EM84 tube - pinout and spec? - UK Vintage Radio Repair and Restoration Discussion Forum

Martin G7MRV · 22nd Jan 2011, 10:26 pm

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

Cobaltblue · 22nd Jan 2011, 10:32 pm

Hi Martin This might be just what your after: http://www.r-type.org/pdfs/em84.pdf Have fun Mike

AlanBeckett · 22nd Jan 2011, 10:33 pm

http://www.r-type.org/pdfs/em84.pdf
Alan

Martin G7MRV · 22nd Jan 2011, 10:35 pm

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.

paulsherwin · 22nd Jan 2011, 11:19 pm

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.

Martin G7MRV · 23rd Jan 2011, 9:42 am

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?

paulsherwin · 23rd Jan 2011, 10:58 am

Just use a PP3 battery.

Sideband · 23rd Jan 2011, 1:44 pm

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.

SB

Coastwatch · 23rd Jan 2011, 3:19 pm

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

Cheers n Beers,

David......

Martin G7MRV · 27th Aug 2012, 4:43 pm

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?

MrElectronicman · 27th Aug 2012, 8:57 pm

Here is a circuit I have used, I skipped the transistor part however
http://www.solderingpoint.com/projec...cator_tube.php

Martin G7MRV · 27th Aug 2012, 9:09 pm

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?

Martin G7MRV · 27th Aug 2012, 9:16 pm

Quote:

Originally Posted by MrElectronicman

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)

MrElectronicman · 27th Aug 2012, 9:22 pm

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.

Martin G7MRV · 27th Aug 2012, 9:36 pm

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!

22nd Jan 2011, 10:26 pm	#1
Martin G7MRV Pentode Join Date: May 2008 Location: Selby Posts: 206	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 __________________ I got food in ma belly and a license for ma telly

22nd Jan 2011, 10:32 pm	#2
Cobaltblue Heptode Join Date: Dec 2010 Location: Exeter and Poole Posts: 676	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 __________________ I don't care if it was a bargain what's it doing on my kitchen table? Mike T BVWS member.

22nd Jan 2011, 10:33 pm	#3
AlanBeckett Octode Join Date: Dec 2008 Location: Burton upon Trent Posts: 1,712	Re: EM84 tube - pinout and spec? http://www.r-type.org/pdfs/em84.pdf Alan

22nd Jan 2011, 10:35 pm	#4
Martin G7MRV Pentode Join Date: May 2008 Location: Selby Posts: 206	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. __________________ I got food in ma belly and a license for ma telly

22nd Jan 2011, 11:19 pm	#5
paulsherwin Moderator Join Date: Jun 2003 Location: Oxford, UK Posts: 6,978	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.

23rd Jan 2011, 9:42 am	#6
Martin G7MRV Pentode Join Date: May 2008 Location: Selby Posts: 206	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? __________________ I got food in ma belly and a license for ma telly

23rd Jan 2011, 10:58 am	#7
paulsherwin Moderator Join Date: Jun 2003 Location: Oxford, UK Posts: 6,978	Re: EM84 tube - pinout and spec? Just use a PP3 battery.

23rd Jan 2011, 1:44 pm	#8
Sideband Dekatron Join Date: Oct 2003 Location: Croydon, Surrey Posts: 3,580	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. SB __________________ 'And, in the end, the love you take is equal to the love you make' The Beatles.

23rd Jan 2011, 3:19 pm	#9
Coastwatch Pentode Join Date: Feb 2010 Location: Berwick-upon-Tweed Posts: 114	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...... __________________ Ouch! who turned that soldering iron around My other hobbies

27th Aug 2012, 4:43 pm	#10
Martin G7MRV Pentode Join Date: May 2008 Location: Selby Posts: 206	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? __________________ I got food in ma belly and a license for ma telly

27th Aug 2012, 8:57 pm	#11
MrElectronicman Hexode Join Date: Feb 2011 Location: Washington DC, USA Posts: 409	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 __________________ David

27th Aug 2012, 9:09 pm	#12
Martin G7MRV Pentode Join Date: May 2008 Location: Selby Posts: 206	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? __________________ I got food in ma belly and a license for ma telly

27th Aug 2012, 9:22 pm	#14
MrElectronicman Hexode Join Date: Feb 2011 Location: Washington DC, USA Posts: 409	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. __________________ David

27th Aug 2012, 9:36 pm	#15
Martin G7MRV Pentode Join Date: May 2008 Location: Selby Posts: 206	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! __________________ I got food in ma belly and a license for ma telly