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Re:Glowing mercury thyratrons: inside a 1940s Teletype switc... by Animats on Monday September 10, 2018 @06:55PM (#17955850)
Yes, the usual Teletype loop current supply is a 120VDC supply with a 10K 2W resistor, to get 60mA constant current. Efficiency is below 10%, with over 90% of the power going to heating the resistor.

That's partly why I built a switching supply interface. Far less heat. Small box, no ventilation required.

Re:Glowing mercury thyratrons: inside a 1940s Teletype switc... by chiph on Monday September 10, 2018 @08:45PM (#17956337)
I briefly considered taking one of the cards along to my high-reliability soldering class just to provoke a reaction from the instructor.. But figured it might bring down attention from On High into the state of our circuit cards and that wouldn't have been good. Besides, they lasted 5+ years like that before needing replacement. We pretty much only replaced them when the boards became brittle from the heat.