It's well known that single bulb F40T12 LPF magnetic rapid start ballasts can be wired up to run in preheat mode. Unfortunately, the preheat current is very low to non-existent...

Is there a way to boost the preheat current by wiring something in series to the starter wire?
Maybe a diode or some sort of starting compensator?

Maybe capacitor of some value

It can be estimated. Measure the OCV when there is no starter, and the current through the starter when you are shorting it

It's well known that single bulb F40T12 LPF magnetic rapid start ballasts can be wired up to run in preheat mode. Unfortunately, the preheat current is very low to non-existent...

Well, that means that "well known" is a nonsense.
The problem with these low ballast factor ballasts in nonstandard connection is not only the preheat current, but generally the too low cathode heating during the normal operation as well.
In the RS connection the extra cathode heating  is supplied separately, but with preheat all cathode heating comes only from the cathode fall. And that means at lower current higher cathode fall voltage and that means faster ions hitting the cathode surface.

Quote from: good223 on July 12, 2016, 11:07:05 PM

It's well known that single bulb F40T12 LPF magnetic rapid start ballasts can be wired up to run in preheat mode. Unfortunately, the preheat current is very low to non-existent...

Well, that means that "well known" is a nonsense.
The problem with these low ballast factor ballasts in nonstandard connection is not only the preheat current, but generally the too low cathode heating during the normal operation as well.
In the RS connection the extra cathode heating  is supplied separately, but with preheat all cathode heating comes only from the cathode fall. And that means at lower current higher cathode fall voltage and that means faster ions hitting the cathode surface.

So even when it's hooked up in preheat mode the limiting factor is the standard amount of current from the 3.3v rs filament heater circuit?

@Ash,
I'll check into that soon and get back to you.

No, it is the short circuit current of the "arc" source of the RS. Which is not designed to be anything particular, as the RS ballast was never meant to work in this mode, not even for the moment of starting (as RS)

So even when it's hooked up in preheat mode the limiting factor is the standard amount of current from the 3.3v rs filament heater circuit?

The heater circuit is not connected in a preheat configuration, so whatever is on those windings does not matter at all.
And as Ash wrote, the preheat is done by the ballast current. And because the ballast is nearly a constant current source (even when the short circuit means 30% above the normal running arc), when the rated arc current of that ballast is just 0.25A (talking about the shoplight ballasts feeding the F40 by just about 25W or so), the preheat current would be barely above 0.3A. To compare, the 3.5V winding in the RS configuration feeds the heaters above 0.5A or so (it is actually steered by the lamp itself) and yet it is just barely able to heat the filaments to emission.