chrismb wrote:Fair enough, I thought that might be the case.
Nonetheless, is there any harm in trying it, and/or what should I do to prevent any harms either way to this from any tube I might put on it? Is the HV/detector output short-circuit protected, or if not then what is the internal resistance likely to be that makes this thing 'tick'.
You could probably put any tube it was designed for on it. You might need additional series resistance for some types (geigers and others) that need a current limit and make big signals anyway.
I have no way of knowing what the internal series R is (heck, the connector for the input isn't even in the picture?) That's what you ask the guy you bought it from about, or google for the manual and schematics. The ones I've looked at use 10k or 100k. That will instantly fry most corona or geiger tubes on the first pulse. The corona tubes need a huge current limit series impedance, they reccomend 40 megs or so, way way more than is going to be in that box, likely.
chrismb wrote:I was imagining that if you run the output via a resistor with a small cap across it, then you limit the current by that resistor yet if the tube 'fires' and you get a potential drop, the cap will pull the detector line down, giving you a count. That way, you can adjust that resistor until you get the signal you want (that is, on any unknown tube that you are probing), thereby avoiding excess load on the HV drive.
If you put a cap across a series resistor, the impedance becomes a short circuit at HF (like a tube pulse) and makes the resistor no longer a current limiter for fast signals, like all tubes produce.
(the pulse is the equivalent of a few mhz during the risetime)
So you don't limit tube peak current -- it can still draw plenty while charging the cap -- and get no benefit from the resistor except at low frequencies, which don't exist in this situation. There isn't that kind of "free" in electronics. The corona tubes like to see about a 40 megohm impedance, though with a small enough cap, you can use 40 megs in the DC supply and about 100k preamp input impedance. Say 47 or 100pf in that case, they can take the extra current it takes to "charge" one that small. But you still have a tiny signal, so you still need a preamp unless you can set that thing for millivolt pulses and I didn't see any knobs for that on the front. Looks like it's looking for volt-level stuff like a scintillator would make into 10k or so. However, with less effort than typing the question, you could hook it to a signal generator through a small capacitor and see what it takes to count it.
Teach a man to fish....
chrismb wrote:Is it not safer to do something like this with an unknown tube and unknown supply?
You seem to not understand some very basic electronics, as your question implies that you think a cap will pass the signal (so it doesn't attenuate like a resistor alone would), without passing the current it takes to make the signal! You can have one, or the other. Not both.
Reactance (works like resistance for computing currents and such) is X = 1/2*pi* F * C (c in farads). A swift risetime is a high frequency component, in this case in the mhz range most likely. Just like with a current limiting resistor -- you need to allow some drop to protect the tube, which kind of negates the signal pass through function of the cap. You just have to get the gain some other way (and in any case, these only put out a few mv and even if you got it all into your box, its unlikely to be enough to make it count.) As mentioned above, these are noisy tubes, so you also need some sort of settable sharp threshold to separate the baseline noise from the real signal pulses. I see no knobs to set up that kind of stuff here.
We've documented a dirt cheap working circuit for these here already...you can mod this to look like that, or build what we know works already.
Posting as just me, not as the forum owner. Everything I say is "in my opinion" and YMMV -- which should go for everyone without saying.