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Joined: Wed Apr 29 2009, 12:22AM
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This is mostly the same story that I posted on another forum the other day. At June flea market, I was compelled to get 5 electronic vacuum tubes for $1 each. Roughly half of them work (the magic eye counts as a half). My favorite is a slightly used 866-A mercury vapor rectifier, built more than 50 years ago to do a job using plasma. It's maybe the tenth specimen in my collection of 866 and similar tubes, but now the most studied.
Last night I put it through its paces with 2.50 volts AC on the filament and DC with series resistor on the anode. Warmed it up in free air at 100 mA. The following pictures were taken with these currents and voltage drops:
1 mA, 12.5 volts (zoomed in to show the filament glow. No photo of permanent magnet play.)
10 mA, 10.49 volts
100 mA, 10.37 volts
1 ampere, 10.43 volts (incl. any IR drop in filament, interconnection, and transformer)
Given the PIV rating of 10 kV, those forward voltage values would be not too shabby in a silicon diode.
Condensation of mercury inside the lower part of glass envelope at, say, 50°C tells us the internal pressure is about 20 microns. Most makers of 866-A specified condensed mercury temperature range of 25°C to 60°C (pressure 2 microns to 26 microns). Some have extension to 70 or even 80°C, with a doubling of maximum current and a greatly reduced PIV. Datasheets for plain 866 from Hytronic and Eitel-McCullough (later Eimac) go down to 10°C. 866-A's have a metal cylinder blocking our view of the filament, allowing greater emissive area at same temperature and power.
At 100 mA, vigorous fanning of the tube envelope eventually brought the plasma voltage up to 10.9 V. With a paper envelope loosely wrapped around the tube for thermal insulation, I let the voltage get as low as 10.15 V.
The partial pressure of Hg in fluorescent and "neon" lamps must be similar. The lamps contain on the order of 10 torr of noble gas. Anybody know if the rectifiers also use a noble gas? I suppose their glow spectra could give us a clue.
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Some mercury vapor rectifier measurements and glows
