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Old 3rd January 2008, 04:20 PM   #1
poynton is offline poynton  United Kingdom
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Default Simple Magic Eye Circuit Wanted

Hi.



I have several magic eyes but I like the look of the EM71.

Does anyone have a simple circuit to drive and power on of these??


Andy
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Old 3rd January 2008, 04:48 PM   #2
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http://www.akh.se/tubes/htm/em71.htm
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Old 3rd January 2008, 05:00 PM   #3
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I have a little magnavox amp with an unused tube socket that i'd like to do something similar with. I have a bunch of NOS EM84's laying unused.
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Old 3rd January 2008, 05:08 PM   #4
poynton is offline poynton  United Kingdom
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Quote:
Originally posted by jeffdavison
http://www.akh.se/tubes/htm/em71.htm

Hi
Thanks - that will probably do the job !

What resistor / drive arrangement will be required on the input ?

I assume a capacitor will be required so it's AC not DC ?

Andy
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Old 3rd January 2008, 06:17 PM   #5
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If using it as a level indicator to detect a signal that contains no DC. You will have to rectify the signal to produce a negative voltage. Then use a cap to filter it to -DC. So a diode and a cap or a bridge rectifier and a cap. And you will probably want to put a bleeder across the cap so it doesn't just hold the lowest value for ever.

If detecting a signal that also contains a positive DC. You must first use a coupling cap to block the positive DC before doing the process mentioned above.


If your using it to detect bias I'll have to see the circuit your adapting it to.
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Old 3rd January 2008, 07:11 PM   #6
poynton is offline poynton  United Kingdom
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Thanks

I just want to put a level indicator on the front of an amp.


Andy
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Old 3rd January 2008, 08:17 PM   #7
Jeb-D. is offline Jeb-D.  United States
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From that spec sheet it looks like the range is 0-20V. So you will have to tap it off a pretty strong signal. And you may have to make a voltage divider to give you good range on the display.
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Old 3rd January 2008, 08:29 PM   #8
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The attached circuit is an excerpt from one of my projects. EM71 has an interesting non-linear, degressive characteristic somewhat suitable for output level (power) "metering", showing movements at low levels already, while not running into full reading too fast.

Tom

P.S.: The C3 value as shown is vast overkill; depending on B+ quality this cap even can be left out altogether.
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