Jon Howard has been visiting here and we've been busy fusing and making and testing things this week. When we got those Russian B10 tubes we luckily also scooped up a couple of 3He tubes as well. Y'all keep eyes peeled -- we paid -- $20 for each. Not quite like the whining on fusor.net. Sadly, there were only two in that batch, and Jon's getting one.
Still working on the "ultimate preamp" design, with some success. This is a two transistor asymmetric risetime low quiescent current design, 700 uA on a 9v battery that makes 5v or so pulses without clipping, and has feedback for great linearity. I will probably be using this on phototubes as well, as it's really fast. In this design, both transistors are biased barely on, and the incoming pulse turns them on further. This gives very fast response, with a slower turn off time, perfect for a signal that does the same thing. The signal gain for AC is roughly 50, set by the ratio of the npn collector transistor to the emitter resistor on the pnp input stage. Input transistor is a 2n5087, npn is a 2n3904. I took some care in design and testing this to have it work decently over the whole voltage range of an alkaline 9v battery -- from about 9.5v down to about 5.8v or so.
Here's a sketch of the preamp design. It's neater than I usually post, because I didn't draw it, Jon did.
Here's what the assembly looks like. We tested in the same moderator as used for the CHM-14, which is a little short for this, but it works fine anyway.
And here is what the top looks like with the coffee can shield removed. We'll punch some holes in the can for the connectors for HV and signal, and the power switch of course (and paint the thing).
In a test on the main fusor here, this tracked our 22" 3He tube quite well -- at about 90% of the same count rate, when positioning the new one where we usually put the neutron oven. That's only a little bit closer to the action than the big guy, so sensitivity is more or less equivalent. Here's a scope shot of all three of our detectors going at once.
The new preamp in combination with this tube gives much shorter pulses. We took this shot without the can shield, because the radar wasn't hammering us so badly, but did pick up a bit of noise on the baseline due to that -- and this scope sees that due to sampling at 2.5 ghz in peak detect mode.