Most of us experience this sooner or later, but if you experience it really close to where your machine was delivered, here is the two top reasons why your laser tube died.

1: Overpowering the tube

While the tubes are called 40w, they cannot really handle 40w and has a effective output of about 30-35 watts depending on tube quality.
This mostly happens on the digital versions of the K40, where the milliamp. output isnt visible, only a percentage of the power.
Theese machines cannot handle 100%, they cannot handle 90%, you will overpower your tube at 90%, thus lowering the life span drastically.
If you have a digital version, i suggest installing a 30mA amp meter.
Keep your tube below 18mA at all times to be safe. You can go up to 20-25 but the gain is very little and you are strangling your tube for every second doing it.

2: Overheating the tube

This is pretty common, not having a clue about the water temperature in your tank and just running the machine.
The laser tube works the best at 15-20c water temp, anything above will degrade your tube.
If you run at 23c your tube will not die tomorrow, but you may lower the expected life time by a couple of weeks or month. So for some its ok to run at 23c as they know they need a tube change later on.
There is also a link between using wrong coolant to unexpected tube malfunctions.
Read more about that in the watercooling article.

But if you want to use your tube the most, keep temperature around 20c.
Whats happening when the temperature is too high is the regenerative gases has harder to regenerate inside the tube wich lowers the power output with time.
There is a deeper scientific explanation, but i keep it simple and understandable for now 😉

Quick info: Never go beyond 18mA and keep water att 20c or lower 🙂

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Hello! I have a heavily modified K40 (290mmx570mm cutting area, Z-table). I am still using the same tube it shipped with. When I received it, the “tube temp” thermistor was taped to the side of the tube. This placement, I believe, is probably not the best location since it is not measuring the water temp, but the surface temp of the exterior glass tube. I’m cutting 5mm ply at 10mm/sec and it gets through just fine, but I also notice the temperature rising (tube wall). However the water temp doesn’t seem nearly as warm.

My question is, should I move the thermistor to the water container rather than the tube wall? In other words, can the tube temperature exceed 30-35 deg C as long as the cooling water stays down in 22 deg C range?


The tube was getting up past 32 deg C after just a few minutes of cutting, yet the water was basically the same temp as when I started. THANK YOU for confirming this! I have a decent stock of ice packs ready to go 🙂



First off thank you for this amazing site. It has been immensely helpful and helped me commit to getting into this hobby.

I purchased a laser that was cutting fine for the first dozen or so cuts and then stopped producing a beam that would cut through even 2mm soft plywood. I replaced the mirrors, lens, realigned, and set it for the optimal focal length and it still will not cut.
Thanks to your site I installed an mA-meter from day1 and I know I never over powered the tube and it has operated at no more than 22C.
With replaced parts I assume it must be the tube or the PSU at this point (but the mA-meter does show the same power that it had shown when it was cutting). Is there a way I can troubleshoot the tube? I have gone to the company that sold me the laser and I was able to get £100 refunded from them to avoid returning the cutter. If I need to buy new parts I am happy to do that. Thank you for your time and while I tend not to use AliExpress (lean towards Banggood) I would be happy to donate to your page if you have a link I have not seen. Even just for all the help this site has already been.

Last edited 1 month ago by Ramon

So I went to try this just now and there was already a bridge (like those on HDD to determine slave drive) on the P+ and GND (PSU Type 2). There was nothing else connected to the P+. From day 1 of receiving the laser it has been bridged. The laser was firing away with this bridge in place back when it was cutting instead of just engraving but being unable to cut. The meter has been reading above 5mA since I first used it. Does this mean it is most likely the tube?
Really appreciate the tube brand recommendations. This site has been a real stress relief in this endeavor.


Excellent, thank you. Do you have any links for the tubes to get you a kickback from vendors? You’ve been more help than the actual vendor of the laser so I would love to support you besides seeing ads on the side if I can. Thank you for your time and all your assistance.


Bought and installed the new tube (thank you). While I was test firing it to re-align everything I noticed that the power does go over the previously mentioned 5mA…but then drops back down to below that 5mA. I guess I did not fire it for long enough before. With it starting strong and then dropping back down could it be some capacitor/flyback issue? I think I will still replace the whole thing. I just want to get back to trying to cut cool things for people! Thank you again for your help.



