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I replaced the T8 fluorescent tube in our utility room with an LED 24w watt tube retrofit replacement back in 2016. I arranged for an electrician to bypass the ballast for me at the time. The tube worked great but just last week I went to switch it on and it started flickering so rapidly I had to switch it off. Then I put it back on later and the flickering continued until it eventually went off completely.

Now when I go to switch it on you can hear a slight hum from the light fixture - (the type of hum you would hear before a fluorescent tube has managed to power itself on) but the tube never lights up.
I'm just slightly surprised the tube did not last longer. It wasn't on for prolonged periods of time very often. In total it has lasted us 5 years.

My question is has the tube failed or has something in the light fixture failed or burnt out?

We have another one of these tubes in the same fixture in our garage and it still works.
Both tubes are Lumilife 6ft LED tubes

Without being there to test it, it is not possible to say definitively. You could however swap the "tubes" over and see does the flashing go with the swapped "Tube" If it does you will know for sure it is the "tube"
My money is on the "tube" has failed, specifically a capacitor.
LED lights often claim to last a very long time, but they don't last THAT long

I was surprised how short lived my LED tube was, around 18 months, and for most of that time house was empty. As with yours ballast removed, and second tube lasted until son swapped it all for 2" down lights.

New house no LED failures and I do wonder how much due to having a surge protection device fitted?

For the replacement for the fluorescent tube to work either with or without the ballast is needs to have a wide voltage range, to get that either pulse width modulated or a switch mode driver is required inside the tube, it seems likely it is that which has failed, since built inside the tube nothing one can do, other than ensure no surges on the supply.

With an electronic ballast a fluorescent lamp gives out around 95 lumen per watt, and it's life is around the same as LED, so only advantage of LED is you can get same light coverage but lower lumen output. So in corridor's for example we don't need the amount of light from the fluorescent we just need coverage, and likely using PIR switching which fluorescent does not like, so LED is good, but I have seen LED tubes in double fluorescent fittings, which seems pointless as if half the light was good enough then simply removing one tube would have been better.

You can get LED at up to 120 lumen per watt, but in the main they are only 95 lumen per watt, so no better than fluorescent with electronic ballast, the wire wound ballast has been discontinued but the comparison tables still compare to wire wound ballast, at around 75 lumen per watt.

that would make sense a 100w "normal" bulb is around 1300lm[lumen] so that at an led at 120lm per W would be perhaps a ratio off 9 or 9.5 to one so a 10.5-11w bulb would = 100w call it 10.75 for simpicity so
1w=9.5w
2=19w
3=28.5
4=38
5=47.5
6=57
7=66.5
8=76
9=85.5
10=95
11=104.5

and at 95lm perw is 20% less at 7'5w
1=7.5
2=15
3=22.5
4=30
5=37.5
6=45
7=52.5
8=60
9=67.5
10=75
11=82.5
12=90
13=97.5
14=10.5w
i think most off my bulbs are somewhere in between

Every bulb needs a leakage resistor so any capacitive or inductive leakage does not cause bulb to flash this does not really change with bulb size, so smaller bulbs will be around 60 lumen per watt and this likely raises to 100 lumen put watt with the larger bulbs, DC bulbs tend to be better lumen per watt, and often the smart bulbs are better with lumen per watt, likely as they have a pulse width modulated chip to regulate rather than simple capacitor.

So a 24 watt replacement for a fluorescent tube, likely 100 lumen per watt, but with 6 x 4 watt bulbs, likely down to 75 lumen per watt. Also the light must be used in the room, putting it simple a window will allow the light to exit the room, a white curtain will return it to room, and a black curtain with convert the light to heat, a bulb in a chandelier pointing up onto a white ceiling the spread is good, pointing down it will likely be absorbed in the floor. Also with the base below the LED chips the capacitor will run cooler, and electrolytic capacitors don't like heat.

I think the small GU10 bulbs over 4 watt waste much of the light as so small, my son tried using them in a kitchen, it replaced my 24 watt LED fluorescent replacement, so more like 36 watt and same amount of light, but better spread and it looked better, and that is a good point, it's not all down to light output per watt, but what looks right in the room.

i also found the narrower the input voltage range the better the light output seemed to be purely by sight so not very scientific
a say 85v-263v was likely to give less actual light out compare to a say 185- 253v
and indeed i noticed bigger bulbs punch more above there weight[size] and small ones struggle
i also try all bulbs with or without diffuser domes [the translucent bit to make it look like a bulb]as they are desighned to give a greater spread off light so iff a bulb is in a shade the light can be wasted as it gets deflected onto the shade rather than down and a bit sideways with no backwards projection

When I was in university we had to experiment with LED's and one thing was to switch them on/off and over drive the on time, average still within limits, they seemed brighter to look at, but lux meter give same results, using them to read with no advantage.

So using a simple capacitor as driver and full wave rectifier they are flashing at 100 Hz, can add a parallel capacitor to smooth output, but looking at the light one is easy miss lead. Best way to compare is see the auto settings a camera selects.