This is the tube in particular I am refering to...
http://www.ekran-os.ru/epm66geng
a non inverting glass window with a S-25 PC Gen2+ tube...

DISCLAIMER: Adjusting the tube has the ever so slight potential of ruining your night vision device, do this is at your own peril!!

The older ATN Night Spirit 2 scopes seem to have one. (or at least mine does) I am pretty sure (but can't confirm) the Armasight Avenger has the same tube...

This the Image Tube exposed....


Get the tube lock ring out, (it should come off with finger pressure) Then remove the seals and it should expose some silicone sealed holes; to remove the silicone, gently sink an exacto and just start turning it like a screwdriver, very gently, the silicone should come right off, exposing the set screws.

From RIGHT to LEFT on the above picture:

-Set screw #1 Overall Gain
 Turning this one in a clockwise rotation will increase overall max gain, counterclockwise will lower overall gain
-Set screw #2 Min gain when bright. (seems like the BSP will overide this value tho and lower the gain when a bright light hits the tube)
 This controls the lowest percent gain setting on the tube, (most street lit scenes will fall into this adjustment)
-Set screw #3 Max gain when dark.
 This controls how much the gain percentage is increased when the scene goes real dark. Max gain is determined by Set Screw #1.
-Hole #4 I am not sure if there even is a screw inside this one; so I don't know what it does.

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Comments:
-Cranking the gain to the max won't render better scenes, it will be a noisy mess; and contrast will go out the window... the factory setting gain increase is way too low, and there is plenty of room for improvement without maxing out this setting.
-Cranking the min gain setting too high will make the scope too bright when looking at light sources... I recommend cranking this one up enough to make a difference but don't overdo it!! IMO the stock setting didn't render dark shadows outside street lights; now it does.
-Use common sense, but there is plenty of lot of room to play with different configurations and you can tune your scope to suit your needs. I wish these settings were externally adjustable...

Once you start playing with these settings you'll hit the PhotoCathode light sensitivity limit real fast, 350 lm/amp is quite low compared to some of the gen3 stuff and that is the real limit of these tubes (since they work the same as Gen3 except the PC) The low SNR these tubes have will also be quickly noted when gain is set at a higher level and be prepared to give your resolution away when looking at really dark areas with high gain; now, you'll see a clear clear image, but it just won't be as sharp as when light hits the tube; which is an improvement where before you just saw a black image with some shades of something in there...

Hopefully this guide helps someone with one of those scopes/tubes to squeeze every last bit of performance out of their scope.

G.

Here is a video I took of my NSP2 with a F1.3 25mm lens and the corrected distortion lens. I finally bit the bullet and purchased the goggle kit for these devices with an F1.2 lens and the headmount so these should arrive sometime this week hopefully...

In the scene there are just a few LEDs on the room, which are visible on the first shot that I took with the ISO3200 on the phone camera (which is a little better than my eye can see)... These LEDs are enough to lit the whole thing up when you put the scope on. The windows and blinds are closed shut and there are no direct street lights that shine inside our house. (There is a tree line where the windows face) I removed all the night lights that I have so it was dark as a cave. The light coming from underneath the door is from my computer monitors... I can't even see it with my naked eye. The rest is just ambient LED light that shines. Before I made this modification to the tube, doing this test would've rendered a nearly dark image with some shades of dark but nothing compared to what I see now. The two video takes haven't been altered in brightness nor contrast. The image looks a lot lower res than it actually is, however, there is a lack of resolution when it gets that dark, so don't expect to read size 4 fonts with these. Inside the hallway there are zero lights, just whatever shines from the kitchen LEDs. I also left a few things on the floor to check if could see there was something on the ground as well... which you can see.

Here is the link to the video:
https://www.youtube.com/watch?v=F2adE-6VJgg

Hope this helps!

G.

I made another video, but this time inside a 100% dark room with just a single red LED from the space heater. This small and barely visible red led puts out enough light that the tube can render an  image of the room. I am working on getting a Gen3 now as I am eager to pit it against my modded Gen2 tube, just to see how these perform indoors. At some point I'll probably get a thermal viewer as well... but for now my eyes are on a Gen3...

Video here...
Ekran EPM66G dark room test video

When I first got the tube this test yielded an image, but with just shades of dark green, and the red LED was about 1/2 the brightness it has on the video, and you definitively couldn't see the shadows the LED projecting on the walls. So,  I'd say I've probably doubled or tripled the light gain of the tube with this little mod. Obviously the noise is apparent, but considering I've paid 500 bucks for it, I think its good enough for my application.

Test was done using an F1.3 lens 25mm.

G.

People who manufacture tubes optimize all settings on tubes so they give the best image possible  at the lowest light level possible, after all that is what they are selling. That said tubes lose photo cathode sensitive over time. Increasing gain will hide the problem for a while and you may not notice that the image is nosier than when new. This manifest its self as not being able to make things in very dark places the way you would if the tube was new.

But if the tube does what you want, good.  

Yeah, the not as good PC sensitivity is already apparent, no surprises there; and looking at the specs I kinda knew before doing any of this.

I guess making up some PC sensitivity by cranking up the gain seems like a cheap way to increase performance. That's how the cascade Gen1 tube works @ 100,000x light amplification its going to render images no matter what, even with low PC sensitivity.... The reason why I posted this was that perhaps some people might just want to up the performance of their cheaper stuff without having to shell out the cash for a new NVD.

My understanding on manufacturers was that when making products they are made to match a spec sheet; for example; in order to have a better than 20 SNR they limit the gain at 20,000x... but what if someone doesn't really care much about the SNR? Then by cranking up the gain he now gets to see an image, noisy, but he gets to see something that otherwise would've been even more useless because he couldn't see anything at all.

I am on the hunt for a Gen3 now, I would like to see with my own eyes the difference; kinda like when I first got my NSP2 after using a Gen1 for a long time...

G.

When a manufacture builds a tube they build it with much more gain than necessary to simply make there gain spec in a given type  tube. No two tubes are alike PC's, MCP's and screen's all vary, the gain is used to optimize the performance with a balance of light out and resolution of the image. If you need more light out to see the image that is fine, turn up the gain but what you might not notice is that detail in the picture is going away. you have to have a balance.

Engineering is a balancing act of the laws of physics.

You're right, its a balancing act, and with more gain the images wash out, as in there is less contrast to pick details. That's why I think people should play with the adjustments, to find out what is acceptable to them. For example, I changed mine quite a bit, from a solid green image with the cap on, to a Gen0 performance (zero gain) and now its at a point in which I can still make details but the images are much brighter and clearer than they were before. I am still going to get a Gen3 at some point... but I felt that before I move on, I needed to play with what I had to see how much better it could be made...

Like I said, this "mod" is geared towards people that have a scope with an Ekran tube inside, and hopefully it awakens other people's curiosity and start tinkering with I2  tubes to see what they can really do when pushed. Some of these Gen2+ scopes can be acquired for real cheap nowadays and they could be modded in a very simple way that is not obvious, since these holes are closed shut from factory.

In the end, I am  just trying to help others get the best out of their scopes.

G.