Simulating tube amplifiers

Discussion in 'Amateur Radio Amplifiers' started by W8JI, Dec 29, 2011.

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  1. G0HZU

    G0HZU QRZ Member

    Tom, I think it would also help me understand if you defined what you think C1 consists of. Are you referring to the ballpark 5pF figure in the datasheet for the tube? eg 3-500Z.
  2. AF6LJ

    AF6LJ Ham Member QRZ Page

    This is going to get interesting.
    I'm looking forward to seeing your model as well as your way of looking at this.

    When I was in college in the seventies my logic systems instructor said that MOS devices would make all bipolor devices obsolete. Forty years later and you can still buy a 2N2222. :)
  3. G0HZU

    G0HZU QRZ Member

    Hi Sue
    I hope it does get interesting.

    I'm in an extremely weak position because I don't really know much about valve physics. My experience is all solid state so I expect to make a few blunders.

    But I don't understand Tom's anode/grid feedback analysis (because it breaks even my most basic model of the tube)
    So I may have to revisit the model but I need to understand why it is broken before I can fix it.
  4. AG6K

    AG6K Guest

     It already has.
    • Rich, ag6k

  5. AF6LJ

    AF6LJ Ham Member QRZ Page

    I'll do what others haven't done...
    Go find yourself a copy of one of the vacuum tube manuals from the sixties I would also add a copy of the RCA Transmitting tube manual.
    In both the RCA receiving tube and transmitting tube manuals there are really good sections on theory.
    The Receiving tube manual goes into this subject in detail.

    Don't forget to read up on Miller Effect in Vacuum tubes also.
  6. WA4OTD

    WA4OTD Ham Member QRZ Page

    There are several spice models of tubes on the internet which will likely prove good starting point. Then adding the parasitic elements is easier.

    I can appreciate a tube will be a difficult model but having used modelling tools all of my career I think this can be done.

  7. G0HZU

    G0HZU QRZ Member

    Hi Leroy,
    The way I see it I don't think I need a very advanced SPICE model to describe the instability mode adequately. I think I can do it with a basic linear analysis using a very basic maths model for the tube.

    But I will try and look for some advanced SPICE models on the web as they could prove very useful for more advanced modelling. Can you provide a link to the better ones? :)

    Do the RCA manuals cover instability analysis? I think this is the area that is giving me the most confusion.

    Tom appears to be the technical resource on here and I'd like to know why he thinks the feedback path is due to capacitance from anode to the grid structure. To me that seems to fly in the face of basic theory but I'm just a newbie.

    The rest of his analysis seems just as fundamentally flawed. I'm basing this on my schoolboy knowledge of how triodes work so I'm a little confused.

    Tom, can you PLEASE give me even a five line explanantion about how anode/grid capacitance is the feedback path?

    If it is the feedback path then my tube model is seriously BROKEN and I need to find a quiet corner to rethink my position on this :)
    Last edited: Jan 2, 2012
  8. AF6LJ

    AF6LJ Ham Member QRZ Page

    In my opinion;
    It's best to have a complete understanding of the theory before you dig into eccentricities of vacuum tube behavior.

    Tom and rightfully so is stating the complexities of tube modeling........
    With that said; I don't think anybody can walk into the middle of any given technology and expect to have a full understanding of what is going on with that technology.
    A little history doesn't hurt ether.
  9. G0HZU

    G0HZU QRZ Member

    Yes but anyone can draw a capacitor across A-G, call it C1 and say it is a feedback path. I just want to know WHY it is a feedback path.

    Can't somebody on here just tell me why this capacitance is the feedback path that causes instability? My model is BROKEN if this can happen. Somebody help with a 5 line answer please :)
  10. KL7AJ

    KL7AJ Ham Member QRZ Page

    Hey Sue.....a while back I ran across Miller's original treatise on the Miller effect. What amazing analytic thought...i don't think modern engineering schools are equipped to produce folks like that any more. I'll see if I can find the link....I've got the hard copy somewhere around here. I was just going to mention the Miller effect, but you beat me to it. :)
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