These are HX, so lowering the input voltage will bring the current down, indeed. But you would need an extra transformer. But on the other hand a low power 120/12 or /15V rated at 0.6..1A or so will do the job, by subtracting the secondary voltage from the mains (you have to find a correct phasing to get subtraction and not addition).

Nice! Thanks for the info, that might just be the ticket.

Aside from that, would changing the cap value do any harmless changes?

Leave the cap value original to the ballast. It does not influence the lam circuit at all, but it does influence the input power factor. When the power factor is worse, it means higher loading of the small transformer...

Okay I will leave it as is. If the lamp sees 60 watt, will that cause any risks? I'm fine with shorter life

"Shorter" may mean here even 1/10. Really don't do that. MV's are not made anymore, better preserve them and so do not damage them by overdriving. Underdriving (e.g. by the MH39W ballast) may lead to inferior performance, but does not cause any extra wear. The ballast input voltage reduction is also a good way. Just check the arc current (after it thermally stabilizes), if it is the 0.6A (or a bit below is still OK).

I tried one on the ballast for a test. I will repeat this when I get a clamp meter.


But at 118v ac, the ballast is pulling in 65 watts at .58a and pf of 0.94. The lamp was stable and did not seem overly bright. OCV 240v and running is 102v

I'm going to repeat this on 110v and 100v ac using my variac using the same bulb. The killawatt meter will be on the variac as well but it works fine at 100v ac

------
At 110v, brightness is noticeably less. Kill a watt reports:

110.8v ac
0.58a
59 watts
Pf 0.91
Running 106.2v
Ocv 226

--

101.4v ac
0.56a
50 watts
Pf 0.87
Running 107v
Ocv 211

What matters is the current to the lamp, that should be the 0.6A.
Not what the ballast is pulling from the mains.

Indeed, but until I have the clamp meter I'm afraid I'm stuck with what I have. Will retest all when I do get it but that'll be a separate post

a 100W CWA H38 ballast should do just fine for an 80W mercury lamp, has to be CWA however, not HX, as only CWA is constant current

an 80W mercury lamp runs at 115V 0.8A a 100W mercury lamp runs at 130V 0.8A, and CWA ballasts are not super fussy about lamp arc voltage and are pretty good constant current drivers

that is no matter how the lamp voltage fluctuates they will keep the current the same

with a HX ballast the lower arc voltage would cause an increase in lamp current, overdriving both the lamp and ballast)

otherwise if you dont have a 100W CWA H38 ballast a left field approach would be a single lamp rated F72T12/F96T2 HO ballast and just stick the lamp between 1 red and 1 blue wire again these ballasts tend to be pretty constant current, its how many are able to drive multiple sizes of HO lamp, since to a fair degree, they dont care for lamp arc voltage, and a HO tube runs at 800Ma just like an 80W mercury lamp!


there is no real good PSMH ballast for driving an 80W mercury lamp and indeed a H43 75W mercury lamp runs at 130V 0.6A

In my experience, I have managed to run a European 80w mercury vapor lamp decently well off of a North American 70w M98 pulse start metal halide HX autotransformer ballast with the ignitor disconnected, but the lamp is slightly overdriven at around 880mA and an arc voltage drop of roughly 100v or so. For me, I prefer to run lamps somewhere around 10% of their rated current.

See here:

https://www.lighting-gallery.net/gallery/displayimage.php?album=6271&pos=42&pid=210388