Circuit Sculpture Based on an EM34 Tuning Indicator Tube




Introduction: Circuit Sculpture Based on an EM34 Tuning Indicator Tube

I've always been intrigued by these "Magic Eye" tubes.

When I got the chance to scavenge one on an old radio, my first thought was to power it on.

I bought this high voltage converter and quickly confirmed that the tube was working as expected, albeit a little dim.

I recently became fond of so called "circuit sculptures", and the idea emerged to transform this unused old strange tube into a weird brass creature.

I wanted it to look like an insect about to jump at you.

But first let's look at the finished product.

Hopefully you'll want to know how it's done.


Here's what you'll need to make your own:

  • An EM34 Tube (can be found on ebay). You may use any other tuning indicator tube, just adapt the wiring and voltage accordingly.
  • 2 1M-Ohm resistors
  • 1 switch
  • 2 9v battery cell connectors
  • 1 DC-DC boost converter based on a MT3608 (to produce the 12v expected by the high-voltage DC-DC converter)
  • 1 High-Voltage DC converter
  • A bit of insulated copper wire
  • about 1m of 1.5mm brass wire
  • a few cm of 1mm brass wire (for the antennae)
  • 4x 10x10cm 5mm-thick plywood for the base
  • dark oak wood stain and wax
  • A 100w soldering iron (or any other quite powerful one with large soldering bit)

Step 1: Front and Rear Legs and Rings

I didn't take as many photos as I should have to produce a detailed tutorial. 

Anyway, here are the main steps.

Start by setting-up the output voltage of the 2 converters using a bench power supply.

The low voltage converter is set to output 12v with a 9v input.

The high voltage converter is set at maximum output voltage (about 210v) using a 12v input.

Then clean-up the tube!

I've removed dust and sanded the rear connectors in order to facilitate soldering.

Warning: Tubes should always be put in a socket, and soldering on the pins may damage the tube. But this is a circuit sculpture and a socket won't look nice at all here ;)

I'v started by forming and soldering the two brass rings that will be located at the extremities of the tube.

Then I did a rough sketch of the legs using copper wire, setting the overall dimensions.

I drew a leg shape and formed the front legs using this drawing.

The front legs and front ring were assembled for soldering using a rudimentary rig, as shown in the photo.

Then I used hot glue to temporarily glue the legs to a temporary wooden base, and glue the front ring to the tube.

This helped orienting the beast in the right position.

Once in position, I was able to determine the rear legs length and position.

Soldering the rear legs was not easy!

The left rear leg was soldered to the rear ring (this is going to be the 9V supply ground).

But the right rear leg must be isolated from the rear ring (since it's going to be the 9V supply VCC).

So its end was first wrapped with a bit of shrink-wrap, then super-glued to the rear ring.

The tube is oriented with its marking on top.

This provides a nice view into the tube and a nice orientation for the eye's glowing iris.

Step 2: Wiring-up the Tube

I've used this schematic.

I've started by soldering the resistors, then the filament power supply.

I chose the power supply polarity in order to facilitate soldering GND to pin 8.

The filament is a bit overdriven at 9v, but it will never be on for a long time so it's OK.

I took care of pre-measuring each wire length, pre-adjusting each brass wire before soldering.

Next came the high-voltage power supply lines.

The high voltage converter is stuck onto the tube with a bit of hot glue.

Luckily the high voltage tube pins are almost aligned with the converter output pins.

Step 3: Low Voltage Converter and Antennae

Next is the low voltage DC converter.

A bit of hot glue to the tube and it's time to connect its output to the high-voltage converter input.

This required bending the brass wires around the tube and crossing over each other.

The low voltage DC converter input is connected to the rear brass ring (ground), and to the right rear leg (VCC).

Don't forget to connect pin 7 to to pin 8

In order to make this beast alive I also wanted to be able to make the iris move when touching the brass.

The trick is to use the grid pin (4) to set it to a negative voltage. This will be -9v, easy to obtain with two 9v battery cells.

I thought it would be cool if this beast had antennae that served this purpose.

So pin 4 is connected with a long 1.5mm brass rod, to a short 1mm brass rod shaped as antennae, that can touch the front ring (connected to the -9v power supply) when pressed.

