Reci or Spt tube replacement?

Good day all!
I am looking for opinions from those of you that have replaced their worn Reci tube with an SPT (red dot integrated). I can actually buy one locally from a Thunder Laser dealer. I was told that all the Thunders 100 watts and above come stock with an SPT tube and all lower powered models come with Reci tubes. I have seen posts from 3-4 years ago stating the SPT cuts better but the engraving isnt as good because of a larger dot size. Just wanted some opinions from those of you who have actual experience using tubes from both manufacturers. I could really use some advice! TIA!
FWIW, I know people say that 100 watt machines do not engrave that well, I have had zero issues getting amazing engravings from my 100 watt reci W4 tube…Id like to keep it that way if I decide to run the SPT.

I doubt that the spt tubes have a smaller output beam diameter. All things being equal, the output beam must be smaller to create a larger spot size on the material.

There are a number of calculators that will give you a good estimate of the spot size… I use this one the most.

Look at the math in the calculator link and you can see the D (input beam diameter) is in the denominator, so a bigger D value results in a smaller spot size. Shorter lenses and/or a beam expander will reduce spot size along with dof (depth of focus).

Each machine will perform a bit differently, that’s to be expected. Both of these are great tubes, I was going to put an spt in my machine when the tube failed. I watched a video by Russ Sadler on making an accurate visible framing led. I decided it wasn’t worth the extra cost. I used mine a couple of days, then changed lenses, that changed my point, so I ripped it off and increased the acceleration. Haven’t had one of these basically forever.

It could be just the fact that most tubes won’t actually lase at low power levels. I know a number of people claim their larger tubes won’t lase below ~20%… also heard of 1%, but I find it hard to believe.

The bottom line is fine engraving generally takes very little power. If I can easily do the job at 10% on mine (5W) – 10% on a 100W is 10W or twice the required power… it gets worst if it will only lase at 20% then it’s 4 times the power needed…

Tubes are analog devices, they have a range and don’t work as well at the extreme ends. High power is for cutting, that’s really all it’s good for, at least in the hobby world…

If it works for you… great … in the end, that’s all anyone really cares about…

Pick the one you want… the most simple and direct solution.

Let us know which you decide on and how well it works out…

Good luck


Good morning Jack! Thanks s much for the detailed answer.
I am still tossing the options around as I really like the idea of the integrated red dot. I am sure I had seen you post somewhere that you were gonna give the SPT a shot next time you needed a tube, I guess that hasn’t happened yet?
On another note, maybe you can shed some light on something else that I have been pondering…
With the way my power supply is set up, if I run around 67% in lightburn, my current is sitting around 23mA. I believe the safe current level is 26mA max. I never go beyond 23mA, but I guess I’d be comfortable at 25 even though it wouldn’t make a huge difference in output. It would be nice to adjust my PSU to run safely at 25mA max when I set lightburn to 100%, just so there cant be any accidents…wondering if you have any input on that? I’m the type that doesn’t go adjusting things unless I am 100% sure I know what I am doing.
Thanks again for your time, I really appreciate it!

I found out about SPT tubes watching one of the Russ Sadler videos on leds for framing. I had run a few years with no led framing pointer. There are other ways to align things on these…

I sent a note to Russ about using an spt tube and he advised he’s heard a number of complaints where it was off from the lasers impact point. Of course you can’t correct it and I don’t know what Russ considers off in these situations. I’ve never had a led spot for framing, so I figured I don’t really need one that costs me and may not be accurate…

I ended up replacing it with a 45W tube from CloudRay. Works great, easy swap…

This is relatively easy to do. When you set Lightburn to 100% power, the pwm is on all the time or at it’s maximum value, so it’s relative simple to set the lps to limit current for your maximum.

Anytime you’re adjusting things, use a lower power… during testing you can drive components out of their safe zone. I set the power to 50%, this produces a 2.5V current control at the lps, unlikely that will be too much current.

With a 50% pwm, you can now adjust the lps to limit it’s current to 50% of you maximum.

This photo is from the back of my machine looking up towards the control panel… So you have to be a contortionists to get in here and adjust it… You only have to do it once… but depending on how the lps is mounted will determine how accessible this pot is. They are 20 turn pots and all I’ve dealt with have had them set at maximum current or fully cw.

Once this is set to 50% of the your maximum current, then the percentages of power will directly related to current value through the tube…

Tubes are negative resistance devices, meaning that a change in voltage doesn’t necessarily relate to a change in current… In other words, you can’t apply Ohms law to these devices. If you plot the current input to the wattage output, it isn’t really linear as we expect and some folks find that maximum wattage output doesn’t occur at the maximum current rating.

If you plot watts/current, then you can set your percentage to relate to mA or watts… The previous setup I described will give you a mA scale… I prefer to use wattage, that’s up to you.

These higher end tubes usually have data on output power and a maximum working current value along with a do not exceed current value. Sometimes it’s expressed as watts…

I suggest:

  1. find your tubes maximum working current
    a. pick your maximum comfort safe operation value
  2. use a 50% pwm power setting from Lightburn or via your console
  3. using the determined current, adjust lps for that value

This will sync up the 100% in Lightburn to 100% of what you set…

Make sense?


Most of it makes sense Jack, I did some reading on PWM and Duty Cycles. It seems fairly straight forward but I’ll wrap my head around it a little more before I start messing with adjustments. I do know that my handheld power meter shows around 80 watts when I am running at 24mA. I run a 100 watt Reci w4 (130W Peak if I recall)…perhaps its losing its grunt and needs replacement??
I’ll check back if I need to harass you again!
Also…thanks again for your time, Jack. It’s more than appreciated!

This is really irrelevant when it comes down to it. You can put 2.5V on the IN terminal and emulate 50% pwm… that’s the output of the LAn-1.

A tube is an analog device not a digital one. When you turn a tube on at 50% it will run at 50% power, a diode laser (ssl) will be on for 1/2 the time and off the other half.

It’s good to know, but it’s more of a red herring or a side bar in this situation.

You’ll have to pick what you want as a 100% mark… mA/watts. Some find that maximum watts out occur at a lower current level than the tubes working current… No one really cares about mA, but it’s what most people have and should be monitored. I always felt a good plot to see where peak power occurs would be more useful.

Good luck


OK, I read over your instructions again and I completely understand. It is simple. I misread the first time and assumed you meant that I need to adjust the PWM frequency to 50% of what it is set at, hence the unnecessary research. On the plus side, I now have a better understanding of the machine.
Note to self…read twice and harass Jack once! lol
You’re the best! Thanks again…