Here's a fellow that has his act together. Great writing. Thanks for
sharing it with us.
73
Bob K4TAX
John T M Lyles wrote:
>
> We see Xrays emitted from tetrodes such as the 4CW250,000B at 28
> kVDC. They are very low energy and can be stopped by an aluminum
> chassis. But they are a problem for an open screened system, where
> people can stand there and watch the tube. They appear to emit around
> the plate to screen ceramic area, pointing slightly downwards from
> the center "plane" of the tube. (if you put a horizontal plane
> through the midpoint of the tube)
>
> In our RF tubes such as the 7835 triode, the voltage runs about 22 kV
> pulsed, with RF swing around that. There is definite yellowing of the
> alumina ceramic in two bands, where stronger Xrays are beaming out.
> They happen to be areas that are also in line with the filament
> electron emission, not shielded by the anode. Its most evident on
> white (Wesgo and Kyocera) ceramics. We have analyzed the yellowing
> using the technique of thermoluminescence and proved that it is from
> Xrays. It does not apparently weaken the seal, nor ruin the
> dielectric properties.
>
> When we have high potted tubes such as the 4CW250,000B, they usually
> produce stronger Xrays at about 50 kV and above. Older tubes
> sometimes seem worse. We can tell that they are emitting by watching
> the current on the power supply, which begins to indicate current
> from the field emission (electrons) inside the vacuum tube, even when
> cold. When we get a few mA or so at 55 kV, the power in the Xrays is
> quite dangerous. Our tester is now lead shielded for this. We have a
> written proceedure when high potting, which addresses this and other
> hazards such as high voltage.
>
> With planar triodes such as the CPI-Eimac Y820, we see a few mRem of
> Xray dose at 3 feet away, when holding off about 15 kV. This
> surprised me at first, when I was snooping around with a survey meter.
>
> So be aware, that Xrays can and will be produced by vacuum tubes,
> even when cold, if high voltages of over 10 - 15 kV are used. It
> depends entirely on the tube geometry and the voltage, so a
> generalized rule or chart would be difficult to produce. If you plan
> to build systems with high voltages such as this, and run them
> without metallic covers, you may be getting a 'nice' dose. Also if
> inadequte cutoff bias is used and a small leakage DC plate current is
> occuring, you may be generating Xrays. The e-beam is depositing
> energy somewhere within the tube....
>
> It is good that most amateur and broadcast radio amplifiers are not
> running at such high voltages. Technicians using klystrons and IOTs
> at higher voltages, as well as users of superpower tubes for radar or
> scientific applications are almost guaranteed to be getting exposed
> if they don't take precautions. A measurement of the radiological
> hazard is highly advisable in those applications with a calibrated
> Xray/gamma meter.
>
> John
> K5PRO
>
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