Three prong crystals can be summarized into three types:1) Two crystal elements per holder.
2) One crystal element per holder without heater.
3) One crystal element per holder with heater.For crystals with a heater, one side of the heater and one side of the pressure plate are connected to a common pin. For holders which host two crystals, one side of each crystal shares a common pin.
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| This collection of three prong crystals came from the SPARC Museum [1] via Bruce Macmillian. What a variety! The Bliley crystal has two crystal elements but only marked with one frequency, namely 2110 KHz. Note the FT-249 case designator on the US Army Signal Corps holder. (Photo by Bruce Macmillan) |
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| With covers off. The Bendix MX-2 could not be opened. (Photo by Bruce Macmillan) |
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| To add to the mix, here are two more manufacturers of three prong crystals and one variant. (Photo by Jerry Proc) |
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| The heater/thermostat assembly on this crystal was made by Klixon , a name which can still be found in today's marketplace. That's a beryllium alloy disk that's being used in the thermostat portion. For purposes of the photo, the crystal holder is upside down and is providing support for the heater assembly. (Photo by Jerry Proc) |
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| Side view of heater/thermostat assembly. (Photo by Jerry Proc) |
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| Here, the heater grid (right) has been liberated from the heat radiator plate. The heater is held captive by two claws on the radiator plate. Mica insulation covers the heating element. One crystal had to give up its life in the name of science to make this photo possible. (Photo by Jerry Proc) |
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| This is how the crystal element and heater assembly share the crystal holder cavity. (Photo by Bruce Macmillan) |
Notes:[1] SPARC = Society for the Preservation of Antique Radio in Canada . The museum is located in Coquitlam, British Columbia. Web: http://www3.telus.net/radiomuseum/
Credits and References:1) Bruce Macmillan <bruce_macmillan(at)telus.net>
Nov 2/07