I have a 39W electronic MH ballast coming in the mail. I've recently read on here claims that the ignition pulse of such a ballast is unlikely to harm an MV lamp, as long as you don't try to hot-restrike the lamp.

With that said, I'm wondering if there's a way to "quench" the pulse to make it safer under all circumstances. Like, putting a 10 nF capacitor across the lamp or something. Anyone ever try that?

The safest way about this is a magnetic ballast with the igniter removed. There exists magnetic 39W ballasts even if they are less common. However, you might be able to open the case and disable somehow the ignition module. Unfortunately, I don’t really know how to go about this.

That said, there are plenty of really nice ceramic metal halide lamps that make nice ambient indoor lighting.

...as long as you don't try to hot-restrike the lamp.

In the theory yes, but you can nnot avoid thet from happening from time to time, out of your control. Prime example is a glitch in the power feed (e.g. a lighting hits the HV line somewhere near by, causes flash over on the overvoltage arrester, the substation sees that as a short circuit fault so cuts the power, by that quenches the arc at the arrester but causes you losing power at the same time, then an automatic recloser in the substation turns the power back ON) and you have a hot restart cycle started even without you even able to turn around.

I'm making a fixture for my Mom's indoor gardening that takes a 39W MH lamp and can screw straight into a ceiling fixture, hence the need for a light electronic ballast It would be nice if I could make it take a 50W MV lamp too, without having to worry about it undergoing a hot restrike condition.

Based on the weight of the ballast for its tiny size, I can only assume that the ballast is potted, unfortunately.

First the electronic ballast will most likely adjust the current so it either to maintain the 39W (that won't be the problem), but some may also be attempting to "stabilize the arctube temperature" (that assumes certain lamp characteristic), which would go crazy with MV.
As second, electonic ballasts feature a lamp EOL detection/shut down feature (designed to reduce the risk of arctube explosion in the first and preventing color shifted EOL lamps from lighting at bad color in second priority). Problem is, the MV characteristic will most likely trigger this protection feature so cause the ballast to shut down.

Plus the ignitor functionality uses to be incorporated into the circuit rather deeply and sometimes even plain impossible to disable by any simple HW modification without interfering with the normal ballast functionality. You would have to study the exact ballast circuit you have at your hands, there are way more topologies in use to implement the HV ignition than with magnetic ballasts (where all ignitors are basically variations of 2 or 3 circuits; when part of electronic, there are at least half dozen more)

And generaly you can not make any MH ignitor "MV safe" in any other way than plain disabling it. So if you want to alter lamps there, you can not avoid at least a configuration switch. The bare minimum voltage required for the MH ignition (>2..3kV) is way too high to be MV safe (<750V), so a fixture able to safely feed both MV and the (pulse) MH without any reconfiguration or switching is not possible.

I'm making a fixture for my Mom's indoor gardening that takes a 39W MH lamp and can screw straight into a ceiling fixture, hence the need for a light electronic ballast (...)

Cool ! Could you post pictures when it is complete ?

I think it'll be fairly difficult to disable the ignition circuit without going in full reverse-engineering mode.

I'm playing with the idea of designing an electronic MV ballast one day, especially for the ballast wattages that are already unobtainium these days. Or I should say, adapting an existing ballast design to mercury lamps LOL.

Cool ! Could you post pictures when it is complete ?

Certainly. It'll be ugly and minimalistic, but it should be a neat project.

To update, I put a 50W MV lamp on the ballast, and it works great with one exception - during warm-up, the ballast will cut out, presumably due to how slowly the MV lamp warms up. I have to unplug it and plug it back in. I have to do that twice, and then the lamp warms up the rest of the way and works very well. I also didn't consider the possibility that the ballast would regulate output power as well as it does - it puts *exactly* 39W into the lamp. So I should have just gotten a 50W version. The 50W variant might solve the cut-out issue too, since the extra current would warm the lamp up faster.

The good news is that for ignition, it's perfectly safe under normal conditions - this ballast applies OCV for a full second or two before firing the ignitor, so the MV lamp starts without the ignitor ever firing.

Did you managed to disable the ignitor?
Be aware even wgen the lamp may seem to work well, even appear to exhibit instant hot restart, the high voltage during that hot restart may, instead of inside of the arctube, ignite the arc between the wires in the outer bulb outside of the arctube. And that will destroy the lamp in less than a second.