Thanks for also bringing up the whiskers inside vacuum tubes. This is
common, and the debarnacling process is
part of standard processing of new tubes with handles, per
instructions from CPI/Eimac, and Econco. Other companies also include
it as part of conditioning or seasoning a new tube. Out of the
carton, they are referred to "green" tubes until they are
conditioned. Kind of like drying out lumber I suppose - hi hi. Eimac
is referring to large tubes (i.e., multi kW to megawatt sized) but
there are no fundamental physics which remove these limitations from
glass or hand held vacuum tubes, except to say that they have less
frequent breakdowns due to the lower voltages used, and the small
internal volume. Gettering is built into all modern tubes to my
knowledge.
A five page Eimac Engineering Newsletter "Conditioning of Large Power
Tubes" explains this. I scanned only two of the pages, and the
summary paragraphs to post here. Missing are the safety instructions
and the power supply sources which were listed by Eimac back in 1973.
CONDITIONING OF LARGE POWER TUBES
GENERAL
Large power tubes are subjected to very rigorous processing during
exhaust pumping at the time of manufacture. Active elements are
processed at temperatures several hundred degrees C higher than that
expected in actual use. This is done to drive off surface and
sub-surface gas from the metals to minimize possibility of these
gasses being released during service life of the power tube.
Free gas molecules will always be present to some degree in a fully
processed tube. There are two obvious reasons why this gas, in excess
quantity, can interfere with proper service from the power tube. Gas,
particularly, oxygen 'containing compounds, may combine with cathode
material chemically to either permanently or temporarily destroy the
electron emission capability. Free gas molecules when struck by
electrons moving from cathode to anode may be ionized by having one
or more electrons knocked from its system. If enough such ions plus
the freed electrons are present, a conduction path is provided which
is not subject to control by the grid. This can result in runaway
arcing which may involve all elements. Current may be limited only by
source voltage and impedance, since space charge to some degree is
neutralized by the presence of both electrons and positive ions.
Electrons from other sources than the heated cathode provide low
current paths between elements when the voltage gradient is high
enough at the negative element for pure field emission. Voltage
gradient at the negative element is determined by applied voltage
between elements, spacing between elements, and surface contour of
the negative, or cathode, element. High voltage gradient can exist in
front of a point on the negative element, or in front of a particle
adhereing to the negative element, or conceivably in front of a clump
of gas molecules on the surface of the negative element. Field
emission occurs readily from a cold surface if the conditions above
provide the voltage gradient.
Ionization of free gas may occur from bombardment by field emitted
electrons. Arcing is likely to occur as a result of field emission in
operating equipment because plate voltage is at a maximum during that
part of the signal cycle when ordinary plate current from the
filament is shut off by the control grid. For this reason an
important part of tube processing when the tube is made is high
voltage conditioning to remove sharp points or small particles from
tube elements. This part of the tube processing may, and sometimes
should be, repeated in the field after shipment or storage, if the
tube is intended for use at plate voltage above 10 kV.
High voltage conditioning is sometimes called spot knocking, or
debarnacling. The process consists of applying successively higher
voltage between tube elements, permitting the tube to spark
internally at each voltage level until stable, (no sparking), then
raising to the next higher level until the tube is stable at a
voltage approximately 15% higher than the peak signal voltage it will
see in service.
The equipment for tube conditioning is simple but specialized. It may
provide DC, or AC voltage, or both. Current required is small.
Voltage should be continuously variable from practically 0 volts, to
the highest value required for proper conditioning of the tubes of
interest. Energy per spark is controlled by the internal resistance
of the supply, plus any external series resistor used. In DC
conditioning it is valuable to have a DC milliammeter to measure the
level of field emission prior to sparking, or simply to determine if
the field emission is within the specified range for the tube being
tested. Also in DC conditioning a capacitor may be used across the
tube under test to closely control the energy released for each spark.
????
During HV processing, particularly between grids and between grid and
filament, some of the redistributed gas molecules may be deposited on
the cold filament causing a temporary loss of emission. If this is
observed, the tube should be operated for an hour or so with normal
filament power to drive off the volatile material. The normal
emission of electrons from the filament will be re-established by
this procedure.
Use of the high voltage conditioning technique in the field will
often save valuable time when installing new tubes, or when placing
spare tubes in service. Some transmitters will provide the
conditioning but at the expense of kick-outs, or at the expense of
bringing a new tube up to full plate voltage slowly, while the
conditioning process is going on.
W8JI said:
>Actually, you can go further than that. Arcs inside tubes are almost
>always NOT caused by parasitics.
>What typically does cause a "glitch" is outgassing in the tube as
>elements heat, seal leakage allowing air in, or metallic "whiskers"
>or debris inside the tube. In most cases, the arc itself will remove
>the problem. If it is a slight outgassing, energy in the arc will break
>down the gas and getter the tube. If it is a metallic whisker or
>debris in the tube, the arc will normally vaporize the stray material.
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