Weekend puzzle

Discussion in 'General Electronics Chat' started by studiot, Jun 20, 2015.

  1. studiot

    Thread Starter AAC Fanatic!

    Nov 9, 2007
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    Is there anyone left here?

    If so would they like to say when and under what circumstances one might make the plate in a (special) vacuum tube negative with respect to the cathode (emitter).
     
  2. WBahn

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    Mar 31, 2012
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    Seems like a pretty wide open and ambiguous question that is calling on people to read your mind.

    If I suggest that this was a pretty normal occurrence in a vacuum tube rectifier, is that going to be wrong because those weren't "special" vacuum tubes?
     
  3. studiot

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    I was thinking of another circumstance, but you would be correct a rectifier is cut off or blocking by making the plate negative.
     
  4. #12

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    This old guy says you can make the plate negative all you want, and electrons will not run backwards through most vacuum tubes. Now, there are cold cathode tubes, I think neon bulbs, fluorescent light tubes, and a couple of voltage regulator tubes. That might be a case of reversible flow.
     
  5. BR-549

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    Sep 22, 2013
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    I'm not sure, it's been awhile. I don't recall the plate being negative with respect to cathode.

    But I think I remember of the plate being negative with respect to the suppressor grid.

    If I remember right, it biases the tube to the negative resistance region.


    ...they used to make a diode inside with a triode, but I wouldn't say it was special.
     
  6. ian field

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    There is a specialised type of oscillator that has negative plate - but I don't recall its name.

    Apparently there is also an electrometer technique that does too.
     
  7. studiot

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    Keep them coming, I'm definitely interested if anyone has any real details of these, or are they just myths from the past?
     
  8. ian field

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    The specialised oscillator was mentioned in a very old issue of Wireless World.

    The electrometer technique was mentioned in Practical Wireless in the 00 decade - cant remember which year, let alone month.
     
  9. studiot

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    Hello Ian, did you perhaps mean a reflex klystron oscillator?

    In an ordinary klystron the collector plate is nearly at the emitter potential, but still slightly positive with respect to it, but the reflector in a reflex klystron needs to be at a suitable potential to repel the travelling electrons, and could be negative.

    That is actually quite close to the device which originated this question.
     
  10. ian field

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    No - it was originally noticed as a nuisance phenomenon around the filament in lamps running on DC. They could cause interference to radio reception. In the early days it was the only way to get up to UHF, but power output was very small.

    It may well have been at least part of the inspiration for klystrons and the like, but it wasn't one itself. Apparently you can do it with a regular valve and a couple of Lecher lines.

    If I could remember the name of it I'd tell you - I had a quick poke round in Google but came up empty.
     
  11. ian field

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    Had another crack at Google and found it:

    Barkhausen oscillator in 1933
     
    #12 likes this.
  12. studiot

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  13. ian field

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    The article doesn't actually say its a klystron of any sort, but does support my idea that it inspired further research that led to transit time devices such as the klystron.

    Its probably just semantics though - most people don't remember that the magnetron existed before the British cavity magnetron, it was an amazing new development that could produce centimetric wavelength for high resolution radar - it helped win the war, and that's what people remember.
     
  14. studiot

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    If you look at the how it works explanation you will see that it is a primitive klystron.
    The plate is biased to set the electron stream oscillating to and fro, not exiting via the plate.
     
  15. ian field

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    I didn't make any assumptions from what I read in that text. You can decide that an early experiment with the transit time effect was a primitive klystron - but some experimenters have duplicated the results with regular valves and a couple of Lecher lines.
     
  16. studiot

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    and how did they bias the plate?

    The negative bias reflects the electrons, causing the standing wave system in the electron beam.
    That is the essence of the klystron, and Barkhausen's experiment.

    I am still grateful for those pointing me to things I did not know and where to delve into the past.

    This thread has been enlightening because it shows many more negatively biased plate applications that I thought of at the outset.
     
  17. ian field

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    Just guessing - but I assume they'd bias the plate the same way as in any Barkhausen oscillator.
     
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