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Re: tube coils

>Message-ID: <199609072225.QAA05011-at-poodle.pupman-dot-com>
>Date: Sat, 7 Sep 1996 16:25:05 -0600
>From: Tesla List <tesla-at-poodle.pupman-dot-com>
>To: Tesla-list-subscribers-at-poodle.pupman-dot-com
>Subject: Re: tube coils


  [ snip ]

>
>Actually, it's the high voltage on the plate which *attracts* the
>electrons from the filament.

   True.

>Current then flows if the grid has a
>sufficiently positive charge on it.


   Not exactly.  Most tubes run with the control grid NEGATIVE to
   the cathode.  This is so that the grid will not draw current 
   itself.


>With a positive grid, the electrons
>are 'helped' via attraction to the grid charge, but are pulled all the
>way to the plate

 
   Basic idea is correct.  As you said, the control grid does not make
   the electrons go to the plate; rather it is the high positive
   voltage of the plate (and the screen grid, if your tube has one). 
   The control grid, on the other hand, is basically used to *reduce*
   the flow of electrons from the cathode to the plate.  Some power
   tubes and tubes in "grid-leak" bias circuits actually do allow the
   grid to go slightly positive to the cathode, but only on the peaks
   of the  input waveform on the control grid.  The operating range of
   most control-grid -to-cathode voltages is from cutoff (large
   negative) to zero volts.   Again, this is to keep the control grid
   from drawing a lot of current.  The moment the control grid becomes
   positive with  respect to the cathode, the grid circuit draws
   considerable current.

   Some huge power tubes that are designed to operate with
   the control grid going positive.


>When the grid is negative (cutoff), the cathode
>(filament) electrons are repulsed, and hang around the cathode in a
>small cloud...
>
>Remember - electron movement is from negative to positive, ergo, current
>flows OUT of the plate, not the other way around.
>
>- Brent

 Fred W. Bach ,    Operations Group        | Internet: music-at-triumf.ca
 TRIUMF (TRI-University Meson Facility)    | Voice:  604-222-1047 loc 6327/7333
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