I know i keep asking questions about Preheat lamps and luminaires, but...

Does running a two-lamp Preheat luminaire with only one lamp significantly shorten the single tube's lifespan? How are these fixtures effectively wired? Does current get adjusted accordingly for just one lamp? Are there two separate ballasts? Or does the lamp just run at 2x the amperage? Reason why I ask is, one socket on my desk lamp is partly broken. Lamps don't want to light on it, so I wonder if it's safe to run the other (vintage) lamp by itself without another lamp completing the circuit.

Also, if someone could provide me a wiring diagram of how these fittings are wired, that would be wonderful.

For North America, most multi lamp preheat luminaires ran the lamps in parallel.

For the shorter lamps, F20 and under it was most common to use either separate chokes for each lamp or those dual lamp ballasts which were really only two chokes in a single enclosure. Even with the more elaborate lead-lag F20 preheat ballasts, they were still largely independent of each other. For larger lamps, like F40 preheat, the typical circuit was a lead-lag ballast. The primary step-up winding was shared but the ballasting windings were independent.

In countries with 240v line voltage, some shorter preheat fluorescent lamps were ran in series off a single choke, for example 2x F20/F18 lamps would run as a series pair off a F40/F36 ballast. Each lamp would still have its own starter but shared a ballast. Those setups require two lamps in order to work.

 

@Maxim
Assuming that this is a 120V lamp, the two lamps almost certainly both have their own starters and ballasts. This is effectively two independent lamp circuits in one lamp, so it is usually completely fine to only connect one lamp. Here is a diagram of what this lamp most likely looks like inside:

@Maxim
Sorry I put the wrong link into that last post, the image should be there now lol

@Multisubject , all good. Now I wonder- does this mode of operation / wiring change when you introduce a "manual" preheat switch, one where you hold the button until the ends start to glow?

@Maxim
I doubt it, they would probably just use a 2-pole button (2 buttons connected physically as one) so they both preheat at the push of one button. May I ask what size tubes are in this lamp?

@Multisubject - F15T8

Yes, in that case it would be impossible to run them in any other way since the voltage per lamp is 50V, so the diagram I drew is almost certainly accurate.

Sometimes if you press the edge of the On button on one of these two-lamp manual desk lamps you can get it to only preheat and start one lamp.  It's not uncommon to see hot and neutral reversed from what @Multisubject depicted when the switch is in the head of the lamp and ballasts are located in the base.

@Patrick - interesting...