I hope someone here can verify/clarify this for me. I read someplace (cant find it now) about engraving performance with a high wattage tube. Something about the high watt tube (130w) does not fire “as well” or “at all” at lower % as say a lower watt (80w) tube would. I read it on the internet so its got to be true 
Higher wattage tubes have higher firing threshold - it takes more power to get the plasma to ionize. My 40w will fire at 2% power, but my 100w needs about 11%. And the wider tubes have a wider beam, so you get a larger dot size, which means larger kerf and less detailed engravings.
What brand of tube (length/diameter) and power supply are you using?
A glass tube laser will also have a rising response after use at high power; you may be able to get lower power ignition after you have run the tube for a few minutes at a high power setting.
“wider” You mean the 80mm diameter of tube?
The Reci W2 80w & Reci W6 130w are both 80mm, would dot/kerf be the same for them?
I haven’t purchased anything yet. I am still waiting on quotes so I have time for research, maybe too much time. I have no idea what I plan to do with the machine so lets assume 50/50 cutting and engraving.
(To anyone reading this… Do your own research, I am no pro here, learning as I go)
I am still not sure what I want. My intention was to get a 100w as I felt it was the middle of the road. So that is what I searched for. On ebay it seems like they exaggerate/contradict themselves a bit. A lot of them state “Reci W2 100w” which is peak power (shortens tube life). If you look up the specs online (lightobject) for a Reci “W2” you will find:
RECI W2 80W/90W CO2 Sealed Laser Tube
* Power: 80W/90W (105W peak)
* Triggering Voltage: 24KV
* Operating Voltage: 18KV
* Current: 26mA (28mA max)
* Length: 1200mm/47"
* Diameter: 80mm/3.15"
But a W4 is a bit different…
RECI W4 100W CO2 Sealed Laser Tube
* Power: 100W (110W peak)
* Triggering Voltage: 28KV
* Operating Voltage: 22KV
* Current: 26~28mA (Recommned 26mA)
* Length: 1400mm/56"
* Diameter: 80mm/3.15"
Not true - it’s more about understanding the power curve, the nozzle distance, lens quality and what size. I use a 175 watt CO2 tube everyday. I cut mainly wood and plexi. I engrave on everything. Point size of the more powerful tubes can be mitigated by the lens and distance to work surface. There is no magic formula - you have to test, test, test and find the sweet spots. Look at our work and details if you think a higher watt tube is a problem.
Would love to see some samples of your work 
With your 175w, do you have any examples of photo gray-scale/halftone work? The standard picture of your dog will do
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You may need to think about the axis speeds you can achieve without losing steps as well. If you can move fast enough then the high power tubes will be fine for engraving, but if you are restricted because of the mechanics/ drive system you might not be able to get the power down low enough for fine engraving. In my case I have a home made 50W CO2 with drive belts and a Mana controller. I am limited to about 166mm/s (10000mm/min) before the lines get a bit wobbly or I risk loosing steps on very small moves. At this speed I can set the laser at 40% power and this also gives some margin for the dynamic power control to reduce the power at the end of the line without dropping below the firing threshold.
Just something else for you to research:grinning:
Cheers
David
Fabulous work Dean!
I have a 100W laser and can’t engrave Trotec TroLase Lights because even when the laser is barely firing (8% power) and at highest speed (400mm/s) the laser burns the top layer instead of ablating, causing it to peel back from the base, and also leaves a lot of char.
It engraves thicker TroLase beautifully though.
I was considering using less efficient moly mirrors to dial the power back a tad – 10% is the sweet spot for engraving on my machine. But my machine has a dual-laser controller so am also toying with the idea of adding a 40W tube just for engraving.
I am finding that my 150W Reci works just as well as my 100W Reci for engraving. I was concerned at first, but everything worked out fine. I also agree, the dot size is a lens / focal length determination (not a Reci W2 v W6 v W8 or power element). CERTAINLY the ignition voltage is higher in higher wattage lasers, but this is easily compensated by… faster jobs anyway (and faster movement). 
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Sorry, but this is categorically untrue. Yes, larger diameter tubes do typically have a larger diameter beam. That part is correct. But according to all known laws of laser optics (as well as actual real world application for those that shun all things academic), you get a tighter, higher quality focus when you have a larger beam input diameter, which is fundamentally the opposite of the above statement. That is why some very high quality machines use beam expanders early in the beam path. An additional benefit of beam expanders is that it lowers the energy density of the beam as it passes through mirrors and optics, which reduces heat buildup and increases working life of the components. A fundamental property of lasers is the smaller the raw beam diameter, the larger the divergence, which ultimately affects the ability to focus.
“Enlarging the beam to twice its original diameter will cause a two times reduction in the size of the focused laser spot at the work. If the size of the focused spot at the work can be reduced by a factor of two then the resulting power density at the work will increase by a factor of four since all of the power of the laser through the lens is now being concentrated into a spot that has ¼ the area of the focused spot without the beam expander. This can be a pretty big deal.”
https://www.jimani-inc.com/blog/beam-expander-for-laser-marking-system
"A laser beam - due to diffraction and beam divergence which in turn are related to dimensions being finite - will focus to a spot size that is directly proportional to the focal length of the lens and inversely proportional to the diameter of the laser beam at the point it meets the lens. The formula is:
spot dia. = 1.27 x f x wavelength x M2 / D
where D is the beam diameter at lens position."
https://www.parallax-tech.com/faq.htm#f_size
You can play with the parameters and see for yourself. Put in your system characteristics, and then change only the diameter of the input beam (the size of the beam as it would exit your tube). You can see firsthand, that all things being equal, a larger raw beam diameter will focus to a tighter spot with higher energy density, and hence, a smaller kerf. The downside is a smaller depth of field.