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I am replacing a ballast for a single tube. The new ballast I purchased is designed for two tubes but states that it is also compatible with a single (more powerful) tube corresponding exactly to what I need.

They apparently forgot to draw the wiring diagram for the single tube setup though. The only diagram I have is for two tubes and seems to be typical:

             Power Switch    +--------------------------+
 Line 1 (H) o----/ ----------|Black    Dual Tube     Red|-----------+
 Line 2 (N) o----------------|White      Rapid       Red|--------+  |
                       +-----|Yellow     Start      Blue|-----+  |  |
                       |  +--|Yellow    Ballast     Blue|--+  |  |  |
                       |  |  +-------------+------------+  |  |  |  |
                       |  |                |               |  |  |  |
                       |  |       Grounded | Reflector     |  |  |  |
                       |  |      ----------+----------     |  |  |  |
                       |  |    .----------------------.    |  |  |  |
                       |  +----|-     Fluorescent    -|----+  |  |  |
                       |  |    | )      Tube 1      ( |       |  |  |
                       +-------|-       bipin        -|-------+  |  |
                       |  |    '----------------------'          |  |
                       |  |    .----------------------.          |  |
                       |  +----|-     Fluorescent    -|----------+  |
                       |       | )      Tube 2      ( |             |
                       +-------|-       bipin        -|-------------+ 

How do I convert this to a single tube?

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If it has 2 wires to each bulb end, the grounded reflector does not matter. Yes, there are little incandescent-like filaments in each end of each bulb, those are used to preheat the cathodes to 700C so the bulb will start more gently with less arcing damage to the cathodes. – Harper Jan 14 at 1:23
up vote 7 down vote accepted

EDIT I'm doing this from memory and don't have a ballast to look at so I checked a manufacturer bulletin. I was wrong in the beginning so I edited it out.

Wiring diagram

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Works fine, thanks! – Marc-André Lafortune Jun 18 '12 at 21:02

I bought a 2 Lamp ballast and it has 2 yellows coming from the same side as the black and white wires and 2 red with 2 blue coming from the other end.

The ballast that I replaced had 2 red coming from one side and 1 white, 1 black, and 1 blue coming from the other end. The 2 red went to one side of the tube and the blue and the white went to the other side, where the white also connected with the common. The black of course went to the hot.

For the new ballast, I hooked 1 red and 1 blue to the side of the tube that the old 2 reds went to and 1 yellow to the other side. Then black with black and white with white. Light works fine now, instant on and off.

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Your old ballast was instant-start. They have 1 wire to each end of the bulb, and the lampholders' 2 pins are shunted together internally, which cheaply blasts the bulb with high voltage to start it. You have a "Rapid start" ballast which uses 2 wires to each end to preheat the cathodes for a gentle start and -much- longer bulb life. You surely have this wired wrong, question is what fails and when. Get non-shunting lampholders and wire it to manufacturer spec, you'll get your ballast's full capabilities and a better light than you started with. – Harper Jan 14 at 1:18

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