I have had instant start ballasts fail on me a lot. They have been from EOL lamps taking out already crappy ballasts or me being maniacal and using them to overpower and kill cheap f34s. The modes of failure I've witnessed were: blown caps, overheating coils and tar leakage, and just blown fuses. What was the worst failure mode for you?

Every lamp EOL is an overstress for the ballast. The ballasts are normally designed to endure some amount of them, but if you take the median life of 100khour for the quality ballast and 10khours for a quality lamp, the ballast is suposed to endure just 10x the lamp EOL over it's life.

Now when you start to play with the ballasts and finish off one lamp after the other by it, you may reach the expected "number of EOL lamp"-life winthin just one day. So no surprise they last so short.

And another note: If you are using an F32T8 ballasts, you are loading them by way lower arc voltage than they are designed for: F32T8 is nearly a 140V arc lamp, the F34T12 is just barely 90V lamp. Then it depends on the ballast design, if that causes higher arc current (then it will overheat) or not (then no problem at all).

But does pushing ~600v into the tube damage it worse than the ballast? I've also seen a member here who left some f40t12 bulbs in an instant start conversion because they were short a few f32t8 tubes. He didn't see any issues, but the tubes banded quickly due to running cold cathode.

The 600V can not be "pushed" into the tube, the tube always limits the voltage to whatever it "wants".
The ballast is pushing the CURRENT into the tube, unless the tube still does not want to conduct below the 600V.

In other words if the tube ignites at 400V, there is no difference, whether the ballast has OCV of 401 or 1000V, in both cases the tube will see the same - becoming fed by current at 400V.

The extra wear on the instant start came from the fact, the cold electrode tube ignites and mainly operates at higher voltage than when the electrodes are hot. That means, the hot electrode ballasts do not need to be made to generate as high voltage for the lamp ignition, so e.g. F40T12 ballast suffice with 240V for a RS, but needs about 500..600V when the electrodes are not heated. But even when the 240V happens to ignite the cold lamp, the wear is exactly the same, whether the ballast is 240V or 600V OCV.

Of course, when the ballast uses the resonance to boost the voltage, generating 400V when starting a good lamp means less stress for the ballast components than generating the full 600V to start the worn out one or when the lamp refuses to start at all.


And with the F40T12 put into a F32T8 fixture means the F40 are severely underdriven, so their cathodes are not operating at correct temperature. That means higher voltage fall on the colder cathode, so more voltage to accelerate the ions there, so higher energy of the ions sputtering the materials off, hence the faster blackening.
And even if it does not sputter during normal operation, it takes longer time to warm up the heavier cathodes of the F40, so they operate for longer time in the damaging cold cathode mode, so the wear progresses way more.