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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