also I am not saying it is more safe to operate a lower wattage lamp on a higher wattage ballast, even though the ballast usually survives this well, lamps can rupture violently if overloaded enough. I am not too familiar with the series choke which is common for 35w, 50w, 70w and sometimes 100w HPS, but they may work better for dealing with a higher lamp load like what youve been doing. My experience has mainly been with the CW-Autotransformer and HX-HPF type ballasts used for metal halide, mercury and higher wattage HPS. These are the ballasts I advise against "experimenting" with lamps far outside the rated wattage of the ballast

Hey thanks for the reply to my question. It would be nice to be able to find matching MV bulbs and ballasts  but it a toug thing to come by anymore,  I guess thats thalf the fun is hunting this old obsolete MV stuff. So if im understanding correctly, its ok to use a MH ballast after removing the ignitor and capacitor  to power up a MV bulb? 

I see that for a 70w MH ballast you use this to drive a 100w MV lamp, what would happen if I used this same 70w MH ballast to drive a 175w MV lamp? It would be great if it  only illuminated the  175w bulb to lets say half lumination, just lkke it is now with my 50w HPS Nema head driving the same 175w lamp but  without the weak link of the 50w HPS ignitor giving up the ghost keping the MV bulb continuously lit.

I really appreciate everyone sharing their information with me because no electrician or electrical supply place would ever be willing to share or even know about this knowledge.

Most electricians are not trained too deeply with discharge lighting technology, I know as I took electrician courses to get my certification. Matching ANSI codes is basically all that is taught. So I suggest use the internet to seek information as the majority of electricians out there do not know this stuff in depth. You can use a 100w Mercury lamp on a 70w MH ballast...but not a 175. Almost all MH ballasts are of autotransformer type and what will happen is exactly what I explained before. The ballast will overheat and fail as the lamp draws more current from the ballast than it is rated for. But 100w Mercury lamps are still readily available, almost every hardware store around here carries them, both DX and clear. Last time I was at the Home depot they had Philips 100w DX (H38) lamps for $12. Menards has the mogul base Westinghouse DX 100w mercury for about the same price. The pulse start 70w MH ballast you have would work perfectly just disconnect the ignitor (as mentioned above) cap off the ignitor leads (blue, red or X3 and Com) and youre set. Keep the capacitor as that improves your power factor.

Have a look on Craigslist, there are always at least a dozen listings where I live for used mercury bucket/area lights, usually super cheap. I do alot of lighting retrofits and sometimes remove mercury fittings and fixtures but almost never see the lower wattage ones (50w, 100w) Im most often replacing 250w, 400w and 1000w Mercury fixtures with MH, HPS, Fluorescent or more recently LED.

And to add: The 70W HPS belongs to the "low voltage" family as well (the arc voltage is around 55..60V if I remember well; I'm talking about 120V territory, the 230V market area is different...)
That means the ballast is just a series choke. And that mean just 120V OCV, way too little to operate the MV (it will either not ignite at all or cycle), unless aided by the ignitor.

With the ignitor it will work, only the ignitor components would be consumables (the previous apply as well; so just as a decoration).
If the ignitor is accessible, it could be easier to maintain it...

Otherwise with this combination (series choke on a 120V feeding an 100V arc MV) there should be no risk ov overheating anything (again, except the ignitor components), first the 70W HPS, as well as the 175W MH are designed for 1.5A and because of the series choke and the conduction phase angle the current (and there is just one circuit loop, so only one current) would be even way lower than that designed value.

A few years ago I ran a regular HX-NPF 175w mercury yardblaster fixture on a standard incandescent TRIAC wall dimmer (the ones in your dining room LOL).  It DID work for the most part, though I later discontinued that setup.  As I recall it would light down pretty low too.  If started with the dimmer all the way up it could be dimmed  but had to be done so slowly, otherwise the arc would extinguish.  If started really low/dim, it would never really warm up to full pressure (a hot restrike was pretty much instant sometimes and other times it would not restrike by itself, but sit in what I will now dub "starting probe glow mode" until the dimmer was turned up a bit to "spike" the voltage.
I also always kept an incandescent lamp running on the same dimmer to maintain at least a somewhat resistive load (incandescent) as opposed to inductive (ballast) since TRIAC dimmers aren't intended for the latter.
So it DID work in experimental use, but I wouldn't try it long-term as I have no idea how safe/practical it really is...if you do I will not be to blame if something goes seriously wrong...LOL.

