Do 3-way HID bulbs exist? It would revive the idea of 3-way incandescents. But a problem would be arcing between the contacts of the base.

Stan aka trianero2012 has one designed to run on three phases I think. The thing id say is the ballast will have to have different taps and a switch to work low, med, hi.
http://www.lighting-gallery.net/gallery/displayimage.php?album=2480&pos=45&pid=70036 

Stan aka trianero2012 has one designed to run on three phases I think. The thing id say is the ballast will have to have different taps and a switch to work low, med, hi.

Except that's just a concept design, and there would be 2 arc tubes.

The problem is, when it's on hi the arc tubes are in parallel, so one will run and the other would be off on hi setting, unless the ballast had two coils for the different arctubes. Something like 50-75-125 would be cool, but those ballasts are rare as balls and making a design would be a bit challenging.

The problem is, when it's on hi the arc tubes are in parallel, so one will run and the other would be off on hi setting, unless the ballast had two coils for the different arctubes. Something like 50-75-125 would be cool, but those ballasts are rare as balls and making a design would be a bit challenging.

On high, both of them would be on.

I'm talking about on a single coil ballast, since they're essentially wired in parallel.

And why not use 2 separate HIDs ? That would not need anything special but common parts for luminaire construction, ordinary mass produced lamps, and you dont have to throw out anything "still good" when one of the HIDs eols

Yes, exactly. A simple three way switch, respective ballasts and lamps, and power will make this a cool concept.

The real power transferred to a bulb is dictated by the ballast, so if a reduced light is necessary, it is matter for the ballast to deliver lower power and not the lamp.
So instead of a "3-way" lamp, there are "x-way" ballasts (most common are with two power settings, or the electronic ones smoothly dimmable over their range)

Working two arctubes on a common ballast would be technically feasible, but they would have to be actually wired in series, with one of them bypassed by the selector switch for the lower setting.
But such creation will be highly impractical: In order to work on full power on regular ballasts (both arctubes in series), the voltage drop across each arctube would have to be kept rather low, something around 60V or below. But the cathode fall would be there twice, so double losses just from that.
And when on low setting, the arc voltage will be very low, so current a bit higher, what will cause even higher losses on the ballast than at full power and even overstress an ordinary ballast.
So a special ballast would be needed anyway.
And when we have a special ballast and all the extra losses, just dimming an ordinary lamp is way easier and even when the lamp losses efficacy, it won't be worse than the low voltage arctubes, but at least at full power it will have it's full performance.

With parallel connection the current will always select just one of them and the second will stay OFF (by the way that is the concept of the "stand-by" twin arctube lamps - when one fails so it does not want to ignite, the second becomes the one with lower voltage and so takes over the current)

Well, I know what I am going to be building when I get enough money. A 50-100-150w MH fixture, probably a wall light.

Well, I know what I am going to be building when I get enough money. A 50-100-150w MH fixture, probably a wall light.

With a single lamp with dimming, or two lamps?
For a single lamp I doubt the MH can go so low (50W, so 33% or so), the 50% is quite well feasible...

Maybe no. At lower temperature there may be some liquid pool of halides forming, so eventual corrosion of the arctube..

With  50/100/150 HID, i'd use not 50+100 lamps but 50+150 lamps and have only the 50 and 150 states. Thing is, the 150W lamp is significantly more efficient than 50W and somewhat more than 100 too, so why compromise on efficacy in the high setting ?

What I'm talking about is essentially a 3-way incandescent but with the filaments replaced with arc tubes, and then have a ballast for each arc tube.
Then you need some kind of switch to control them. If an arc tube burns out, you still have the other one still going.

The Incandescent concept is not all that good for HID due to the efficiency considerations (not too good for the Incandescent either, but thats for another time)

Do whats efficient, and that is when you want X light, get it with 1 arc working and not with 2 smaller ones

Use double arc tube lamps for reliability but so that only 1 arc is burning at a time

The thing with discharges is, they are usually very tight with thermal balance. Having two arc tubes close to each other means the rated power would have to differ quite significantly whether the other tube is lit or not. And even whether the other tube is above or below it. Mainly for the lower wattage tube the difference would be rather large.
Yes, the incandescent filaments are relatively sensitive as well, but they are way farther separated relative to their dimension. The usual 2cm distance for a 1mm diameter filament spiral is 20:1, that ensures pretty well at least some isolation.
So for an equivalent with 1cm diameter arctubes it would mean the arctubes would have to be 20cm apart. Not that practical within one bulb. And you may get a bit better with just two lamps side by side.