I would say get the used one. Obviously chokes do fail, but not very often. I think a used one will work okay. And it is made in the USA which is nice. Obviously if it is untested you are taking a gamble, but I am assuming you know it works.

At some point I want to make a F4T5 preheat fixture and I need your opinions on the ballast, I'm gonna use a Robertson SP48 ballast. There are two listings I'm looking at. A brand new one made in China for 24.95 or a USA made one used with some scuffs for $12.99.

@Medved

So you have a transformer with two identical windings (X-Y and Y-Z) connected in series so that their total measured inductance is effectively zero across X-Z. When you apply current across X-Z, the center node Y will gain voltage relative to the points X and Z (assuming X and Z are at equal voltage)? That sort of makes sense, but anything after that I am lost.

Yes, assuming the transformer has ideal coupling, so magnetic flux is exactly the same through both windings, so none could leak between them.

In reality you get some nonzero inductance, because the magnetic flux generated by one winding can squeeze without going through the other winding, this property of a transformer uses to be called "leakage inductance". How much it is depends on the exact arrangement. It could be few percents, or even 20..30% if there is really a space between both coils and/or even a magnetic shunt (a piece of iron core designed to provide that path).
With standard transformer you want that to be as low as possible, but with SRS some leakage inductance is beneficial, as it boosts the voltage across the lamp for ignition above the mains voltage.

European T8 lamps have krypton. European "Eco T8" and "Eco T5" lamps have xenon. American VHO T12 have Neon. F34T12 have krypton and external conductive coating.

We have now huge rain storm and thunderstorm, after days of dryness: https://www.youtube.com/watch?v=zegtCgONRdQ

I don't know about fire alarm strobes, but I have some strobe light drivers and I know how they work. Usually they consist of a small step up transformer, the primary of which is driven by the discharge of a small capacitor. This sends an HV pulse to the antenna outside the tube which discharges a larger capacitor through the tube. I would say the most likely thing to go wrong is in the triggering circuitry. This is sometimes done with a neon indicator, but could be more complicated with fire alarms if they are supposed to strobe synchronously. Usually they aren't that complicated, just start checking the different components until you find a bad one. Do you have a picture of the board?

In addition to collecting lighting related things, I also collect fire alarms (technically lighting related as well. Among my collection I have many devices that contain strobes. Most of them work, but I do have two that don't work that I would like to get working. Specifically, a Simplex 2903-9150 120V strobe plate, and a Space Age Electronics A32 white strobe plate. Does anyone on this site have experience in strobe circuits or repairing them?

@LightBulbFun
Excellent point, I did not consider that. Of course an RMS ammeter has the final say, but if you can find chokes that are just for 18-20W tubes it would probably be better than those 14-20w ones.

@funkybulb
That is a very good idea, common 18-20W chokes provide 240 ohms at 118V, while (according to my calculations) the tube needs ~125 ohms. Two 18-20W chokes in series gives 120 ohms which is close enough. Plus preheat is a lot cooler than RS in my opinion.

a 20W *Reference ballast* is 240 ohms, but many production ballasts are not, especially not your common 14W-20W choke some of which dont even drive a F15T8 at full power, just keep that in mind when trying to work things out like this

@funkybulb
That is a very good idea, common 18-20W chokes provide 240 ohms at 118V, while (according to my calculations) the tube needs ~125 ohms. Two 18-20W chokes in series gives 120 ohms which is close enough. Plus preheat is a lot cooler than RS in my opinion.