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I have a two-socket fluorescent lamp. Initially only one light tube was inserted and it worked fine. At some point, it started to go off seconds after being turned on. I thought the tube went bad, so I bought another F15T8. Surprisingly, if inserted alone, the new light tube would also go off right after being turned on. However, if I put both tubes in, BOTH would stay lit. The OFF switch then gets very hot (>100 C), which did not happen when I was able to use just a single tube. The lamp has two switches. The ON switch has to be kept pressed down for a few seconds until the light is turned on, while the OFF switch only needs to be pressed briefly.

How should I understand what happened here? Is the switch going bad and did that cause my first problem that a single light tube no longer stays lit? How could the switch get so hot?

lamp thermal image during lamp operation

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A quick search of ballasts revealed electronic ballasts that support (2) F15T8.

Go straight to that, don't waste any time with old magnetic ballasts that buzz and flicker on a good day.


An instant-start ballast does that by shocking the tubes lit, but this wears the tubes faster. It typically needs 3 wires for a 2-tube setup, and they can run independently.

A rapid-start or programmed-start ballast makes use of the preheat coils in the tubes (automatically doing what the start switch did), causing an easier start. The rapid-start ballast is guessing. Both of them need typically 6 wires for 2 tubes, so you'd need to add wires to the neck conduit. They usually need both to be present/working.

  • Thanks. That's likely what I will end up doing. I didn't realize that the tube would still light when one of the ballasts had gone bad, and found all the series of my observations interesting. I'm going to open the lamp up again and test the ballast. – Roc White Aug 25 at 15:13
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First off, these lights are not designed to work with only one bulb. That will cause premature failure of the ballasts. Those three little "boxes" in the base are ballasts. The ballasts sends the proper amount of electricity to the fluorescent bulbs. The ballasts provides enough voltage to start the fluorescent bulbs, and once they are started, it quickly lowers the current to produce just enough electricity to produce a steady light source. When the ballasts fails a fluorescent bulb will either attach directly to 120-volt current and quickly burn out or it will just fail to light. Your ballasts are probably bad and I doubt you'd be able to get replacements. Better off just getting a new lamp.

  • Since if I put both tubes in they both light up, the ballasts are probably still good? And actually one of the three boxes is just a junction box. The other two are each rated for 15W and appear to be connected separately to the two sockets. I find the entire series of observations quite peculiar. – Roc White Aug 24 at 21:39
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    Yes, the ballasts are hooked up to their own sockets but if they are energized, they need a load, but the switch should not be getting real hot like you described which leads me to believe the ballasts are failing. – JACK Aug 24 at 22:03
  • I see. I'm going to open the lamp up again and test the ballast. Found some instructions for that. If the ballast are bad, I will likely get a new one or a new lamp. – Roc White Aug 25 at 15:09

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