I went over the wiring and everything looks solid. I have a 3 prong wall outlet and there was a plastic washer so I will be taking that out. What gets me is that this thing worked fine for half a dozen or so cuts after purchase. That is how I know that at 8% (I believe around 5mA) I was able to previously cut the 2mm soft plywood. After checking the wiring the power is staying above 5mA (thank you). when I turn it up that high and with the new tube it is doing better but while it engraves well it isn’t cutting unless I am at 10mA+ and moving at around 5mm/s. This is for that sam 2mm ply that previously cut at half that power easy.
I have checked the focal length by cutting something at an angle I and I pretty bang on. Is there anything else that can be impeding the laser’s power? I have new silicon mirrors and a new focal lens in the machine (feel like I am getting closer and closer to a Ship of Theseus situation).
I checked the troubleshooting guide you have and did not see anything about this problem. Since I have the money, would you recommend I just swap the PSU for an HY-T50 and start from scratch (rewire)? Thank you again for your time. Depending on your recommendations I can also easily take you up on your generous video posting suggestion. I will say when pushing the test button the purple beam is pretty much instantaneous, and the mA meter now holds steady.


Thankfully due to your site, I installed an mA-meter before I even fired the laser for the first time. It is how I was sure I was not firing the tube at too high a power level. Thanks to your tips I have been able to get it to cut somewhat and I believe now a lot of power is being lost due to some reflection going on in the laser head (getting huge ghosting line now). I also purchased a new Cloudray laser head that I will be using your modification guide to adjust before I mount it (I have to widen the bracket anyways).
Power supply: The HY-T50 has already arrived and I have a 24v/4A little external PSU from other projects. I was planning on installing it after reading your 24v external psu article.

I noticed that you have an “upgrading the power supply to HY-T50″ article in your upcoming articles listing. Is there anywhere I could currently find such a guide? The labels on the HY-T50 are different from the stock PSU and I would rather not fry something that was easily avoidable. Even a wiring diagram would I think get me where I need to be (I have “PSU TYPE 2 pinout). Thank you again for all you have already helped me with. You have an American in Scotland in your debt.


You must be psychic. I also replaced the mirrors with SI mirrors. I picked up a couple of lenses from ebay but unfortunately cannot remember the brand. Thank you for the tip on the nozzle, I will keep that in mind when I try it this weekend.


Your tip about the hot nozzle head really helped! I was not getting anything after adding the new laser head but I could feel how hot the head was getting. I have the beam coming out the nozzle now and am hoping to align in better when I next have time off. The alignment is all over the place since I replace the head and the new one sits higher than the old one so I raised everything else a bit too.


I do have a Prusa but I was able to drill out the original plate and then adjust the height of all the other mirrors. My main alignment problem at the moment will be figuring out how to get the beam centered coming out of the nozzle. Since the mirror in the laser head is not adjustable that is proving to be trickier than I anticipated.


I have a dual head Chinese laser, with 150 watt reci tubes, today laser 2 it not cutting through (3 mm Mdf ) amp gauge reads 6mA , laser 1 is fine and working well, im assuming it’s the tube failing, as they are both running off the same PSU.
Is there a way to test this first.

Helmut Schlechmair

My Laser has changed the beam spot from TEM00 to TEM01 , i have used the Laser with 0 degree water this Winter and after some jobs the beam is like a 8

Tracy M

Hello can u help me understand why my laser skips when powered above 3-5 ma. Is it losing power. The best is still slightly purple

Wayne Hooper

My laser tube failed on me, I’ve now replaced it and it fires on test. But for some reason I can’t connect to my laptop anymore. I’m running windows 8 with K40 whisperer. Any ideas?


What are the symptoms of an overheat? Changing from TEM00 to TEM01? Or just a reduction in power?

I had kink in the coolant line and the temps spiked to what I would imagine to be “catastrophic levels.” Something like 45C…

The laser still fires and can still cut plywood to some extent, but nothing like when the beam was in TEM00.

I guess I’m wondering if the change in the mode equates to loss in power or if the TEM01 mode distributes the power into the classic donut shape.

Also, what is it that actually causes the change in mode? Does it have anything to do with the gas inside the tube, the optics inside the tube, or the physical shape/construction of the tube?


What is the recommended % of laser power for quality of clean rubber engraving for making a personal seal with complicated logo without causing damage to the CO2 laser tube?


So, 1 px/mm and 1 pass is enough or the best is 2 px/mm and 3 passes at the 25% power and 200 mm/s for the Kehui KH-3020 Laser Engraver to engrave a 40mm personal rubber stamp?

Christian Forasté

Bonjour HP Persson

Pour Un Bon fonctionnement Laser CO2 100w

Quelle Température ?? °c

Quelle Puissance Max ?? mA

Merci De Votre réponse

Christian Forasté

pour la réponse
HP Pression


When you say the best temp is between 15 and 20 degrees , does that mean there is a loss of power when running the tube under 15 degrees? Was just wondering why this would be the case… cheers


Ok Thanks, I see your point. Will definitely keep that in mind.