Indeed, majority of the problems come from the fact the triac dimmer does not operate correctly on an inductive load (mainly when it it is in the form of a transformer, so high DC bias happen when there is even slight asymmetry).
The parallel incandescent does make it way more stable in operation, but the asymmetry problem still persist.

Otherwise the pure inductive ballasting (so a series choke or the HX autotransformer) is quite tolerant with phase cut dimmers (if the OCV remains sufficient), but the lead phase ballast (e.g. CWA) won't operate (either at all, or would require way higher wattage of the dummy resistive load).

For the dimmer operation the MV's were even rated to be used on phase cut dimmers (in 230V areas, so with just series choke as a ballast and a dimmer specially designed for such operation, so not needing the dummy load), the minimum power level was 50% (and that means just slight phase cut). These systems were usually not using triacs, but either magnetic amplifiers (for smooth control), or saturating inductors (two step), with those you could be sure it will work well even without any dummy load.

hi, everyone.

i have a question similar to this topic: isn't a standard (ie: not pulse start) metal halide fixture nearly identical to a mercury vapour fixture? meaning, if i had an 100 watt MH fixture, wouldn't it be perfectly suited to run an 100 watt MV bulb?

Yes as far as method of operation a probe start metal halide works the same as mercury vapor. However the operating current and voltage of the two differs slightly especially in the lower wattage sizes. Mercury lamps of 50w/100w and 175w will be overdriven a bit if operated on MH ballasts of the same wattage rating. Therefore a 50w mercury lamp would run best on a 39w MH ballast, 100w MV on a 70w MH ballast and 175w MV on a 150w MH ballast. Anything above 175w,...250w, 400w, 1000w can be safely paired with a MH ballast of the same wattage as the lamps will be overdriven just very minimally and shouldnt affect lamp life.

When limited to probe start MH's vs MV's of the same wattage, the MV lamps are 100% compatible with MH gear.
But the point is, the lowest wattage probe start MH I've heard of is the 175W. Maybe some experiments were carried out with lower wattages as well, but they were not commercially viable, so no standard was ever created for them.

The 150W and lower wattages are all pulse start. And that mean not directly compatible to operate any MV (the HV ignitor poses a risk of an arc in the outer bulb in the MV).

what about this?

http://www.wayfair.com/Cooper-Lighting-MH-Dusk-to-Dawn-Light-AL70MH-CPK1053.html?

the manual doesn't say anything about it being a pulse-start fixture.

All 70w MH are pulse start, probe start lowest wattage available is 175w. But it takes two seconds to disconnect the ignitor and essentially then it becomes a "probe start" ballast. Philips did make a 100w probe start halide lamp for a couple years (I remember seeing them occasionally) but have been discontinued since 2008 I believe, the same year mercury ballasts were banned here in the US. 

All 70w MH are pulse start, probe start lowest wattage available is 175w. But it takes two seconds to disconnect the ignitor and essentially then it becomes a "probe start" ballast. Philips did make a 100w probe start halide lamp for a couple years (I remember seeing them occasionally) but have been discontinued since 2008 I believe, the same year mercury ballasts were banned here in the US. 

for future reference, how would one find and disconnect the ignitor? i'm not familiar with electrical conenctions or anything.

There will be either two or three components to the ballast (in the case of a pulse start MH) the ballast core and coil, the ignitor, and in the case of a high power factor rated ballast there will be a capacitor. In a NPF (normal power factor) ballast there will only be the core/ coil transformer and the ignitor. There will usually be a sticker on the ignitor case that reads "lamp ignitor", lamp starter, ignition etc.. with a Kv rating, wiring diagram and sometimes a max. ignitor to lamp distance. When located clip the ignitor leads and cap them off. In some formats the only lamp output lead from ballast is connected to the ignitor and then a separate lead from the ignitor goes to lamp, in that case just rewire the lamp lead from the ballast direct to the lead from the lampholder bypassing the ignitor. The "com" and X3" leads dont matter.

Of course this cannot be done with potted F-can type magnetic ballasts or electronic ballasts. Ive tried to remove an ignitor from the PCB of an electronic ballast before and somehow it caused the OCV to drop to 40v  electronic HID ballasts are junk anyway and will usually kill mercury lamps in a few weeks or months, metal halides seem to fare a bit better but nothing beats good old magnetic