From: ABSOLUTELY_NO@SPAM.ORG   
      
   Filament inrush current _is_ a big deal with high power transmitting   
   tubes.  I have known of cases where the inrush current was so high that   
   the intense magnetic field created by the inrush current shook the   
   filament so much that it was broken.  This was with a tube whose filament   
   requirements were 7.5 volts at 104 amps (a ruggedized version of the   
   3CX10000A3).  The tube was used in industrial dielectric heating.   
      
   Burle Industries, the successor to RCA's transmitting tube business   
   published a number of guidlines and application notes to maximize the   
   life of their tubes.  The following are direct quotes from these   
   publications.  These notes are rather long and not directly applicable to   
   the _miniscule_ tubes used by the readers of this newsgroup, but I   
   thought they might be interesting.   
      
   If anyone here wants to build audio gear using the tubes I mentioned   
   earlier, I have 5 NOS ones with a matching filament transformer and   
   sockets!  A pair of these will loaf at 40 kilowatts output.  :-)   
      
           73, Dr. Barry L. Ornitz  WA4VZQ        {my ham call} at   
   Hotmail.com   
      
   From the "Application Guide for BURLE Power tubes."   
      
   A filament starter should be used to raise the filament voltage   
   and current gradually in order to limit the high initial surge of   
   current through the filament when the circuit is first closed. The   
   starter may be a system of time-delay relays which cut resistance   
   or reactance out of the circuit, a high reactance filament trans-   
   former, or an adjustable auto-transformer. Regardless of the   
   method of control, it is important that the filament current never   
   exceed the value specified in the published data. In equipment   
   which utilizes automatic "run-up" of filament voltage with volt-   
   age regulators, provision should be made for a limit-switch cut-   
   off of the filament-voltage supply in the event of malfunction or   
   "runaway" of the regulator to prevent damage to the filament   
   due to overvoltage. The limit switches should be set at or below   
   the maximum permissible filament voltage for the tube involved   
   and should be checked periodically to assure proper operation.   
      
   From "Techniques to Extend the Service Life of High Power   
   Vacuum Tubes."   
      
       On/Off Filament Cycling   
      
   When a cold filament is turned on, damage is   
   caused by two effects: (1) The current inrush into a   
   cold filament can be up to 10 times the operating   
   current if the filament supply is of very low impedance;   
   (2) Grain reorientation occurs at about 600 to 700   
   degrees centigrade; this is called the Miller-Larson   
   Effect. The grain reorientation will result in a   
   momentary plastic state that can cause the wire to   
   grow in length and therefore become thinner.   
   To illustrate the first effect, one can use an   
   ordinary 60 watt light bulb. The light bulb should   
   operate at 0.5 amps at 120 volts. This translates to a   
   hot resistance of 240 Ohms. An Ohmmeter reading of   
   a room temperature 60 watt light bulb will be less than   
   20 Ohms or, a hot to cold ratio of greater than 12.   
   Power tube filaments will have about a ten to one hot   
   to cold ratio because they operate at a lower   
   temperature than light bulbs.   
      
   The second effect is aggravated by variations   
   in the cross sectional area of the filament wires along   
   their length. This will cause “hot spots” in the thinner   
   cross sections due to the greater current densities.   
   The higher temperatures at the hot spots cause   
   increased growth during the warm-up through the 600   
   degree Centigrade temperature when they have a   
   higher current density than the rest of the wire, and a   
   much higher power dissipation per unit length due to   
   also having a higher resistance. Each time the   
   filament is turned on, the wire becomes thinner until   
   the hot spot temperature gets into a runaway   
   condition. During the runaway, the filament   
   temperature reaches the melting point during the turn   
   on surge, and the light bulb (or filament) produces the   
   familiar flash bulb effect that signals its demise.   
      
   Light bulbs nearly always burn out during the   
   turn on inrush surge. Each Off/On cycle removes   
   several hours of life from the light bulb (or filament)   
   and this trauma is approximately proportional to the   
   cube of the inrush current. By limiting this inrush   
   current or, by preferably eliminating it, the life of the   
   power tube filament can be lengthened several times.   
      
   The Egyptian Ministry of Information   
   operates a two megawatt transmitter in the medium   
   wave band. This Doherty transmitter uses four tubes,   
   two carrier tubes, and two peak tubes working through   
   a combiner. Even though the transmitter was   
   operating more than 22 hours per day, the filaments   
   were turned off and back on every night. The very   
   expensive tubes (about $100,000 each) were lasting   
   only 5,500 hours, and a stock of 10 spares was   
   expected to last 18 months for the four sockets. After   
   leaving the filaments on continuously and rotating the   
   spare tubes annually, the station has yet to have a   
   tube failure after seven years.   
      
   In Greenville, North Carolina, VOA has four   
   500 kW transmitters with vapor cooled tubes. The   
   transmitters are used for 12 to 15 hours per day.   
   Initially, the filaments were turned off and on two or   
   three times per day during gaps in the schedules.   
   Three years ago, VOA began leaving the filaments on   
   at all times except for major maintenance. As a result,   
   the expenditures for tubes for these transmitters has   
   dropped from about $420,000 per year to under   
   $100,000. This was done at a cost of $15,000 per   
   year in added electrical power cost. For every extra   
   dollar spent on added filament power, we had a return   
   of more than 20 dollars in reduced tube costs — eat your   
   heart out Wall Street!   
      
   Calculations for both of the above situations   
   indicate that each Off/On cycle of the filament was   
   reducing the life expectancies by over 75 hours. An   
   unexpected result was that it appeared to be   
   independent of the filament construction - straight   
   wire, hairpin, and mesh filaments all benefited nearly   
   equally by leaving the filaments on continuously. The   
   mesh filament was expected to suffer more from   
   Off/On cycling because of flexing of the many welds.   
   Also, the mesh filament structure will grow in overall   
   diameter as the wires lengthen. It is constructed in a   
   manner similar to a Chinese finger trap which   
   changes overall diameter as the axial length is   
   changed. This mesh filament construction causes an   
   additional failure mode because the overall mesh   
   diameter can grow enough to cause a grid-to-filament   
      
   [continued in next message]   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)