I was doing some silly experiments today with fluorescent tubes, I was playing around with mercury migration on DC using some high hour tubes. Long story short, I had fun and learned a few things, but accidentally fed about 100 watts through a 36W tube, which means that one of the cathodes didn’t enjoy it and blew up quite spectacularly. So I did an old school trick, using some very thin wire and shorted the two pins on the end that blew. This allows the fluorescent fixtures to heat the still working cathode heater, and to my great amazement, the tube strikes and lights up normally with a fresh starter. It’s been running for a few hours now and is not showing any signs of problems.

The tubes I experimented with are 4 old and well used General Electric PolyLux XLr’s, the much newer ones with reduced mercury that don’t like cold weather too much.  The 3 others survived my experiments, but probably have very reduced lifespan now. And well, the 4th with a blown kathode also technically survived since it still works, but I’d say that it’s on life support right now heh. All tubes got fairly dark ends, but I learned a few things so I don’t feel sad about it. These tubes were already well used and I’ve got so incredibly many new ones, those ones I am not touching! These were worn!

I also experimented with a new osram tube, which survived without damage, a very new and sadly very low quality one that had a rattling cathode inside. It got very dark ends but survived just fine. I have a few more of those tubes without cathode issues, which of course I’m not going to do stupid experiments with!

Here is a picture of the tube with blown kathode, it works, and you can very much see a large fragment of the kathode heater laying in the bottom of the tube there!

I’m going to run these tubes for a while, and I’ll keep you all updated on  how they fare. Especially the blown one.

It may survive up to originally intended lifetime, assuming most of oxide coating is left intact. Broken electrode of course will always cold start, so it will die prematurely if the tube is on short on-off cycles.

Interesting! I have noticed that it is difficult for it to strike with traditional starters when I’m starting it from cold, so I tried an Aura Quickstrike electronic starter which seems to start it reliably every time. I suspect my relatively short runtimes (2-3 hours per evening) will probably shorten its life significantly, but I also suspect it might get mercury migration and get dimmer towards the broken end. That happened to another tube that had one broken cathode that I used for a while, but later gave away. The person I gave it away to apparently used it for a few months and then it started behaving in that way which I thought was very interesting! I suppose time will tell! I’m going to move them to some other light fixtures, as I prefer to run some colour 35 T12 bulbs in my main lights )))))) I have some extra light fixtures mounted on the walll for additional lighting, I will install these into those instead and then I’ll keep you all updated how long it lasts, I am very curious myself too! I’ll certainly be happy if it lasts more than a few months

I suspect that the cathode must have at least some of its emissive coating intact since it lights up relatively easily despite just having one heater. I think the heating coil probably was what exploded, and not the actual cathode structure itself. I suspect that most of the emission normally happens from the thorium coated heating coil , so we’ll see how long it lasts

but I also suspect it might get mercury migration and get dimmer towards the broken end.

Not if the emission layer is still there, once it starts there is no difference whether the filament is broken or not in your circuit.
The difference is only for starting: You can not preheat the broken one. But once it starts, the discharge itself will warm up the cathode spot, the same way as it keeps the cathode spot warm during normal operation of a normal good filament.

Update: Still working, even after almost daily useage.

And yeah, the blown cathode heater doesn't appear to have any effect on the tube once it strikes and is lit. The blown side has gotten noticeably more blackened from sputtering during startups, but the tube still strikes on the first try of the starter. The other tubes also have very black ends due to being near EOL at this point. I have a feeling that one of the 3 others with still good cathode heaters might fail due to being near EOL before this one fails, hilariously enough!  Will be very interesting to see how long before it fails to ignite. There's a good chance that I get bored of these tubes and swap them for something else before they fail, heh. I recently got some really awesome 965 colour tubes that id love to use instead, so right now I'm having a dilemma if I should let these 840 tubes run until failure or if I should swap them with some fresh 965 tubes!

Saving the new tubes for the future is of course also a lovely option

wait, thats a good trick to remember, especially now that getting new flurescent tubes is not as easy as it once was

How does this even work, I thought u need both heaters intact for a tube to strike

I wonder if its possible to somehow start a flurescent tube with both heaters blown

Depends on what you mean by 'blown'. If just mechanically broken, yes. If EOL = all activation spent, or the whole filament missing, no way.

Yes it is possible to start a fluorescent with both ends “blown”. My brother actually did that with a 15W T8 tube, which had both heaters open circuit, for some reason. He just shorted both sides of the tube with thin wire and sure enough. A Philips S10-E starters starts it with the inductive kick. It of course cold-cathode starts so it ain’t gonna last long but it sure works, great way to get some use out of damaged tubes! It’s been working for months now, so yeah heh.

Of course it won’t work if the cathode material is spent , but this shorting trick works on tubes that have been for example shipping damaged, been exposed to too much vibration, stuff like that.

So when I said blown I didn’t actually mean blown as in…. Kathode gone. The heater is gone yes, but of course not the whole cathode