I had a total of five 2x F40T12 fixtures in my house. Two of them (above my workbench) have already been converted to LED. At this point we are just using up the tubes we have left and then gradually converting to LED. Well another fixture lost a tube yesterday, but we don't have any more fluorescent tubes or LED tubes right now. So I pulled off something a little sketchy.

The fixture has a 2x 40W Advance ballast (not the residential type), and importantly it is thermally protected. I removed one set of sockets so the fixture can only accept one lamp at a time, and I connected the red pair of wires to one socket and the blue pair of wires to the other, and capped off the yellow wires. This way there will be one lamp connected across where normally there would be two in series. Now this on its own was not enough to instant start a lamp, so I tried something even more sketchy.

I hooked up a 100W metal halide semiparallel ignitor to it. Connecting X2 and LAMP to the red pair of wires and the COM wire to one of the blue wires (or the other way around) makes it work pretty much like it normally would with an MH ballast. This worked beautifully! I was able to insert a lamp with a broken cathode, and it started up like new! And of course it was a little brighter than usual since the lamp is being overdriven. I have quite a few "dead" lamps, and I will be finishing them off in this fixture. Since the running voltage of an F40T12 is pretty much the same as a 100W MH lamp, the ignitor triggers only until the lamp lights and then it stops.

Does this risk damage to the ballast? I know it isn't rated for the HV pulses from the ignitor, and I know it isn't rated for the current of running one lamp where there would normally be two. Is this a horrible idea? I ran it pretty much all day yesterday, and the ballast didn't seem to get any hotter than usual.