[AMPS] Xray levels

[John T. M. Lyles]

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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