It really depends on the type of setup you have. If you have a vented hood you can get it much closer to the plant than if you don't. The easiest way to judge is to put your hand at the same level as the top of the plant under the light palm down, if you can leave it there for 60 seconds you're good if not move the light up until you can. Obviously the closer the better but you need to be careful not to burn them! I hope this helps.
Is your hood vented? If not anywhere from 14 - 24 inches. Mine (unvented) stays about 16 inches away and I don't have any problems. Edit: I could probably put my hand considerably closer to the light than the 16 inches, what I mean is leaving it there for 60 seconds comfortably without feeling like you're getting a sunburn.
Like I said, I leave mine about 16 inches from my tops, and have no problems. I'm using a 400w non vented MH light. Try it about there and if you see signs of heat stress then simply move them up.
If you have an HPS ballast you dont need a conversion bulb or switchable ballast. You can run any MH or HPS lamp in the lamp holder as long as the bulbs wattage does not exceed the ballasts output. . Ballasts are like car alternators/generators and only give out the required wattage for what you are running.
My setup came with a new hps bulb and a mh conversion bulb for a good price, I didn't purchase them separately. The op wasn't really asking about ballast choice though. I only gave the bulb type for reference of heat output. I'm giving it a try this time, and maybe try something else next time.
What bulb and wattage are you using ? Do you have a fan cooling the lamp and do you have an extraction fan ?
Yes, but a HPS ballast also has an igniter, which the MH doesn't need and probably would fry it. That's why they make MH conversion bulbs, it has something in it that absorbs the pulse from the igniter. A "switchable" ballast simply has a switch to turn the igniter on or off. The ballast part inside is indeed the same either way. If it's a digital ballast then it isn't specifically HPS or MH and can run either.
It a 400 watt MH, setup does include ballast (conversion), fans blowing in room and a small fan blowing directly onto light. Temp stays around 80 and the humidity 40 -50 % Everything si going fine right now, i just want to know the correct distance from bulb to plant to get max potential.
400 watt MH in a conversion ballast. Fans in room and on light itself. Temp stays around 78-80, humidity stays around 50 %