dm160 magic eye help

[makaze]

Hi, I am hoping to add the dm160 as an indicator only eye to a micro amp I have built.

I have some help in getting the right voltages for the filament and plate I am a bit stuck on the grid.

The data sheet describes the grid as requiring -3v (turns fully off) and the 0 volt for fully on.

So my thoughts were to first of all create a -3v supply to the grid which the has it in an off state.

Then tap into my ac audio signal in the preamp, rectify it to +only, then using an opamp try and get an output of 0v - +3v.

Finally connecting this opamp out to the grid resistor would then allow the audio to drive the grid from -3 to 0v.

Is any of this right?

Thanks for any input.

[Arnulf]

Originally Posted by makaze

Is any of this right?

All of it That's how it is normally done, you might not even need a separate opamp though. Use low drop rectifier (schottky) or opamp-based rectifier with zero drop if you want more sensitivity at normal listening levels.

[Steerpike]

The DM160 was a logic state indicator for early digital computers, as such it is only an on/off kind off display, not a moving shadow like the more common tuning eyes or level meter tubes. If you have DM160s, always good to use them, but they aren't particularly pretty eye-candy. All the EM8* or UM8* series are prettier, although physically BIG! in comparison, and need abouy 3x the anode voltage

[makaze]

wow Arnulf, I'm amazed and chuffed to bits that any of it was right let alone all of it. Can I just check.......the tube is directly heated requiring 1 volt, so I'll have +1v on the positive side and 0v on the negative side. it the 3v swing I need in relation to the +1v or 0v.

If I tap into the post/or pre preamp signal is there a best method for creating a tap? As the amp is only 0.3watt I presume I need to ensure I dont take too much signal.

The dm160 is v small and will go nicely into my micro amp, I'm just on a learning curve and this seems hard enough. I of course dream of using the beautiful eyes available on my bigger amps. I had an old selmer with the small magic eye for the tremolo and had a matching (homemade I presume) light box made from many larger eyes. Of course I was too young to know what a fantastic thing it was, I just remember being hypnotised by it.

the dm160's dont have shadow effects but they do at least glow at different levels dependent upon input into the grid. Check this funky little put together.

DM160 frequency indicator video by merlinblencowe - Photobucket

Thanks.

Woz

[Chris56000]

Hi!

The DM160 has a 1.0V 25mA filament and is designed to operate off transistor circuits with a voltage change of -5 to -0.5V with a 30V banode supply. As other Members have posted it does not have a variable-length display!!!

The DM70/DM71 has an exclamation-point display which varies in length with the grid voltage, being a maximum at -0.5V and extinguishes at -10V with 90V anode supply. The DM70 has long wire leads whilst the DM71 has cropped leads designed to fit a subminature holder, the 1950s Elpico Geloso Trape Recorder used the cropped lead version!!! Other equivalents 1M1, 1M3, Y25.

Lead connections:- 1 - g, 4 f-, 5 f+, 8 - a.

For Mains use earth Pin 5 and operate the filament from 6V with a 220 ohm resistor.

You can get octal-base eyes still and these are easier to fit to panels than more modern types, but most need 200 - 250V if you're going to see them properly and unless you can find a good NOS or used one, the modern manufactured replacements tend to have very poor phosphors compared to those of the genuine valve era!!!

[Arnulf]

Originally Posted by makaze

wow Arnulf, I'm amazed and chuffed to bits that any of it was right let alone all of it.

Check your original post again; you wondered if this would work:

- datasheet says negative voltage (-3V) for off, 0V for full on, I'll take your word for it since you read the datasheet

- create some sort of -3V to 0V signal source from your AC source (this is actually two consecutive paragraphs of your post)

- create this signal from audio via rectification (= the only way to do it)

- connect this input to tube grid (this is the point where "grid leak" or "grid bias" resistor connects to as well, again the only way to do it)

So yes, correct on all points you posted there.

Can I just check.......the tube is directly heated requiring 1 volt, so I'll have +1v on the positive side and 0v on the negative side. it the 3v swing I need in relation to the +1v or 0v.

In relation to +1V in this case, but trust me, datasheet provides suggestions, you'll need to tweak this value in real circuit anyhow to getr the best possible response (easiest way to do it is by changing cathode resistor). You'll also have to tweak the input voltage swing, you might need a voltage doubler afterall, or a real amplifier with opamp.

If I tap into the post/or pre preamp signal is there a best method for creating a tap? As the amp is only 0.3watt I presume I need to ensure I dont take too much signal.

Signal swing is measured in volts rather than watts. You need to determine signal swing in volts. Normal preamps output line level voltage (1-2V RMS which is almost 1.5V-3.0V once full-wave rectified without voltage doubler).

"Watt" information is useless because you don't know whether the maximum possible signal swing is 0.3V at 1A or 300V at 1 mA which both mean 0.3W.

the dm160's dont have shadow effects but they do at least glow at different levels dependent upon input into the grid. Check this funky little put together.

You could use a neon indicator (such as the venerable NE-2) in place of tube for that. Neon lamps don't require extra heating and they also display varying intensity levels depending on current across them.

For what it's worth, there are also neon lamps which fire up to a different level (physically) depending on current across them, so they work as bar indicators, rivalling magic eye tubes.

[Chris56000]

Hi!

I meant to add this addendum to my post (there's no "edit" button on the forum!) - In my book "Circuits for Digital Equipment" it shows the DM160 with a centre-tapped 0.5 - 0 - 0.5 V filament transformer, if you'd like to use one of the DM160 subminature indicators, it dosen't particularly matter which side of the filament your source of control voltage is referred to as these are variable-brightness indicators only!!!

However if you want to be sure and you good at tiddly transformer-making, then I'd knock up a small h.f. 5W oscillator (TIP41C etc) with a 0.5-0-0.5V floating filament supply and a 30V 10mA anode secondary. It would need to be fully screened tho!!!

[Arnulf]

AC filament supply will cause blinking of the indicator unless its frequency is high enough. Unless display is distorted (one side visibly darker than the other) there is no point in using AC supply.

[Steerpike]

Have you a photo of the amp, so we can see what style of eqipmemt the indicator is going into?

How many DM160s do you have?
A really unusual display would be a bargraph using 10 or more DM150s, driven from an LM3916 IC. But I guess you don't want to get too complicated when starting out.

The bigger eye tubes (like the 6E5, EM4, EM80, EM87) are not difficult to use and need no complicated circuitry, so don't be scared to try them. The downside is they want about 250V on the anode & target, and about 300mA heater current. The ones with phosphor on the glass (EM87, EM84), rather than on the target electrode seem to be brighter, and last longer.