This is shaped so as to be springy.

The brass rod passes under the +12v line. Touching this line makes the iris "tighly closed", which provides for a nice effect.

Time to double-check the wiring, connect the batteries to the legs and power-on!

Soldering brass can be tricky and may lead to "cold solders" and defective contacts.

I had to double-check continuity and fix some solders espectially on T-joints.

If everything's OK, we're ready to build the base that will house the batteries.

Step 4: The Wooden Base

I've used five 10x10cm 5mm-thick plywood laser-cut pieces, glued together.

A bit of sanding, a hole for the power switch, and a nice touch of old oak stain and wax.

Once the base was ready, I've placed the scuplture on it, re-positionned the legs and marked holes to be drilled.

I drilled 2mm holes.

3 of them will host the power supply cables.

The 4th will host a short length of copper wire to maintain the leg in position.

The wires were soldered to the end of the legs, where they touch the base.

The battery and switch wiring is trivial.

One battery goes through the switch to the filament and DC converter power supply (rear legs), respecting the chosen polarity.

The positive connection of the other battery is connected to the ground of the first battery, and its ground wire is connected to a front leg, thus providing the -9 V power supply to the front ring (and to pin 4 when touched by the antennae).

I then removed the temporary hot glue maintaining the front ring on the tube and replaced it by a drop of super-glue.

And we're done!

A new beast has been added to my circuit sculptures bestiary.

If you're ever interested in my first attempt at a circuit sculpture, it's here.

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    Question 12 days ago

    The high voltage converter is set at maximum output voltage (about 210v) using a 12v input.
    210 volts on a bare wire from the sculpture. And what if you pick it up just wrong.


    16 days ago

    Nice project, I just want to say that these indicator tubes are becoming increasingly rare and expensive.
    They have a shorter lifespan than any other tube in an old radio, so there is a great demand.
    My tip for you and other creators is to create a circuit that does the opposite, a modern screen that mimics the function of the old tube, which we can use for our old radios!

    It is also common practice to never solder on the pin of a tube, but always use a socket!

    By the way, if you are going to renovate an old tube radio, you must not only replace all the electrolyte capacitors, but also all the paper capacitors, so that the end tube is not driven too hard and burns up the output transformer ....


    Reply 15 days ago

    You're right. Unfortunately AM broadcasting is slowly disappearing. Lots of old radios are converted to bluetooth-enabled speakers.
    I converted an old rotted radio+turntable into a very nice bed table (keeping the original 6V lighting coupled to the bed light).
    I understand the need to restore fully functionnal ones, but I gave up the idea :)


    Reply 14 days ago

    Hi again.
    Tell me if you think I'm leaving the subject.
    But being able to repair tube appliances is on the verge of extinction.
    We are still many who know how to do it, and are more than happy to teach the technology to you younger ones.
    As for the conversion to bluetooth, I think it is unnecessary.
    Almost all radios have a gramophone input, which is perfectly suited to operate from any music player.
    It is therefore completely unnecessary to destroy a radio that only needs a few new capacitors to play again for several years, with unique sound!
    And if you scrap a radio, you have sockets to re-use, so you do not have to solder directly on the tubes pin ...


    Reply 14 days ago

    I understand your point of view.
    However, at least here in France, AM broadcasting is slowly being phased-out. People who enjoy the family's old radio cabinet may want to find another use for it. There's a local company refurbishing them into moden bluetooth speakers while keeping the original cabinet.
    I do know tubes are meant to be plugged into sockets. But my sculpture is...a circuit sculpture, an original piece of electronic art.

    I'm sorry I guess you won't like at all what I'm going to build with another AM34 tube :)

    I have another old radio that only needs capacitor replacement to be working again, but...there's not much to listen to in AM anymore here so...I'll keep it under my bench for now.


    Reply 14 days ago

    I understand.
    But why destroy the radios, you can install bluetooth without removing anything.
    And even without a transmitter, you can use almost any radio as an amplifier, with the gramophone input.
    Can you not help the younger ones by write in your text that, usually the tube are put in sockets, but I solder immediately, warning for that the tube can break from the heat ...


    Reply 13 days ago

    I've added a warning :)


    Reply 13 days ago

    Nice, I wish you good luck!