>
>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.
// Where is the getter in a 3-500Z?
Where is the getter in an 8877?
Can gettering take place while a tube is cold?
tnx
> ...
>
>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.
>
// True, Mr. Mr. Rauch. Big-bangs undoubtedly take plsce in the
atmosphere.
>
>>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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>
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>
>
- R. L. Measures, 805.386.3734, AG6K, www.vcnet.com/measures.
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