what is all this ball ache about Plate resistors? How do tubes even work?

Discussion in 'The Workbench' started by john hammond, Aug 20, 2019.

  1. john hammond

    john hammond Well-Known Member

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    Ive discovered a really good video, it's authored by a teacher so it's no surprise that it's so good.

    He teaches us in a very teacherly hypnotic, re-assuring voice that plate load resistors have two very important and different functions.

    1) the first I already knew about, which is to simply step down the voltage off the b+ so that the plates of the tubes can work at a voltage level they're happy with.

    2) the second, ( i knew there was a second function but all i knew was it had something to do with some sort of blocking function.)
    The gentleman explains with very easy to understand diagrams how the plate load resistor stops the ( now increased ) AC signal current from simply joining the b+ voltage back on the rail.
    This sounds like a pile of horseshit from me just reading it here, but watch the video and it's as clear as day.

    As a bonus, he carefully explains with excellent diagrams how tubes ( and amps) work.

    For those who get real bored and just want to see the money shot

    14:33 on the video explains this second AC 'blocking',( moreso, Hindering) function
    10:20 on the video , he explains how tubes work in the first place with excellent diagrams.

    the first function of a plate load resistor is explained from the beginning of the video.

     
    Last edited: Aug 20, 2019
  2. mickeydg5

    mickeydg5 Well-Known Member VIP Member

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    Tubes are just something you pass things through unless used for structural support.
     
  3. john hammond

    john hammond Well-Known Member

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    forgive me mickeydog5
    " thermionic valves"
     
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  4. mickeydg5

    mickeydg5 Well-Known Member VIP Member

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    Oh, ooh.
    Ok then. Carry on. :)
     
  5. mickeydg5

    mickeydg5 Well-Known Member VIP Member

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    Well yes, a plate resistor is part of the load resistance. You got to have something from letting all your signal get zapped to ground through the power supply filter capacitors.
     
    Last edited: Aug 21, 2019
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  6. john hammond

    john hammond Well-Known Member

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    yeah it seems this resistance thing is used a lot, without resistance the electricity cant hang around long for it to ' do' anything...light a bulb ( an old fashioned tungsten filament bulb)...turn a rotor in an electric motor etc.
    if there was no resistance in between the plate wires on the primary of an ( class ab) output transformer, the current would simply flow straight to the centre tap on that primary side of the transformer, and nothing would be reflected on the secondary side ie no noise.
    Different output tubes want the current to ' stay' going through the primary taps ' longer' or 'shorter' so that they can work properly to give proper harmonic output.

    edit, just walking the dog i realised that if there was no resistance from plate ends to b+, the sound would be as if you'd grounded your first filter cap..CRACK..so ' no resistance IS reflected on secondaries, not in a musical note but rather a sort of thunderclap.
     
    Last edited: Aug 21, 2019
  7. mickeydg5

    mickeydg5 Well-Known Member VIP Member

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    Yes, so true. :shock:
     
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  8. Chris-in-LA

    Chris-in-LA Well-Known Member

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    That was no gentleman, it was Uncle Doug.
     
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  9. Cadorman

    Cadorman Well-Known Member Premium Member

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    That explains it! When I was maybe 5 I had a little battery powered submarine. Stripped the wires and stuck them in the outlet in my bedroom. A little smoke and a big jolt. I obviously needed a resistor! Might have to try it again....
     
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  10. john hammond

    john hammond Well-Known Member

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    i wound up power transformer on an old core once as an experiment.
    Not uh..really understanding this principle of resistance and load..i figured " why do i need so many turns on the primary?" ( after unwinding the original transformer and noting there was so many turns)
    So i just wound this thing with about one eighth the windings i needed. Because there wasn't enough resistance...instead of making a transformer i made a smoke machine instead.
    more wire , more turns = more resistance.
     
  11. john hammond

    john hammond Well-Known Member

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    see now there's at least two types of boys.

    - one type just plays with the damn submarine
    - the other type just has to try things like this

    theres a third type (me)

    - some boys don't bother with wires, they just stick their fingers in via a section of teeth stripped wire.

    edit, it wasnt via a stripped wire, there was a broken power outlet with exposed wires that no-one bothered to fix in the abandoned building behind my home.
    In some sort of delayed reaction update to this near death experiment, one night, as an adult ..while asleep...i pulled out the plug from the wall just far enough to expose the terminals now half stuck out, jammed my fingers in and all i remember of the event now is this hum that nearly knocked my teeth out.
    So since then, i don't sleep near power outlets.
     
    Last edited: Aug 21, 2019
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  12. The Dose of Harmony

    The Dose of Harmony Well-Known Member

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    I love his videos I think he is the best teacher out there!
     
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  13. The Dose of Harmony

    The Dose of Harmony Well-Known Member

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    there are a lot of videos about triodes and pentode that go even deeper in his channel !!
     
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  14. john hammond

    john hammond Well-Known Member

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    for me at the moment i'll stick to trying to imagine current , both ac signal and dc wave travelling through the same resistor at the same time in completely different directions, ill get back to pentodes and uncle doug in just a little while.
    edit- ok i can see just thinking about it that the two currents dont actually go through the resistor. one does, the other tries to, but decides not to, instead going through the cap into the next grid..
     
    Last edited: Aug 21, 2019
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  15. Nik Henville

    Nik Henville Well-Known Member

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    Location:
    Eastern Seaboard, the nook-shotten isle of Albion
    We
    used
    to stick
    magnesium ribbon
    between live and neutral on the mains socket.
    Then we would stand well back and, using a wooden ruler, switch the mains on.
    The wooden ruler? Always better to be safe than sorry.
    The magnesium ribbon?
    Flash bastards.
    :hippie::pirate::uk:
     
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