>
>In 1994, working with a tube rebuilder, Econco, we autopsied several
>4CW250,000B tetrodes, all Eimac builds from the 1980s. These tubes
>were used in our plate modulators as the series tube, a parallel pair
>of them used, with 30 KV DC on the plate, and about 22 kV pulsed at
>the cathode used for the output to the plate of the RF final PA tube.
>This floating deck hookup is sometimes called totem pole
>configuration.
>
>Anyway, the 250K's had started to breakdown frequently, with internal
>arcs, similar to gassy tubes. The screen to cathode spark gap was
>breaking down often, indicating that the screen was rising to high
>voltage. Also, the grid bias power supply would get killed sometime.
>Upon examining the internals, (i have the photos somewhere - it was
>before digital cameras were available) we found that the grid cage
>was warped in the worst tubes. More closely, we found that the inside
>of the plate (anode) had much cutting from electron beam. It was cut
>with tiny slots, that mirrored the beam pattern through the slots in
>the screen and control grid cages. Since then I have found that this
>is fairly typical in high power tubes, although it is worse in some
>conditions of bias and classes of operation.
>
>We surmised that:
>1) the grid warpage was not a manufacturing defect, but a result of a failure
>
>2) E beam cutting was the cause, as it was liberating a lot of copper
>material into the vacuum, and arcs were bombarding the heater and
>grids.
>
>3) The beam cutting was causing hot spots, the grid secondaries would
>make things worse, eventually it was a runaway until the grid warped,
>which REALLY caused a problem - with current hogging at that point.
>
>4) the tube is not perfectly symmetric, radially speaking. It only
>takes a tiny misalignment for the focused beam to be nonuniform
>around the tube. It can be seen in the cutting pattern of the copper
>plate.
>
>5) inadequate cutoff bias was the reason, about -350 Volts DC from
>the original power supples. Focusing of the beam during 'cutoff' is
>possible, and can also generate substantial Xrays.
>
>Looking at the tube datasheet for 800 volts on the screen (there is
>no zero screen voltage curve, although that is the condition that we
>have with cutoff), with -350 VDC on the grid, about 1 mA will flow
>with 10 kV across the tube. With 30 kV in our case, the current is
>even more. Taking that minute current x the plate voltage, one can
>understand that there is significant power being generated in that
>beam to do some damage.
>
>Redesigning the grid power supply to operate at -600 VDC, and buying
>new tubes, seems to have fixed this mysterious failure problem.
>Haven't lost a tube that way in the past 6 years - knock on wood.
>
an interesting account, John. thanks
- Rich..., 805.386.3734, www.vcnet.com/measures.
end
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