Yet more on tube modeling

Discussion in 'Radio Circuits, Repair & Performance' started by KL7AJ, Jan 2, 2012.

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  1. KL7AJ

    KL7AJ Ham Member QRZ Page

    One of the difficulties in modeling vacuum tubes, that really puts them in an entirely different class than other components is the VAST difference in electron velocity with respect to time AND location. SPICE is not very well equipped to model transit times, or even finite current velocities. The best modeling tools for this come out of the plasma physics community, not the electrical engineering community. There are some rather esoteric physical system modeling tools that take these things into account quite nicely, but they aren't readily available to hams

    In a vacuum tube, electron velocities can range from just a few millimeters per second to about 2/3 the speed of light. (The free electron laser is one odd variation of a vacuum tube where electrons actually travel at the speed of light). For MOST of what we experience in the ham shack, what we need to know about electron energies is pretty well described by Newtonian mechanics, however. This is in considerable contrast to normal "closed circuit analysis, which assumes instantaneous transport.

    Eric
     
  2. AF6LJ

    AF6LJ Ham Member QRZ Page

    Those who were brought up on solid state engineering have trouble comprehending what goes on inside a vacuum tube.
    Imagine trying to drive a spice model for a BWO or a a Klystron.
     
  3. AA5CT

    AA5CT Ham Member QRZ Page

    Yeah, you're probably right; doesn't contrast at all with electron 'drift velocities' as experienced in BJT's (Bipolar Junction Transistors et al) either ...

    Modeling comes in two types; 1) Model the physical process that takes place inside a device, considering the underlying 'processes' that make a thing work, as in a tube regarding cathode emissivity/with temperature, space charge, grid spacings, transit velocities etc or 2) 'model' with equations that mimic the behavior of the physical device under specific given conditions e.g. knowing the tube gm at DC/low frequencies, or the Vp/Ip characteristic curves for a set of fixed grid voltages ...

    Jim de WB5WPA
     
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