preamps boards

[Roberto Ferrari]

Hi
Long time not attending the forum.
By the way, do you have any boards to sell?
Mostly looking for preamps for neutron tubes.
Roberto

[Doug Coulter]

I'm all out of those boards. They're not hard to duplicate - the "build it on perboard with through hole parts" version works fine, we just AC couple it to the neutron tube. The tube is supplied with its DC through a large value resistor, then a small cap to the preamp gets it done. Of course the power supply needs good filtering, but since we're talking megohm range series resistance to the tube, a stage of RC in there is pretty easy to add to get the supply very clean. With the supplies I use (volt mutliplied CCFL inverters) all the ripple is HF and easy to filter out with not too much capacity. A thousand to 10 thousand pf coupling cap or so...it's not critical, gets the signal to the preamp.

[Roberto Ferrari]

Hi Doug

OK, will look for the schematic.
Thanks,
Roberto

[Doug Coulter]

viewtopic.php?f=7&t=542&p=4407&hilit=fine+wine#p4407
is what we wound up with. This expects a floating power supply for the preamp - 9v battery.
The board had a coupling cap for AC coupled input, but that tiny thing wouldn't have stood off the voltage required for a neutron tube.

FWIW, I tend to run those tubes at a somewhat higher voltage than they normally are run at to get a bigger signal out of the tube,
bordering on getting geiger action - not quite, but I do get some "gas gain", which is, BTW, easier with He3 than BF3, which I never got really happy with.
I have an old B10 tube I love - it's fairly numb but very reliable and with a big signal. With a fusor, once you get a little past go, it turns out
you don't need super sensitivity, even a low quantum efficiency will still count a copious neutron source. What you DO want is rejection of
all things that aren't neutrons - so you will only see a little background from cosmic rays, but nothing from the average hot test source like a thorium lamp mantle or a smoke detector.

My main "I really trust this" detector around here these days is a hornyak button (from Eljen) on a phototube, using this preamp. It's not very sensitive, but it does discriminate well, can't be flooded and stop counting if there are too many neutrons, is very stable over time, and is time-accurate, unlike nearly all other methods.
Anything that requires slowing down the neutrons to detect them introduces significant "time smear" as to when they came out vs when you got the pulse.
This is important to me as I'm doing non DC fusor drive and want to understand the dynamics better. For DC drive it probably doesn't matter much.

[Roberto Ferrari]

H Doug
Please, point me to your reports on non DC fusor experiments.
Roberto

[Doug Coulter]

I'm still in the middle of the first parameter sweep....so there's not much written yet - I try to wait till I'm fairly sure it's not BS, or a fluke, and I have something to say for sure. There's a little on you tube and a little on here under the nuclear area. Not much, as I don't know much yet.

I have successfully 'bunched" positive ions and accelerated them over some distance, measured some transit times. Over most of the geometry I can get them to act almost purely as positive ions rather than a "plasma". Near the focus, where density is obviously higher...not so much. Also as density gets higher, the charge of the ions tends to bring in the electrons, which accelerate quicker in a given field due to being lighter. The math that will describe this, will be, to say the least, rather daunting, as things are varying in both space and time in ways not covered in the ion trap math at all (it assumes so few particles that they don't affect one another via the charges). And oh, different vacuum levels. Far from focus I have molecular flow, long mean free path - near it, it's viscous flow and has "pressure" behavior on top of having Coulomb repulsion in varying degrees as the electrons flood in during part of the cycle...it's very not-trivial.

I've seen fusion neutrons with asymmetric "RF" down as low as 10kv peak...threshold with DC is around 18kv for the same production (and with DC it goes up REALLY fast with voltage after that). Making RF type stuff at tens of kv is difficult, and "interesting" to handle once you do - it's now easy for me to understand why cyclotrons had their step up RF tank circuit in a vacuum tank - shielding and arcing (or corona or RF in the shack, you name it - every neon bulb in the shop lights up).

I'm right now building a better RF source which will get me at the frequency(s) and volts I calculate will get me to ion-trap-like-containment, but with collisions at focus.
It's not done yet...remoting this kind of thing is also a lot tougher than with a DC supply, and at the production rates I get with DC - and expecting better this way, that's a must. It seems this will only scale down so far, and I'm fond of living.

[Roberto Ferrari]

OK.
No diagram of the setup?

[Doug Coulter]

See my youtube channel...I haven't changed what's inside the tank in a couple years (no need) and it's described extensively with pictures and commentary. And what I do on the outside is described both there and on the appropriate threads here, where it belongs.