So the idea I will go with for this is a 100W MH ballast powering a 100W MV (with no ignitor), a 100W MH, and a 100W European-style SON-T or NAV-T lamp. There will be a single pole switch in series with the COM wire of the ignitor to make it easier to disconnect. However, this setup will over-ballast the 100W MV lamp semi-significantly, and European-style lamps are difficult to come by in the US. I am trying to figure out a better way to do this so people can start out collecting lights without having to spend a whole lotta money just on ballasts. The plans are the following:

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A 150W HX pulse-start MH ballast with a single-pole switch in series with the COM wire of the ignitor, which can power the following lamps (one at a time obviously):

• A 175W mercury vapor lamp (with the ignitor COM switch OFF)
• A 150W pulse-start metal halide lamp (with ignitor switch ON)
• A 150W S56 HPS lamp (NOT S55!!!) (with ignitor switch ON)

All of this can be put in a box or mounted on a wooden board, with a Mogul socket wired to it. Using a cheap porcelain Mogul to Medium adapter, you may connect the HPS and MH lamps which may or may not be Medium base (depending on the lamp you get). The MV lamps are almost always Mogul base.

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This is a good plan right? Am I correct with the 175W MV and 150W S56 HPS being able to run on 150W MH ballasts?
 

I think it should electrically match pretty well...

It will work. A S56 HPS does have a arc voltage 5 volts higher then that of a 150w MH. One issue I see is that 175w MV has a 0.3 A lower arc current then 150w MH, 1.8A for MH but 1.5A for MV, so the MV will be overdriven, but this won't be an issue so long as you use a more readily available lamp, any Chinese one will do, or a GE. You might anger some people (such as me) for running a lifeguard lamp like that.

You might anger some people (such as me) for running a lifeguard lamp like that.

I can't trust myself with even owning one of those and handling it, never mind running it lol.

Well if you do ever get one make sure to treat it well, they are some very high quality lamps.

It will work. A S56 HPS does have a arc voltage 5 volts higher then that of a 150w MH. One issue I see is that 175w MV has a 0.3 A lower arc current then 150w MH, 1.8A for MH but 1.5A for MV, so the MV will be overdriven, but this won't be an issue so long as you use a more readily available lamp, any Chinese one will do, or a GE. You might anger some people (such as me) for running a lifeguard lamp like that.

It won't, because the MV has higher arc voltage (~130V instead of ~95V) and that would lead to lower current from the HX ballast, so it will be very close to the 1.5A (if the ballast impedance would be pure linear and lossless inductor, the current would be 1.6A; but because normally these first have extra losses and second the magnetics uses to be operating very close to saturation, so a bit smaller flux in the shunt because of the higher arc voltage means more significant reduction in the current)...

I did not think of that lol

It won't, because the MV has higher arc voltage (~130V instead of ~95V) and that would lead to lower current from the HX ballast, so it will be very close to the 1.5A (if the ballast impedance would be pure linear and lossless inductor, the current would be 1.6A; but because normally these first have extra losses and second the magnetics uses to be operating very close to saturation, so a bit smaller flux in the shunt because of the higher arc voltage means more significant reduction in the current)...

I was thinking that would help my cause, but I didn't know it would be such a good match with this ballast!

Aallright over a year later and I actually did it, like I said I did the 100W route with a 100W H38 MV, 100W M90 MH, and 100W SON-T, all on a 100W M90 MH ballast.

The MH lamp of course ran up just fine, the MV lamp was of course overdriven (130V at .984A vs the rated specs of 130V at .85A, around 16% overdriven), the SON-T was around 1.2A at 88V (not sure why the voltage is a little low but it never increased higher than that for whatever reason).

Just for funsies I ordered a genuine 100W H38 MV ballast off ebay for a steal. Unfortunately even though it's lower than 150W, the ballast is still CWA since the 150W Dividing Line Of Stupidity does not apply to MVs. Anyway to my surprise it ran up to 130V at .947A! That's only a 37mA difference between the M90 and the H38, and still overdriven! I had no idea this happened with HID lamps, the only times I hear about lying ballasts are with fluorescent. Anyway now I feel less bad using the MV lamp on the MH ballast because apparently ~11% overdrive it just "acceptable" to the Advance Transformer Company (or whoever owns/owned them). Of course this is MV which is tolerant of overdriving but still surprising.

I am really really kicking myself for not measuring the CWA capacitor (supposed to be 10uF), but anyway I might get an ever so slightly smaller one to adjust the current to be more or less on spec. Of course that will introduce even more spiky current than what's already there with CWA ballasts but whatever. Just thought I'd update.

Nice experiment, I haven't really heard of MH/MH ballasts even though they're interchangeable, only HPS/MH.
I have observed this too, one of my SON-Es settled at around 75V on an HPS/MH rated ballast, so I'm guessing it's something similar in your situation. Interestingly, MV lamps are the ones which maintain the most constant arc voltage at most wattages: from 100W to 400W, it seems to be around 130V, while HPS is the least consistent. Overdriving the lamp by 11% is quite a bit but shouldn't be too bad for the lamp.

I often see a 15-18% overdrive in lamp current when I have ran 110V 60Hz HX ballasts on 120V 60Hz.