There's a whole thread on these here:
viewtopic.php?f=11&t=430&hilit=snm (IIRC, we have a few threads on these guys)
Jon Howard and I worked on a couple while he was visiting here as well. It truly helps to have a source of neutrons you know has decent flux to test things with so you can tell the corona noise from the cosmics from neutrons, to the extent possible.
We had "ok" (not great) results with the fine wine preamp topology, which has about 60k ohm input impedance (which the tube kind of sees via the coupling cap, and that low is, well, kinda too low but it worked anyway).
We had issues with them going into relaxation oscillation, lousy quantum efficiency, and yes, noise, even when loaded with a 10m scope probe after AC coupling (which acts like reducing the 50 meg series R too, as far as the AC impedance goes). While I have a good number of these, none are currently in use here - almost anything else works better. The corona noise is normal - and that's what it mostly is, not the series feed R. And you have to get into the corona region (the thing is pretty sensitive to the voltage supply, unlike the claims) and in more or less in the right part of that. We used a variable DC regulator driving a CCFL with diodes and filters for our HV, it was pretty quiet and decently regulated.
They need this high impedance or there's too much energy available to go from corona -> arc. That wrecks the tubes at some point. This includes any energy stored in the coupling cap if the output side is tied to ground with low impedance.
The word is, you CAN make these work, but being both small and with very lousy quantum efficiency - it's not easy. It's truly a huge help to have a known-hot neutron generator for testing. They work well enough on a fusor, but are useless for anything with few neutrons present, as cosmics will then dominate the count rate - and you won't know where to set your threshold - cosmics are all over the place in pulse size, where most of the neutrons are "somewhere in the middle of that". The tiny size of most of these means even cosmic ray hits are rare enough that you might not catch them easily on the 'scope.
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.