Magic eye tube (en.wikipedia.org)

90 pointsby peter_d_sherman4 days ago23 comments

alt227 11 hours ago

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.

VorpalWay 10 hours ago

I think the vaccum tube that it turns out to be is even cooler.

Loughla 6 hours ago

Yeah the fact that we can harness electricity and vacuum to make an indicator that just 'looks right' is kind of amazing.

alnwlsn 8 hours ago

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.

EvanAnderson 9 hours ago

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.

Isamu 6 hours ago

Yes! We had an old reel tape recorder with the magic eye, I loved using the mic with that.

kazinator 7 hours ago

> 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.

frrlpp 6 hours ago

Not that easy, AGC works both ways, if you disable it you need to provide a manual control to not blow your speaker or your ears or both. AGC controls the screen voltage of some tubes with a big factor of amplification. A typical remote cutoff pentode like a 6sk7 can change some 30-40 dB for stage. 70-80 dB AGC action is not rare.

kazinator 4 hours ago

> manual control to not blow your speaker or your ears or both.

Distinct from the volume knob?

It could be that rather than completely disabling AGC, it switches to milder compression.

Another idea is that while tuning, we can delay the volume recovery of AGC. In other words, the same fast compression attack, but very slow release, so that as the user tunes across the center of the band, they can still feel the volume swell.

A simple voltage divider could be introduced when AGC is turned off to attenuate the overall volume, so that the loudest blast is not that loud.

irdc 10 hours ago

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.

andai 10 hours ago

Old electronics had so much sovl.

joemi 9 hours ago

Might be dating myself by not being up to date on the latest slang or abbreviations, but what is "sovl"?

nemomarx 9 hours ago

soul but using the roman v, I think. I've seen it before with cvlt that way

bigyabai 6 hours ago

It's largely 4chan "look what they took from you" cultural panic newspeak, either ironically or unironically.

"sovl" has been variously used to describe LaserDisc, summer camp, bench seat trucks and the McRib.

flyinghamster 6 hours ago

My father had one as well, an old Magnecord unit packaged into a wooden case with transport and preamp, with the preamp having one of these tubes. Nevertheless, it was also fitted (perhaps by the dealer) with a proper VU meter. I learned the basics of recording on that, which served me well at a variety of times.

Naturally, his FM tuner from that era also had a horizontal magic eye tube.

monocasa 9 hours ago

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.

peter_d_sherman 4 days ago

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

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

rbanffy 9 hours ago

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

JackFr 8 hours ago

It's cool, no doubt, but why precisely is it important? I find the engineering that went into late steam locomotives fascinating and much more complex and scientifically designed than the casual observer notices. But building one would be very expensive and serve no other purpose than to delight some nerds (which is a fine result but unlikely to be worth the price.)

m463 10 hours ago

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

ErroneousBosh 10 hours ago

It is likely to have either been the recording level indicator on an old valve reel-to-reel tape recorder, or the tuning indicator on an old valve FM radio.

m463 10 hours ago

thank you.

I suspect it must have been a radio. I remember it said something about eye, but was always dark.

Animats 9 hours ago

The whole point of the Magic Eye tube was that it was cheaper than an analog meter. So it was the consumer-grade replacement for one. On pro gear, you'd find a meter movement and pointer, which is harder to make than a vacuum tube.

In "The Day the Earth Stood Still" (1951, not the remake), when the visitor goes into his flying saucer and powers up a communicator to report to HQ, there are two huge Magic Eye tube like devices. So back then it was a cliche for "radio". Start at 55 minutes.

[1] https://archive.org/details/day-the-earth-stood-still-1951