Wednesday, August 10, 2016

Audio VU Meter

I have a bunch of Magic Eye tubes and was wondering what to do with them.  An audio VU meter with a microphone pickup could make a nice magical flickering display as I originally described here: An Angel Dancing On a Pin Head.

The example Rusky circuit works, but it needs much more gain to work with a microphone and it needs a power supply of sorts.  So, I dusted the old circuit off, hooked up a little triode as an amplifier to drive the display tube and my prototype works nicely.   Moving the resultant rat's nest from the breadboard into a proper display case is another matter though.

The 6E1P / EM80 is super simple, since the Target and Anode are connected together internally, so you don't have to.  It operates at a rather high 250V:
  • Pin 1: Gate
  • Pin 2: Cathode
  • Pins 4, 5: Heater
  • Pin 7: Anode
  • Pin 9: Screen 
Viewed from the bottom pin side, the pins are numbered clock wise, starting at the gap on the right.

The 6N21B miniature dual triode valve pin-out is as follows:
  • Pin 1: k1
  • Pin 2: s
  • Pin 3: g1
  • Pin 4: a1
  • Pin 5: h
  • Pin 6: k2
  • Pin 7: no pin
  • Pin 8: g2
  • Pin 9: a2
  • Pin 10: h
Because the double triode is physically small, the operating voltage is lower than normal at 'only' 100V to 150V.

For a little toy like this, a pair of huge transformers will increase the cost and their bulk will detract from the whole idea, so I made a simple direct mains powered supply - shocking, eh...

A magic eye tube is a tiny cathode ray tube as in old TV sets and requires a very high operating voltage.  So I used a voltage doubler on the 220 V mains.  That ensures that the electrons have sufficient kinetic energy to penetrate surface pollutants and stimulate the ancient zinc oxide fluorescent display properly.  High voltage capacitors are costly and one 630V electrolytic could cost as much as a transformer.  Put two identical(!) 315V or 350V caps in series - much cheaper.

The miniature triode needs a much lower voltage, so I made a simple half wave rectifier for that.  So far - fairly standard.  The triode output is envelope detected to create a negative voltage to drive the display tube gate - any small signal diode will work - a 1N4148 is cheap.  You can put a trimmer on the display tube cathode to adjust the DC bias and together with the input volume control, get the shadow to flicker properly.

Note that when you build high voltage circuits, you should use big parts.  A 1/4W resistor is rated at 50V only.  A 2W resistor is rated at 500V, which will prevent smoke signals.

A single triode has a gain of around 50.  That should work with a dynamic mic.  If you use a condenser mic, then you may need more gain (if it doesn't have an internal transistor amp).  The 6N21B is a dual triode and a two stage amplifier will result in a gain of around 2000, which should work.  However, with a gain of 2000, the amplifier may oscillate if you don't take care with the wiring.  I use RG316 coaxial cable for everything when I play with valves (including the heater wiring), to ensure that all signals are screened properly.

The heaters require about 350 mA (I have measured 280 to 370 mA) at 6.3 V.  So for that, I made a simple triac light dimmer circuit (test it with a light bulb and set it to minimum) with the two heaters in series (I eventually used a quadrac - one less part).  A series choke will smooth the current.  To get the right current level, install a 1 Ohm resistor in series with the tubes, measure the RMS voltage over it and tweak the trimmer - then maybe put a spot of glue/wax on it.

The fuse and chokes provide some protection and spike/triac noise filtering. You can wind your own chokes - a few turns on a ferrite toroid or ferrite cable clamp (easier to wind, since you can wind it while it is open!) is all that's needed, or you can buy a 500mA 27mH common mode choke RN112-0.5-02-27M for a couple dollars. A couple of gas discharge tubes will also help to cut spikes down to size, for example the EPCOS Surge arrestor, Series ES1000XS, PN B88069X6561B502, 1000V, 2kA, from Live to Neutral and from Live to Ground.

For a fuse, I always use resettable polyfuses, since they don't blow permanently.  For example Vishay PTCTL7MR100SBE, a 1A, 600V fuse.

Another (more expensive) solution, which avoids the triac noise, is to use a series capacitor to power the heaters.   You would need to use a capacitor that can handle the continuous current and high voltage.  You need to hook the tubes to a power supply at 6.3 V and measure the current once they warmed up and then select a suitable series element.  One possibility is a 4 uF Motor Run Film Capacitor Mouser Part No: 80-C276CC34400AA0J.  Here is the formula: C uF = 159155 x 0.3A / 50 Hz x sqrt(230V^2 - 12.6V^2) = 4.16 uF

As with any thermionic valve circuit, this one is dangerously 'hot' and noisy.  So when this toy is running, keep yer cotton picken fingers in yer pokkets - else you will be sorry.  Mount the whole kaboodle in a plastic box for good insulation.  I have a rubber mat on my shop floor - got zapped a few times and I'm still here...

La voila!


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