Laser tube hissing loudly after replacement (and not working)

Hello guys,

I have read the posts on this forum but can’t find topic similar my problem. Sorry if I missed it.

I replaced my original laser tube with a new one (40W) and put a little piece of paper tape on mirror in front of the tube to check if is it working. When I tried to fire it up, (about half of second at 50% power) I heard loud hissing from the tube (similar to “normal hissing” when laser is working, but much more louder). Paper tape was not burn. I actually do not know if tube emited some light, because tube lid was closed and I am afraid to let it opened. After visual check, the tube does not looked cracked, but it was dusty on the outer glass from the outside and from the inside. Water was running trough the tube. I am using original 40W PSU (I have second PSU for motors, lights etc.). Next day I tried again with same results. Do you guys have some idea what can cause described problems? Could it be defective tube, or underpowered PSU?

Mines a bigger laser, but I’d never thought it was ‘hissing’.

Could you be hearing a high voltage discharge to the from from the anode of the tube (hv end)?

I’m afraid someones going to have to watch the tube for us to much information. As I stated I don’t have a K40, but I have the next one up.

What do you use for a coolant?

Pictures are nice, a video would let us ‘hear’ the sound you are referring.

You can put the video on something like your google drive, allow reading by anyone and post the link here.

Good luck


When bad the LPS can make a hissing noise.
Post pictures of your control panel and laser power supply, please…

Do you have any additives in your coolant?


Never had the issue. I was hoping for ‘video’ so I could hear it…

Someone posted a diy 80 watt co2 that has a ‘band’ on the cathode end of the tube. Not sure what he can do or if it’s an indicator of another problem… seems to me the only other part here is the lps…

Have you seen this before and what’s do you think the issue is?

Here’s the thread from the Lightburn Forum, both of the video links are posted there also.

Asked for a ‘close up’ and he sent this…

A tube issue, no intent to ‘steal’ the thread… you’ve dealt with the ‘tube’ end of things more than I, hoping for some of your insight.



Hi guys,
thanks for answers.

I took of tube lid and captured on video what happened. Without the lid the sound was very loud. I think now I have a clue what is the problem :smiley: These things are very very dangerous. I am glad I started the test with wireless mouse from the outside of room.

This doesn’t look like the same machine…?

This isn’t what I was seeing earlier, it was at the cathode next to the mirror.

This tube is history, I think.

The tube is arching from the anode (high voltage) to the chassis.

Thank you for reply,

I have standard K40 laser from aliexpress, something like this:

About arching:
Thank you for confirming. I did a little bit of research and found three reasons for arching:

  • dead tube
  • poorly connected anode
  • poorly isolated anode connection

Actually I do not think, that is because of dead tube (maybe now is dead), but rather because of second or third option. I will explain:
When you buy stronger laser (80W for example), you have some kind of connection mechanism to anode/cathode cable on tube covered with electric isolated cover, or, more conveniently, a piece of cable coming from tube and all you need to do is just connect two cables. This is not the case with 40W. From the tube comes out metallic pin about half centimeter long and that is everything. You have to proper connect cable to the pin (you can not solder, because you can crack the glass with high temperature). So you wrap the cable around the pin and use silicon and little peace of hose from your cooling system to cover it up. This can be fine for like 100 volts, but for 20k volts is sounds very irresponsible for me. I know, that K40 is piece of junk, but in my opinion, this is serious security hazard.
So I did my best co connect the anode and it seems like it was not enough. I actually secured the cable to the pin with superglue before applied silicon, maybe that is the problem. Today, I will try to clean the pins with acetone and connect the cables once again.

The silicone cooling tube is actually OK for 20kV.

From the K40 Intro at the top of every page on this site, you can find this page that has some details on high voltage splicing: Laser safety: Light and Electricity

I would not use superglue; it isn’t conductive. I’ve seen small cable ties recommended for holding the wire to the post, fitting inside the silicone tube after the end of the cable tie is trimmed.

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Voltage this high does not need a gas-tight connection. Wrapping the wire on the anode pin is the normal way of connecting. That is followed by a silicone sleeve which you fill with HV silicone.

I’m still confused why he sent us videos of a different machine in the first two with a totally different problem.? This kind of communications makes me uncomfortable… :frowning:


Jack, are you sure it’s the same person? one is an 80W and the other a 40W and different user names…

Huh? Are you confusing @tomas487 with someone else or confusing this thread with another? What do you mean by “first two” here?

Seems so… Need to move back under my rock… my apologies…


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Hi guys,

I cleaned pins with acetone, rewired cables, secured with zip ties as @mcdanlj recommended, put some silicone, then silicone tube and filled the tube with silicone. And after firing up “nothing happened”. So, the tube is dead, but I also do not have arching anymore, and that is important to me. The likely culprit is superglue, which probably did not allow current to pass trough the connection and of course, poor isolation. Now my connection looks more professional.
I actually bought K40 because I like tinkering. I changed or upgraded almost every part in this machine. But from the beginning, I was afraid of changing laser tube at the end of its life. Clearly, my concerns were right. I already ordered new tube and now I know how to replace it properly.

Thanks for the help.


As you figured out, most likely when you put the superglue on it the very thin liquid seeped under, in and beneath the wire and connector. And the gases given off when drying could also coat anything which didn’t get superglue on it. Live and learn right?

So now that you have completely cleaned the laser tube pin connection, did you also cut off the end of the HV wire to the point where there’s no superglue in the twisted wire bundle? It probably wicked into the wire a bit because that what thin liquids do.

IIRC(If I Remember Correctly) others have stated you more likely to have damaged your laser power supply rather than the tube by powering up the LPS with such a poor connection and arcing to the grounded case. At least I think that’s what’s been warned about but hopefully someone can confirm.

I seriously doubt that the CA affected the HV connection. I would expect 20,000 to piece right through that layer. Then again I never tried it.

I think the tube went bad … why?

The arching in the video is typical for a dead tube and there is no amount of insulation that will stop that from arcing.

Yes, that amount of arcing can damage the LPS.

So I guess first replace the tube and then the LPS.


So you think they send him a bad tube?

Dead new tube, unusual unless there is physical damage that’s not apparent


Ya, I looked at the beginning of the thread and this all started with a new tube installation.

And while I know you all here are fantastic at this stuff MY first response would have been to the tube manufacturer after hooking it up and hearing it hissing but not working. On 2nd thought, I would have opened the cover and looked to see if the tube was firing before contacting the tube reseller.


Hi guys,

today new tube arrived. Actually, I bought a set containing tube and power supply unit. At first, I wanted to replace only PSU to see, if the problem lies in tube or PSU. But, when I saw dirty first tube (photo below) and realized, that I can maybe destroy new PSU with arching, I decided to change both just to make things working. This time, job was quick and easy and I had a good feeling of it. After firing up, tube was working as it should have.

In my opinion, this whole problem gives me a good experience how things work and how to deal with them. It cost me one laser tube (about 90€), but I don’t regret it. As I wrote in previous comment, I probably destroyed first tube using non conductive superglue.
Thank you for the help, I very appreciate it.

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