HV CONNECTORS:
Thanks for reminding us about Alden Co for connectors. I have not seen
their literature in years, but somewhere remember seeing them. Sorry for
the high end recommendations yesterday, I realize that we are hams. I had
just spent a week searching for a 15 KV connector that can handle 40 Amps
pulsed,
and settled on a custom unit from Caton that is built in a Burndy-style MS
circular connector. It's the Caton series 16, and we're probably giving
them several hundred dollars each for these. But they have to be as close
as 100% reliable for 10 years, at 7000 feet above sea level. And they would
get unplugged and plugged about once a year, for maintenance.
The problem with 10-15 KV on connectors is that now you have to start
paying attention to gradients and the field enhancement at points where the
dielectric insulation, air and the conductor come together. If the
dielectric insulation of a cable is, say 1/4 inch thick, and the conductor
is 16 guage equivalent, and the dielectric constant of the insulation is
3.5, then for a given 10 KV across this sandwich, there is a gradient
(electric field in V/inch) of 10KV/4 or 40 KV/inch, if it was all
homogenous insulation. If there is an air void, such as a tiny space in the
ground (shield) of the outer conductor of the cable or the connectors, then
the field in this air space is enhanced by the 3.5 dielectric. In the void
the field is over 120KV/inch, even with only 10 KV applied to the system.
This is the triple point effect, not to be confused with ROCKY POINT!
For air at 7000 feet and 40 deg C. (in aircooled equipment) the breakdown
gradient is 75 KV/inch (sea level) x 0.75, or about 57 KV/inch. By
exceeding this in the voids, there will be partial discharge during
operation. In a matter of hours, days or weeks, the insulation will be
degraded by the presence of Ozone, and the polymeric insulation will
finally erode and arc through. This is very common in cables and connector
designs that haven't considered these shortcomings. We encounter this and
design around it constantly. Especially in 100 KV connectors for Klystron
amplifiers. The cable companies actually use a semiconducting layer in the
center conductor (if it is stranded wire) and around the shield, to make it
appear as a solid conductor without voids. Also, the connector companies
(like Caton, Alden, Reynolds) have to pot their connector insulation, often
using vacuum to ensure that all air bubbles are gone. The old shell and
Teflon insert such as an HN do not have adequate margins for 10 KV
operation, at our altitude.
Probably the plexiglass sheet and hole with grommet, and wire coming
through with soldered ends are the most reliable for hams. The banana pins
would be OK too, except would have to be on an insulated sheet. But again,
the safety aspects of it falling loose should be considered. If you are
truly going to put 10 KV through it, think about the gradients around where
the hot conductor and the grounds are close.
ROCKY POINT:
G3RZP is correct that there is definitely a process for high power tubes in
which we slowly increase the DC, while watching the current flicker. You
don't need to worry about cutoff if you do this with the filament turned
off. As you increase the hi-pot, arcs will occur within the tube. If enough
energy is dumped into the arcs, it will clean off the sharp points in the
tube (even in vacuum) and raise the threshold of Rocky Point problem. This
is called debarnicle-ing by some manufacturers. I am talking about
thoriated tungsten tubes here, don't know if it is acceptable for oxide
cathodes. Debarnicaling in an amplifier is not advisable unless the proper
energy limiting resistance is used between the power supply capacitance and
the tube. The folks at CPI/Eimac will test big tubes in a special shielded
device at higher than normal operating voltage, and record the cold
emission as well. This is related to the internal fields around sharp
points, the quality of the vacuum, the cleanliness of the ceramics, and the
production of X rays.
TEST FOR PARASITICS:
If the tube is arcing in an amplifier even during cutoff (no idleing plate
current), then one suspects gas, sharp points, etc. If the arcs occur as
the bias is adjusted towards turn on, then keep an eye open for the
parasites. There is a way I used for broadcast cavities, in which you put
the tube into conduction, in the rf circuit. This means a low value of bias
or maybe zero on a tetrode, with screen voltage normal. Then only pulse the
plate and screen contactors on and off, even just pressing the button for a
short time. The shock of the stepped voltages can make the amplifier ring
in it's VHF or UHF circuit resonance. If you're quick, you can do it
without damage. If not, sorry about that....
John
K5PRO
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