I was doing some experimenting the other day with linear quartz halogen lamps in series with different mercury lamps. Sylvania's 50w and 100w mercury lamps will actually strike up on 120v in series with a halogen lamp. My first test was with the 50w, which extinguished shortly after reaching full intensity on a 300w halogen. I then tried a 500w halogen with the same lamp, it burned longer, but still went out. The 100w lamp does the same thing, I even tried using both 300w and 500w (800w of halogen) in series, and it still goes out.
Anybody know what the correct halogen/incandescent wattage ratio to mercury vapor wattage? I also noticed both lamps did run brighter than they would on a magnetic ballast when on the halogen lamps, especially just prior to extinguishing.

The mercury lamp need twice the arc voltage to maintain the arc. Of course it will strike as lamp ocv
Start around 20 or 30 volts and rise up with pressure
Of the lamp until  it reach the ocv after the halogen.
It goes out cause there simply not enought OCV to
Maintain the arc of the lamp.  Also one note
It  being overdriven current wise at start up, that why
Lamp look bright at startup.  I would expermeint
It would be 150 watt halogen at 240 volt mains to properly run a 50 watt MV .  200 watts for 100 watt mv running on 240 volt ocv.

How do lamp manufacturers get around the low OCV when making lamps for 120v use?

Using lower voltage filament and Also there preheat style mercury lamp, there switch inside to turn off heater inside the lamp once lamp struck the main arc.  For 120 volt  Self ballast mercury lamp.

How do lamp manufacturers get around the low OCV when making lamps for 120v use?

The lamps use preheat start: Instead of the usual starting probe there is a heater filament behind each main electrode. These filament are powered, so warm up the main electrodes, by a thermal switch before ignition. When the electrodes are hot, the 120V becomes sufficient to strike the discharge.
Of course, the arctube is designed for lower arc voltage than the 230V versions.

I got a 175w mercury lamp (GE) to run continuously on a 500w and 150w halogen, it seems 650w of halogen is required to correctly operate a 175. It ran all day without cutting out, the halogens were exceptionally dim compared to the filament in standard SBMV lamps, but nonetheless it works. I may consider retrofitting a floodlight to use this setup (two pairs of R7s lampholders and one mogul lampholder.

The mercury lamp actually appeared to be running close to 175w in output

I got a 175w mercury lamp (GE) to run continuously on a 500w and 150w halogen, it seems 650w of halogen is required to correctly operate a 175. It ran all day without cutting out, the halogens were exceptionally dim compared to the filament in standard SBMV lamps, but nonetheless it works. I may consider retrofitting a floodlight to use this setup (two pairs of R7s lampholders and one mogul lampholder.

The mercury lamp actually appeared to be running close to 175w in output

Do you have one of those plug in power meters? You could verify the output.

Yes, watt meter says 417w, the voltage in my house, for whatever reason is slightly higher than average at 128v. Though I cant imagine that extra 8v has any significant impact on the setup I ran.

The 8V extra is quite a lot" What counts is the margin above the arc voltage. And because the arc voltage is around 110..115V, the difference is just about 50..60V (to the mains peak) and there the 12V (peak from the 8V) is already more than 20%...
ANd if you include the reignition spike, the margin is way lower and there I'm not that much surprised the 8Vrms on the maains makes the only difference between working and not working.

Ahh I see, I didn't realize the arc voltage was that low. Ill try this test again on exactly 120v and see if that extra 8v was the reason it worked.

Try it with a Variac. 

Ahh I see, I didn't realize the arc voltage was that low. Ill try this test again on exactly 120v and see if that extra 8v was the reason it worked.

The arc voltage forms a rectangular waveform, so the 110,,130V means the voltage really does not go higher.
But the mains voltage is a sinewave, where 120V means a peak voltage of 1.414*120V, so about 170V or so. So even with 130V across the arc means the mains is still higher in part of the waveform.
But for the discharge there is one other thing: When the mains voltage goes below the arc voltage (when the mains is going to change it's polarity), the arc get extinguished and so has to be reignited the next half cycle. And that usually means the need for some extra voltage, seen as a leading edge overshoot spike on the arc voltage waveform. And for the reignition to happen, this spike has to remain still lower than the mains voltage at that time. With resistive ballast the spike uses to be higher than on the inductive or so, because there is quite long time gap without any arc. And that is, where I would expect the margin to be that low.

With MV's, the auxiliary probes reduce somewhat the spike, mainly when it happens to be on the cathode side (it forms some starting ionization, so some free electrons on the cathode side already at lower voltage across the lamp). Mainly when the probes are on both sides, the probes will be efficient on both polarities and make the lamp work even on such low OCV as the 120..130V range.

I was doing some experimenting the other day with linear quartz halogen lamps in series with different mercury lamps. Sylvania's 50w and 100w mercury lamps will actually strike up on 120v in series with a halogen lamp. My first test was with the 50w, which extinguished shortly after reaching full intensity on a 300w halogen. I then tried a 500w halogen with the same lamp, it burned longer, but still went out. The 100w lamp does the same thing, I even tried using both 300w and 500w (800w of halogen) in series, and it still goes out.
Anybody know what the correct halogen/incandescent wattage ratio to mercury vapor wattage? I also noticed both lamps did run brighter than they would on a magnetic ballast when on the halogen lamps, especially just prior to extinguishing.

It's a bit tricky with 120V systems, but it can be done. A bit easier with 220V European circuits. Don Klipstein of http://donklipstein.com/ can calculate it fairly easy (Don can calculate just about everything B^{)>)

If you can't figure it out, drop him a note and he'll give you the specifics of what kind of incadescents to use.

Ioannis@