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Subject: [AMPS] more on that topic
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Date: Thu, 13 Sep 2001 12:42:32 -0700
>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 

//   Where is the getter in a 3-500Z?
Where is the getter in an 8877?
Can gettering take place while a tube is cold?
> ...
>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.
>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 
>>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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