Re: HV curve tracer project - need info on vacuum tube X-ray shielding


Keith
 

Hi Ed,
Some types of HV TV rectifier tubes produced near the end of the tube era used to be wrapped in sheet lead to control the X-Ray emissions. I still run into them from time to time in tube lots on eBay.
FYI You can buy sheet lead online from roofing supply companies. It comes in 1 ft. wide sheets, and is typically about .080” thick. A couple of wraps of that around your tube capacitor thingy ought to do the trick.

Cheers!
Bryan
CoolBlueGlow

Sent from the planet Zarnok

On Nov 26, 2019, at 3:43 PM, Ed Breya via Groups.Io <edbreya=yahoo.com@groups.io> wrote:

I'm designing a low power 60 Hz HV curve tracer gizmo, with ranges up to around 5 kV RMS AC, or +/- about 7.5 kV peak in DC mode, up to a couple mA or so maximum short current. The voltage monitor is to be a 1000:1 attenuator around 80-100 megohms in, then either amplified locally, or fed to a scope input. I can set up a slightly different (feature) arrangement where the output can be doubled up to about 15 kV peak in DC mode. My initial plan was to use a 1X2 HV rectifier tube (just cold - no heating) (9-pin miniature with plate cap) as a stable, approx 1 pF vacuum capacitor across the big attenuator resistor, for frequency compensation. This would be no problem at 7.5 kV, but beyond 10 and up to the 15 kV range, it's getting into the soft to hard X-ray emission region. The tube isn't being used as a rectifier, and it's a high impedance environment, so the current is small (few uA peak), and shouldn't be able to produce much emission. But, to be on the safe side, I figured I could enclose that section in a thin steel box, if necessary.

I started searching online for shielding info for this soft/hard region around 15 keV, but so far only found stuff related to the common big equipment shielding like 40 keV and up, at high power. Does anyone know of any info pertaining to emission and shielding in this X-ray region, like HV tube shielding and such, from the old days of tube TV. Also, has anyone ever seen such a thing as info on unwanted X-ray emission patterns from HV tubes (not X-ray tubes, where that's what's wanted)?

I found a bit of info from Kilovac, saying that HV vacuum relays can have an X-ray problem above 15 kV, but no more detail than that.

I could also skip the tube and just make a HV gimmick capacitor from HV cable, but I'd prefer the stability of a nice little vacuum cap. Right now, there is no frequency compensation cap in place, but it works well enough to see what's going on, and actually makes for a pleasant display with low-pass filtering - all the crap present in an arc on the output comes out as nice steps on the scope, so you know a lot of HF info is missing. I need to also first check to make sure the 1X2 acts as a linear capacitor. I set aside a bunch of these years ago for the same kind of use in a 10 kV application, but wondered if the internal structure of the pointy heater and supports may prefer current flow in one direction, and still be kind of a rectifier even cold. I never did find any info on this, and haven't advanced this other project to the point of finding out. Now I'll finally see, and if it is a problem, then bye bye 1X2 (and possible X-ray issues), and hello gimmick cap.

Ed


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