Easy Way to light magic eye tube EM80
This is a green glowing EM80 tuning / indicator / magic eye tube. I just wanted to see it glow. I was able to make the glowing green fan shape slowly open and close. They make (made) round ones called "cat's eyes" that wink! These were cheaper to make than moving needle meters, but as you can see from the Geiger counters in old monster movies, needle meters grabbed hold way back in the 1940s. Luckily the failed and demented priciples of socialism in the USSR kept factories producing these tubes in Russia until the 1990s...instead of food or clothing or useful items...they just kept making these tubes that the entire world (including Russia) atopped using in the 1920s! For under $20 including shipping on eBay and Amazon you can still get brand new in box magic eye tubes.
See the bottom of the page for a video of my capacitance tester that has one.
AC/Furnace and power supply were making a lot of noise in the video. I was also nervous about holding a camera and live wires--so it's loud, weird and shaky.
Here is the stupid simple way I got it to light up, and then open and close:
+250vDC to pin 9
+250vDC to 470k-660k resistor to pin 7
-250vDC to pin 2
5vDC wallwart either output to pin 4
5vDC wallwart either output to pin 5
Touched pin 1 to pin 7 momentarily to work the eye.
When I touched pin 1 to pin 7 the eye closes, but 250vDC power supply sags. I'm only doing it for a few seconds at a time. The grid voltage (g1) on pin 1 is supposed to act to choke off the power to the tube's triode to open/close the eye. It's supposed to be like -1v to -14v, but by connecting the pins I'm hitting it with almost the full -200v I think.
It worked better with a resisitor higher than 220k. Added second 220k to give 440k and it worked but still got too dim when working the eye. A third resistor made it 660k and that seemed better. That's what I uses in the video. I built a decade resistance box which I'll plug in, and then turn dials to easily set different amounts of resistance and see what works better.
My big DC electrophoresis supply was set at only 200vDC in the video: still utterly deadly!
Anyway, the drop in voltage in the power supply is either: supposed to happen (triode shutting down due to control voltage); or it's not a great idea to basically short out your power supply. Most power supplies might pop a fuse/capacitor/blow up, but I was using an electrophoresis supply which is meant to have wires dangling in water/gel for DNA testing...so it might be a little more forgiving.
Here's the schematic:
Here is the replacement of the resistors with my decade resistance box:
I discovered something nobody else online has mentioned: if you use a resistor (or dial up resistance on a box) at 10k you will see a faint ghost image of the open/closed positions on the eye. If you turn the 10, 000 dial right or left you will see these lines move. When you touch pin 1 to pin 7 the lines will be where the eye opens to! Its like a preview! I call these "Logusz Lines".
Here's a photo of my Heathkit DR-1 decade resistance box I bought for a dollar at a resale shop. Heath Inc. used to be nearby in Benton Harbor Michigan.
Extra notes:
All of my posts are just lab notes online. Most are deadly projects! These are not instructions!
Crazy deadly mess of wires. On the left are two of the eventual three resistors in a row. The red wire is +250vDC which splits off: one way goes directly to pin 9. The other goes to 660k resistors which goes to pin 7.
So, what if I put a potentiometer between pin 1 and 7? With it closed it would have to dissipate a lot of power (heat) and possibly melt. I'm not sure if I'd have to ground it (using 3 wires) or not (using 2 wires). Which way would kill me or the circuit?
What if I put a decade resistance box in place of the resistors at pin 7 and dialed it up and down? Power (heat) dissipation wouldn't be a concern, since it wouldn't be connected--only when I touched a wire from pin 1 to 7. This is a great way to dial up a different resistor value without having to solder in/out a bunch of resistors. I had different results with the 3 different values I tried (220k, 440k and 660k). Different amount of eye open/close but also dimming of the entire green output.
Here's the original report: just wires and no resistor so it would glow green:
Here is the pinout for the EM80 tube:
Nothing will happen without a power supply going to the heater pins.
You must add around +200vDC to pins 7 and 9 and -200vDC to pin 2 to get green glow.
To begin:
I needed a 6.3v (AC or DC) supply for heater pins 4 and 5.
I found a wall wart phone charger type thing and cut the end off and attached the bare wires to yellow alligator clip wires. On the wall plug part it had a sticker that said the output was 5 volts DC. Close enough!
I plugged it into the wall and the heater filament inside the vacuum tube gave a tiny orange glow. Success!
Next I needed 250v DC power. This had to be DC.
I was usiing the nifty disposable camera power supply I made for my neon bulb post. But then my 250v DC laboratory electrophoresis power supply ($65) arrived from eBay. So I used that.
As noted in my neon lamp bulb post high DC voltage is very hard to come by: either a camera flash diy conversion, a lucky find on ebay, or splicing wires into a guitar amp or old time tube radio. Lighting up to see the green glow is fine with a camera flash conversion, but I think shorting out to open/close the eye would kill it.
I took a white wire and ran it from the POSITIVE 250v DC output to pins 7 and 9.
Then I ran a green wire from the NEGATIVE -250v DC output to pin 2.
Boom: crazy green glow! So happy.
In the datasheets and magic eye tube tester circuit diagrams the ground symbol actually means the negative DC wire from the DC power supply. A DC power supply only had two output wires: positive and negative. By an annoying convention ground can mean the negative wire of the power supply or battery you're using.
The spec sheets comes right out and says AC or DC is fine for the heater pins, but they don't make clear that the 250v supply needs to be DC.
To get the green "eye" to open and close you can sometimes short certain pins together. To slowly open and close the eye the proper way is to input NEGATIVE -1v to NEGATIVE -14v. I do not yet know if that is AC or DC or if it's relative to the 250v DC? Like: is it -1 volt or 250-1= 249v?
Also pin 1 is the control grid pin. How do you add voltage to a single pin? Some specs seem to show positive and negative wires going towards that single pin. If it's wired in how do you raise and lower the voltage to open and close the eye? Add a potentiometer?
Supposedly, if you ground the control grid the eye will open. Give it negative volts it will close. This is "biasing" the tube.
Without any resistors Connecting pin 9 to pin 1 turns off the green. That makes sense because pin 9 has the full +250vDC and pin 1 (control grid) wants negative DC. It seemed like a bad idea to continue testing this. I won't connect pin 9 to anything anymore. I'll just unplug everything if I want darkness.
Here's more "official" ways to open/close the eye, but I haven't tried them yet, mainly because I don't have an old timey radio with an AVC (auto volume control) to feed into pin 1.
Here is my Sprague capacitance tester that has a round "cat's eye" magic eye tube:
