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Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets.

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Old 22nd Jan 2011, 10:26 pm   #1
Martin G7MRV
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Default EM84 tube - pinout and spec?

Hi chaps,

Just aquired (acquired) an EM84 magic eye tube from a sadly defunct Grundig Modello stereogram (shame really as the cabinet was very nicely kept).

Now, iI now nothing of these, but would like to knock up a test circuit to try it. Can someone please point me to the pinout, circuit symbol, and specification for this device, so iI can work out how to play with it?

Cheers,
Martin
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Old 22nd Jan 2011, 10:32 pm   #2
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Default Re: EM84 tube - pinout and spec?

Hi Martin This might be just what your after: http://www.r-type.org/pdfs/em84.pdf Have fun Mike
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Old 22nd Jan 2011, 10:33 pm   #3
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Default Re: EM84 tube - pinout and spec?

http://www.r-type.org/pdfs/em84.pdf
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Old 22nd Jan 2011, 10:35 pm   #4
Martin G7MRV
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Default Re: EM84 tube - pinout and spec?

Hi Mike,

Yes that's it, iI happened to find it on the net after muchosearching just moments ago!

My knowledge of all things valve is very limited, so playing with this should give me at least a bit of grounding (no pun intended) regards HT supplies etc. The HT transformer of the Grundig should form a good basis for such stuff.
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Old 22nd Jan 2011, 11:19 pm   #5
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Default Re: EM84 tube - pinout and spec?

All magic eyes are essentially voltage indicators. If you apply heater voltage and some HT, they will show the voltage applied to the grid.

The EM84 is unusual in that it has the phosphor applied directly to the glass side of the envelope rather than to an internal metal surface. Because of this it is very accurate, and was actually used in calibrated form in some test equipment. It was the standard record level indicator in most 1960s domestic tape recorders.
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Old 23rd Jan 2011, 9:42 am   #6
Martin G7MRV
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Default Re: EM84 tube - pinout and spec?

Thanks Paul, interesting.

From the datasheet then iI see iI need to give it 6.3v on the heater and +250v DC on anode, but between o and -10v ish on the grid?

OK, so the 250v should be easy enough, and the heater volts, since iI have the transformer out of the radio, but how to generate the negative LT for testing the function on the grid? Perhaps just an extra battery to provide that?

Does anyone have a simple test circuit?
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Old 23rd Jan 2011, 10:58 am   #7
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Default Re: EM84 tube - pinout and spec?

Just use a PP3 battery.
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Old 23rd Jan 2011, 1:44 pm   #8
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Default Re: EM84 tube - pinout and spec?

You will also need a supply to the deflection electrode. A 470K between this electrode and the anode should do otherwise I dont think you will see anything on the fluorecent strip.



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Old 23rd Jan 2011, 3:19 pm   #9
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Default Re: EM84 tube - pinout and spec?

A quick Google and I found this website with a circuit for testing indicator tubes:


Cheers n Beers,

David......
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Old 27th Aug 2012, 4:43 pm   #10
Martin G7MRV
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Default Re: EM84 tube - pinout and spec?

Im (I'm) hoping to finally get this tube tried soon.

But, can anyone check a circuit for me? Ive (I've) worked up this simple circuit that im (I'm) hoping will at least let me light the thing up.

I cant (can't) get a diagram uploaded im (I'm) afraid, but heres (here's) the details -

EM84 tube
pins 4 and 5 6.3v for the heater
Pin 1 - 10k pot, wiper to pin, one side to gnd, the other to -ve of 9v PP3.
+ve of PP3 to gnd
Pin3 to gnd
Pin 6 to +225v
Pin 7 tied to pin 9
Pin 9 to +225v via 470k resistor

Im (I'm) open to suggestions on this of course. Im (I'm) thinking the pot might need to be bigger?
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Old 27th Aug 2012, 8:57 pm   #11
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Default Re: EM84 tube - pinout and spec?

Here is a circuit I have used, I skipped the transistor part however
http://www.solderingpoint.com/projec...cator_tube.php
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Old 27th Aug 2012, 9:09 pm   #12
Martin G7MRV
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Default Re: EM84 tube - pinout and spec?

Hmm, It seems I may have been wrong all along, and its an EM87! Sadly the printed markings all rubbed off as iI got it out this evening! It still has the etched marking 'D7K4',

Looking on the valve museum webpage, the 84 doesn't have the silvering at the top, whereas this one does, as shown on the 87. Another possbile (possible) give away is that the anode to deflection resistor is still fitted to its socket, and its a 100k, as specified for the 87 in its datasheet.

I believe however that theres little else between them, other than the 84 having a wider grid voltage range?
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Old 27th Aug 2012, 9:16 pm   #13
Martin G7MRV
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Default Re: EM84 tube - pinout and spec?

Quote:
Originally Posted by MrElectronicman View Post
Here is a circuit I have used, I skipped the transistor part however
http://www.solderingpoint.com/projec...cator_tube.php
David, yes thats very much what iI was attempting to describe, save for the transistor amp!

Once iI have the power supply built for the Dekatron, iI can also use it to drive this beastie. (I wonder if there would be any way to combine the two? hmmm)
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Old 27th Aug 2012, 9:22 pm   #14
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Default Re: EM84 tube - pinout and spec?

The difference between an EM84 and an 87 is the sensitivity, an EM84 needs -22 volts to close, and the EM87 needs about -10 volts, if I remember correctly.
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Old 27th Aug 2012, 9:36 pm   #15
Martin G7MRV
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Default Re: EM84 tube - pinout and spec?

That looks about right, ive (I've) the basic spec written down here side by side for them both, yes the EM84 says -22v, the EM87 -15v

Ive (I've) also checked the potentiometer size iI was looking at, and it says 5M. That sounds better with the grid resistor value on the spec sheets than does my original guess of 10k!
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