Magic eye indicator in vari mu limiter

[tardishead]

I was thinking about tapping the negative control voltage from a vari mu limiter (STA LEVEL, AM864 etc etc) before the transformer centre tap in order to drive a magic eye indicator circuit.
Will this load down the CV or have any adverse effects on the vari mu action?

 

[PRR]

> any adverse effects

Just do it.

Make a movie.

 

[tardishead]

Sorry to double post but I am not getting much luck in the lab

I hooked up a EM800 magic eye to the control voltage of a Federal AM864
I cant quite get enough negative voltage for maximum light band. EM800 needs about 10 volts to get the full band.
The max negative voltage from the Federal CV is about -4 to -5 volts.
Can anyone think of a simple and elegant way to double the CV voltage to the magic eye without compromising the CV path in the limiter??

My power transformer heaters are 6.3vac 2 amps. I have used 1.8Amps already so running short on heater volts. I think another valve is probably out of the question. Is there a 150ma valve that can give a gain of 2???

 

[clintrubber]

Thread title adapted to the more common & desirable usage of uppercase & lowercase on these forum pages.

Regards,

  Peter

 

[tardishead]

I was thinking of trying a voltage doubler but surely the extra capacitors would affect the time constants of the limiter???

 

[SUPERMAGOO]

I was thinking about tapping the negative control voltage from a vari mu limiter (STA LEVEL, AM864 etc etc) before the transformer centre tap in order to drive a magic eye indicator circuit.
Will this load down the CV or have any adverse effects on the vari mu action?

 Good idea!
and with em84?

http://www.youtube.com/watch?v=IhhOgMvlYL8

http://koti.mbnet.fi/siliconf/JukkaTolonen/ga/peakmtr/peakmtr.html

 

[tardishead]

Yeh its a good idea and works but only fills half the magic eye.
Most magic eyes expect 0-10 or 0-20 negative grid volts - I am only getting 0-5 negative volts from the federal. so i just need to double the volts!!!

 

[SUPERMAGOO]

seeing this post, take the old Vumeter with em84 and I am amazed at how fast it is.
to try to understand a little how to adapt to a varimu, you could make a schematic?

 

[tardishead]

I want to measure the compression control voltage not the audio voltage.

Where is the schematic for a EM-84 vu meter???

 

[PRR]

Change the "+250V" to 125V.

 

[strangeandbouncy]

Hi Guys,


    whaT a very cool idea! I used to run the mix into my Binson Echorec so the clients could see the MagicEye working. They all used to love it. I guess any kind of flashing light would have done, but they were awestruck by the scifi glow!


    I'll be trying this for sure!


      ANdyP

 

[strangeandbouncy]

Actually,


  would an EM87 be better, since it's response is more linear?



  KIndest regards,



      ANdyP

 

[tardishead]

I chose EM800 just because I like the graphic bar rather than an open/close effect. No too concerned with the linearity of it

 

[tardishead]

So ok I reduced the HT to the EM800 and it works to some extent. Increases the extension of the light band. However because there is always a small negative voltage on the CV line in the Federal - the minimum extension of the eye is also getting bigger - so effectively the extremes ratio does not change as much as I would like.
Also when you drop the HT the eye is not as bright.

However the same result of extension ratio would occur if I doubled the control voltage because it would also double the minimum bias voltage. However at least it would be brighter. Can anyone recommend a way of doubling the CV??

Maybe the Federal is not the best vari mu to do this. I need something with a slightly larger CV.

Deep gain reduction on 6386 occurs at -70v so thats a bit much although I think you can get eyes that will cover this.
6BC8 might be a better contender - 436 and UA175 comps.

 

[PRR]

There is NO simple, or even reasonable, way to get a gain of +2, or a negative output, with vacuum tubes on a single rail.

A chip will, if you give it a negative rail.

> because there is always a small negative voltage on the CV line in the Federal - the minimum extension of the eye is also getting bigger

Some minimum length is inherent in the tube. And as you say, in the limiter CV. So you -also- desire to subtract 1.6V before you gain-up.

Just cuz it is EZ: change Ra 200K to 470K or 1Meg. What does that do for you?

To get an offset: you could try to jack-up the limiter cathodes, grid resistor, and diode reference by 1.5V. However the diode ref is also the triode cathode, which has its own diddly adjustment.

You could jack-down the EM800 cathode by 1.5V. Since the total current is hardly over one mA, a simple C-cell would last for years.

I'm still finding it hard to believe that the Federal won't stir up more than 5V.

 

[tardishead]

I'm still finding it hard to believe that the Federal won't stir up more than 5V.

Unless my 6SQ7 is a particularly bad one???

 

[PRR]

> Unless my 6SQ7 is a particularly bad one???

Gosh, I hope not! That's a rare and expensive, tube, isn't it?

You could check the DC voltages on it. A bad triode will show bad DC voltages.

A bad cap is another possibility.

 

[gemini86]

seems to be a slew of em on evilbay...