Laser tube dead on arrival?

My orginal K40 worked for 8 months no problems, then the power supply went (input only, probably mosfet or something). Pressing the test button on power supply fired the laser. Bought a new supply which worked, but the laser tube was putting out TWO beams, not one, right at the laser tube output. Google search told me the tube was faulty. After 6 weeks haggling with Vevor who I bought the unit from, they refused to honor their 12 month warranty. (I used the tube about 50 hours, not near the 1000 hrs the tube is rated for). They offered me $50 because “I used the product”. Ha ha. While this was going on, I bought ANOTHER tube from MKW Laser, via Amazon. Put it in: dead in the water. There are only 2 wires for Pete’s sake, how could I make a mistake? When I had the old power supply on the bench and hit the test button, the dangling HV wire above my grounded metal bench gave out a spark that jumped 2 inches!! So yeah, the supply works. There are no cracks in the new tube, I was very, very, careful installing it. (Since I replaced the tube holders, I used the OLD tube for test fitting). So now I may have another fight over another tube. I find it hard to believe that this tube failed because I’m sure they test them at the factory. Any thoughts on this?

Only 8 months out of a K40 stock tube sounds like typical over driving of the tube and/or over heating. Stock K40 tubes are more like poor quality 35W tubes(cheap price right?) so they can’t be run over ~18mA or they’ll wear out quickly. AND any laser tube should be operated at 20C water temp(entering tube) and should be stopped if the water(distilled water only) reaches 25C.

Before I purchased my K40 I read the FAQ from front to back at and there is also the good K40 Intro at

The tube and power supply in any laser cutter is considered consumable of the laser cutting process. Eight months is too soon by users are often operating these machines in non-ideal conditions and vendors know these tubes can last years or months depending on how they are used. I don’t know if your machine had a current meter on it or if it was one with just the digital push button display where you select a percentage. The digital displays are notorious for killing stock tubes because often even a setting of 40 percent results in over 20ma through the tube.
Again, this is a very common mistake new users make when buying and using K40 laser cutters.

I would validate there is no water in the outer section of the tube. There is a tube inside a tube inside a tube and the water is only supposed to be inside the middle tube(around the beam of light) and not in the outer largest tube. 2nd thing is getting the wire connections isolated with a proper connection and covering. Validate by turning the lights off and looking in the laser compartment and firing the power supply to see if there’s arcing anywhere. Arcing can also damage the power supply.

But you had already said that the OLD power supply worked when you pressed the Test button on the supply so I don’t get how this has anything to do with the new tube and new power supply in the machine. ?

Even a single test firing of the tube without water in the tube can destroy a new or old tube. There should also be no large bubbles in the tube and sometimes the water pump needs to be run for a few hours to work all the bubbles out. Others have slightly rotated the tube so that the water outlet is above horizontal so to let air pockets move out of the tube.

Again, reading the K40 FAQs front to back explain all these things and more in hopes of preventing K40 users from falling down the same holes hundreds of others have over the years.

Thanks Doug for your extensive reply. We just had a hurricane and power was off for 2 days so I couldn’t respond earlier.
FYI, I have a current meter and never went above 17 mA. I also have good water flow, evidenced by a small mechanical rotating impeller indicator and DID stop running it when temp got to 25C. (I’ve since doubled my water supply to get more capacity.) I also go in the bottom of the tube and come out the top to prevent air bubbles and ensure tube is always full of liquid.
I did a STUPID thing. While the old P/S worked with the old tube, I was ASSUMING the NEW power supply should work with the new tube. Dumb. The reason why is that I had HV connectors on both and they were not swappable.
I FOUND the problem when I took the new P/S out to check/replace it. The (input) voltage select switch on the side of the P/S was set to 230 volts, not 115V. Voila. Problem solved.

There were some small issues with the tube: HV wire was too short putting strain on the anode connection and was difficult to work with in the confines of the power supply bay. The bad connection on the anode resulted in spark that jumped from the anode connection to chassis. I fixed that with HV silicone sealant and also put a rubber bushing (grommet) around the hole to keep the HV cable away from metal. The connector that came with the tube was incompatible with a new one I purchased beforehand from reliable source. Troubleshooting by MCW, the laser vendor was non-existent. Telling me the power supply fan didn’t make noise ( in the video I sent), therefore the power supply was at fault was stupid. (The fan turned even with low input voltage, and I also had 5 and 24 volts available.)