[Moon-net] ALL: Burn in a filament

[Kermit Carlson]

Hello Kim;
   
        Yes, the initial condition of power tubes is a essential,
   especially for those types of construction that use
  indirectly heated oxide-coated heaters. One of the details
  is that the oxide coating tends to gett the gas from the
  tube and heating it even into the black-heat temperatures
  (where the filament is hot but below the level needed to
  give rise to emission) will cause the release of gasses
   into the tube. Also, you need to heat the various elements
  slowly since they will outgass at differing temps.
   
  THe manner I reccomend for the conditioning of 
  tubes that have not been operated recently or the history of
  which is unknwon is to use a series of methodical steps 
  for  conditioning;
     First raise the filament current slowly over several hours to a day,
     Then to let it sit for a 24 hours or more without other voltages
     Then slowly raise operating voltages - a low plate voltage with sufficient
         bias with low plate current to start.
     Then a period of low drive power to the grid at a minimal level 
         to heat the grid(s)
     And only then slowly increase the voltages and currents
        up towards the limits being VERY conservative with the
        tube for the first 10-20 hours of operation.
   
   To check the presence of a rough
  vacuum before the application of plate and grid
  voltages one can (with the tube disconnected 
  from everyting else)  look for the presence of a small
  voltage on the grid from the field emmission with a 
  high impedance voltmeter - this will indicate that
  there is sufficient vacuum to establish emission.
   
  THen, if one is interested; the gas current can be
  measured. For triodes; forward biasing an electron flow to
  the grid (small current limited positive grid voltage
  with respect to the cathode) and a look for 
  microamps of flow to the plate with a higher
  voltage which is applied negative with respect to 
  the cathode. 100 volts should be more than
  sufficient. For a large triode such as the 
  RCA/Burle 7835 it takes about 45 volts of grid,
  a 2c39 will be as low as a few volts)
   
      Always be careful with your amplifier with respect
  to electrical safety and be vary careful with total
  power of that grid  current during testing for gas;
  my advice is not to exceed more than half the grid
   power rating or half the amount of average grid
  power encountered during anticipated operation.
  the gas measurement is complete hyperbole for most
  applications, but I have constructed a test stand in the
  shack for this purpose (especially for 2c39 new-old
  stock). At work,  all large tubes are conditioned 
  similarly. If you push a tube too soon it can be 
  counter productive. Other than the interest in
  measuring gas-load, it has helped me find REAL
  gassy tubes before I bother to try to use them....
   
    When I measure a tube carefully I can
  see when you have lost ground when pushing it. Since
  the commercial flow of high-power tubes is lower than
  twenty years ago, there is a much higher probabilty
  that, as hams, we will encounter more tubes with date-codes
  that are decades old. As a matter of fact, I do not now
  know of a single source for timed-pulls for the
  8877, TH328 or the 4CX1500...
   
       I have always tested large tubes before use because
  of the high operating voltages and rebuilding/replace costs
   involved.  If you measure your tube as you follow a 
  slow conditioning you can see an improvement as you 
  condition the tube, with the evidence I have seen I 
  strongly reccomend the SLOW approach.   
  After 30 years with high power tubes  I would 
  reccomend that conditioning not be rushed -
   
       I have built two amplifiers that used pulled 4cx1500's;
  both tubes used exhibited signs of higher gas load than I would
  prefer,  a lot more than one would see from new tubes
  but they do work. 
   
     Standard cautions apply - eg your milage may vary
  and please use care with high voltage. 
   
                                              73 Kermit W9XA
   
   
Kim Liljekrans <oz5iq at privat.dk> wrote:
  HI gang

I need some experience :

have set up a 4CX1000 with only filament ON, and have slowly raised ( 
over a few hours) the votage up to
the normally used ie. 5.8 V measured at the socket.

How long time is nessary for achieving the " normal max vacuum" again ??


I think this has is of general interest , so PSE answer via the net (direct 
is OK as well TOO.

VY 73 de OZ5IQ, Kim ( OZ1EME as well)






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