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Old 2nd December 2004, 04:58 PM   #1
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Default Playing with a Magic Eye.

Hi there,
I am attempting to add a EM81 magic eye to a preamp project I am working on. The specs (what I could find) say that it needs an input of -10V for maximum effect. The problem is that my preamp is designed as a line level unit, so unity gain. I have added a voltage doubler input, but this will still only get me up to a theoretical -4V, but more realistically will only produce about -1V in normal use.
The input amplifying triode is internally connected to the target of the display section, and has a sujested anode resistor of 470K. If I reduce this to say 100K to give me greater gain on the triode this will also have an effect on the operating point of the display section, shifting it by potentially +70V. Will this work or does this tube absolutely need its -10V input.

I really want to stear clear of adding more complication to what is essentially a novalty circuit. If I have to add in another voltage gain stage I might just have to dump it.

Any experience, any ideas ?

Shoog
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Old 2nd December 2004, 05:15 PM   #2
markp is offline markp  United States
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Would floating an opamp with a gain of ten work?
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Old 2nd December 2004, 06:06 PM   #3
Yvesm is offline Yvesm  France
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Quote:
If I reduce this to say 100K to give me greater gain on the triode this will also have an effect on the operating point of the display section, shifting it by potentially +70V.
err, you should increase that value to increase the gain.
However this may just add a "bias" to the display.
Try to use the larger B+ avilaible along with the larger value of plate resistor, or even a CCS ?

Perhaps you could find a point into your preamp where the signal is larger than at output ?

Yves.
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Old 2nd December 2004, 06:18 PM   #4
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If you wanted to put it in the power amp, I'd simply suggest you tap off the driver stage. But since you don't have any signal so high, you'll have to add another stage. Or perhaps you can use another tube, say a high-gain pentode in lieu of the built-in(?) triode?

Tim
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Old 2nd December 2004, 06:21 PM   #5
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Old 2nd December 2004, 06:39 PM   #6
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I found that Circuit and was working from that as the basic idea. But the basic BBC meter is for a different tube with an even lower input requirement of just -5V.
I really wanted to avoid including any further gain stages. An opamp would need another power supply ( or at least a voltage doubler).
The whole project is going in a dead PC power supply case so real estate is at a premium ( especially since it will house a choke).

Its sounding increasingly likely that it just won't play ball .

Shoog
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Old 3rd December 2004, 10:24 PM   #7
SHiFTY is offline SHiFTY  New Zealand
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I rigged an EM81 onto a speaker output that seems to work fine... I used a schematic from the net, hope the author doesn't mind me reposting it. I removed the 390k resistor to boost the signal a little.

This circuit had a lot of gain, might be worth breadboarding...
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Old 13th January 2005, 09:33 PM   #8
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Thought you might be interested in an update.
I finally got the preamp working. The magic eye is rigged up with B+ 200V, an anode resistor of 33K and the grid grounded along with the cathode. This is fed through a 0.47uf cap off the input to the preamp. This produces the eye tube in the half closed position with no signal, and with a 2V source it will move about another quarter, which is just about satifactory for the job its intended for. The cap interacts with the grid in order to only move the beam with the negative half of the input signal. Draws very little current so has minimal effect on the input signal.

Shoog
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