I'm making a standalone stereo VU meter from a pair of surplus "magic eye" vacuum tube signal level indicators.
Sometime late in 2010 I seem to have come to the conclusion that my substantial collection of fluorescent, electroluminescent, and plasma-discharge vacuum display tubes was insufficiently dangerous or exotic, so I went out in search of a suitable example of a Magic Eye indicator tube.
The "magic eye" is essentially a one-dimensional cathode ray tube, and was traditionally used as a tuning indicator for radio receivers. The use of magic eyes dates to the 1930's, which makes them even more archaic than nixie tubes. Unsurprisingly, they have long since gone out of production, and the remaining stock is managed largely by hobbyists and surplus dealers. The nature of the magic eye brings into the world of ham operators, tube amp enthusiasts, and vintage radio collectors.
Like the Russians and their nixies, Maoist China kept the magic eye in production long past its technological shelf life, and the communist economic logic of production that sometimes leads to chronic shortages of vital goods also often leads to absurd overproduction of other goods. Electronic parts with strategic military value are particularly prone to overproduction, so today's collector is often presented with warehouses full of obsolete vintage electronics, decades after factories were shuttered—factories which in turn kept operating decades after their Western counterparts ceased production.
All of that means that it is easy to come by inexpensive Chinese magic eye tubes even though the West stopped using them 40 years ago. Nerds rejoice! There are a variety of sources for these things, but ebay is the most accessible to the novice. A search on "magic eye tube" will produce volumes of relevant hits. At the bottom of the price scale you will see the Chinese-made 6E2, cheap and plentiful enough to risk destroying without remorse in pursuit of Nerdvana.
But there's more. Not only can you find can you find cheap tubes, you can also find cheap modules that include some of the high-voltage components necessary to operate the tubes. Again, understand that these are small CRTs, and they require even higher voltages than nixies to operate. The modules make it easy to integrate these tubes into your own project, but they are still quite inexpensive, thanks to our industrious Asian friends. They come in AC and DC powered varieties, and for my purposes, DC was the better choice.
I had not worked with these before, but after browsing some schematics, the problem seemed simple enough, and for $15 it seemed worth a try, if not just to add to the collection. I ordered a DC module with one 6E2. It arrived in December (2010), prime indoor hobby season. The cathode is powered by a nominal 6.3V source, which is easy enough to provide, but the anode requires a much higher voltage, well north of 200V. I had on hand a boost converter module for nixie tubes that could produce 200VDC, and connecting that quite easily lit up the tube with its eerie phosphorescent green glow.
At this point all I had was a rather peculiar green light bulb. I would have to hook up a signal source in order to see it in action. I did not have a great clue as to what signal level was required, so I connected a line-level audio signal and the tube did…nothing. Expecting this, I took the next step and connected the speaker outputs of an audio amplifier, and the tube sprung to life:
The magic eye is a general-purpose indicator, and can be used for anything that requires a proportional measurement. However, they are traditionally used in RF and audio applications, and I was happy to stick with that. I could not think of a suitable RF application off the top of my head, and that left me with the default choice of audio. As it turns out, the VU meter is the stereotypical example for hobbyist magic eye projects, so why buck tradition? I did determine, however, that if I made such a meter, it would have to be stereo. So I ordered a second unit, put the first unit on the shelf, and subsequently got involved with other business that occupied my attention for months.
Then the sun came out. In Seattle, you make the best of that while it lasts, and the best of it does not happen at your lab bench. But time goes on, and dark days return and the bench comes back to life with all of its glowy instrument lights and experimental fascinations. New projects get started or old projects are pulled off the shelf and revived:
The display will light at 200V voltage applied to the target, but this level is less than satisfactory. The tubes are nominally rated for 250V and will go considerably higher (to the detriment of their lifespan, I'm sure). I knew I would not be happy with the nixie power supply, so I'd have to come up with something else. Now, it is far more common to power these tubes from a mains AC voltage, but as I said before, I did not want to do that. First, AC usually makes sense for magic eyes because they are components in a larger system that also include other things that make sense to power from the wall, like a big amplifier. My project had no such extra function—it is a stand-alone meter. Second, AC is a hassle for hobby projects because of different international standards for mains power. It would be easy to make a design that worked from 220V in Europe, but that didn't do shit on the 110V in America. A wall wart makes the AC problem into a local problem. Finally, the filament voltage is ~6V which means that either way, I'd be making some kind of voltage conversion. From my perspective, stepping up was a more attractive solution than stepping down, and regardless, there was no place in my chosen enclosure for a big ugly transformer.
In light of the inadequacy of the nixie power module, it turns out you can also get DC power modules that will work with higher voltage devices like magic eyes and dekatrons—but I didn't want to do that either. I have rolled my own nixie boost converters, so I decided to give that a try here. Some of my other projects used a microcontroller to modulate a boost converter circuit, but this project did not need a microcontroller, so that was also out. I had a couple of MC34063s on hand, but I elected not to use those as I always had documenting this project for the hobbyist in mind, and I wanted to keep things as simple as possible.
That left me with a clear choice: a 555 timer…
This article is in progress. Check back for updates with more technical detail and even schematics (gasp!)…
The power supply
The audio signal
The display tubes
First working build
Now using sooper-kool EM800 tubes instead of frumpy old 6E2 tubes.
Circuit to the left of center is 555-based boost converter. Circuit to the right is LM358-based preamp.