eye was driven by the AGC voltage rather than the audio signal.

So, have a simple switch to disable AGC while tuning?

Or, a not-so-simple switch. The switch could be linked to a pawl which engages the teeth of a gear on the shaft of the tuning knob, such that you can't turn the tuning knob if you don't activate the switch.

I have a couple of these. Of the ones in a working device, they are in capacitor ("condenser") checkers. You rotate a dial until the eye tube goes from closed to open, then read off the capacitance on the dial.

I don't remember the exact design off hand, but I seem to remember that about all that's inside one of these is the eye tube, a rectifier tube, and a handful of resistors and capacitors. Can also test leakage and shorts. They really knew how to get a bunch of things done with about 10 components back then.

I was hoping this would be some new fangled way of viewing magic eye images mapped onto the inside of a tube through something like polar coordinates and viewed like a kaleidoscope. I was disappointed.

When I was about 12 or 13 I recovered an old reel-to-reel recorder from the trash that had a magic eye tube on it (acting as a recording level meter). The mechanism was broken so it couldn't play or record tapes, but the magic eye worked. I'd never seen anything like it. I'd play with the thing in a dark room and enjoyed the eerie green glow from the magic eye and the warm yellowish glow from the tubes inside coming thru the vents. I saved the magic eye tube and still have it in a box somewhere.

They're real fun.

I'm building a vacuum tube computer and want to use some somewhere, even though everything I've come up with is very shoehorned in. Best I've come up with is showing progress in the reset sequence.

Although right now the clock circuit isn't well defined (I've got a micro controller running the clock and front panel like signals while prototyping), so maybe a pll lock indicator.

I remember playing with my father's collection of tape recorders when I was young and one of them had such a "cat's eye". Maybe that's where my love for glowing phosphors started.

I wish we could manufacture these things again. Preserving the ability to build obsolete machines is important.

Related:

The Magic-Eye Tuning Indicator, W G Morley, The Radio Constructor, March & April, 1969:

https://www.r-type.org/articles/art-137.htm

I remember as a kid some device had one of these, but I can't for the life of me remember what it was.