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What are these Christmas light things?


2006-10-26 5:27 pm
What are these christmas light things?

Here is a link to a few (crappy) photos I took of an oscillator board to an old Silvertone organ I picked up at salvation army. Initially I though these strange little bulbs were pencil tubes, but they only have two leads, and glow orange when the organ is powered up. A friend suggested they were neon lamps, maybe for tuning purposes? Also, does anyone know where I can get a schematic for an obscure, old *** Silvertone?

The neon light bulbs have characteristics like a bipolar zener diode. They will draw no current until the neon inside ionizes (it lights up) and then they exhibit a constant voltage characteristic similar to a gas regulator tube. The ordinary NE-2 variety were designed for use as indicator lamps, an do not have well controlled specs. There were special versions made to have well controlled voltage breakdown characteristics. These were used in timing generator circuits by Tektronix and others.

They have found their way into various electronic circuits over the years, and can even be found in vacuum tube amplifiers as sonically benign voltage clamps or limiters. Look at the schematic of Kevin's 300B P-P amp.

They were used as relaxation oscillators, but in an organ they were often used as the frequency divider using a "locked relaxation" oscillator design. If all of the notes on the keyboard play at the correct pitch, don't mess with it. It is hard to get these circuits working right if you start changing parts.

Mouser lists several flavors of NE-2 lamps with voltages of 65 and 95 volts at several currents.
I know in some organs (namely German Ionika) neon lamps were mounted near a wire with HF that was shielded by shield on keys. When some key was pressed shield gradually opened HF radiation so a lamp started conducting; such a way organ type attack / decay were simulated.