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ohm's law in practice

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  • #16
    AN ideal transformer has no wire resistance, but a real one does. Depending upon the wire size and number of turns the winding resistance could be half an ohm or 150 ohms or whatever. (Not limiting discussion to 6v now) Your 6v winding will have very low resistance, but some. Your B+ winding a lot higher. The wire resistance can be thought of as in series with the ideal current source. If it is only .2 ohms, your meter might not detect it, but as we have seen, as little as .43 ohms makes a diference. At least on the 6v.

    As to our discussion of applying Ohm's law to your example, we can ignore the wire resistance as we explore the basic concept. But in practical application one should consider transformer wire resistance in calculations. For example, guys going nuts trying to figure out why one of a pair of power tubes always idles a few ma hotter than the other. When they measure the resistance of the output transformer winding they find one side is 75 ohms and the other side 125 ohms. Those resistances are in series with the tubes and the extra 50 ohms affects the current.

    That wasn't a power transformer, but consider the B+ winding of a PT. One transformer has a winding resistance of say 150 ohms, and another a resistance of 50 ohms. That extra 100 ohms is in series with the current, so it will have a obvious effect upon sag. Same as putting a 100 ohm resistor in series with your rectifier.


    And remember, the transformer doesn;t make current and push it out, the load draws current from it. No load, no current.


    Wattage for resistors? As you point out, dissipation is not the limiting factor in many places. How about a 1meg grid return? Essentially zero current, so a 1/100th watt ought to work. But a lot of guys prefer to use 1w resistors in their amps. There are a lot of factors other than dissipation. One is thermal stability. Inherent noise. Another is voltage. A larger resistor may well be able to handle higher voltages. And simple mechanical strength, that can matter in something that vibrates all the time like an amp circuit next to a loud speaker.
    Education is what you're left with after you have forgotten what you have learned.

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    • #17
      Thanks for the explanation Enzo

      Just an aside on this: I use the small 1/4 watt size resistors for all tube stuff I build, except where the larger wattage is actually needed. This is because I have a stock of the small 1% metal film resistors in all values for my solid state work.

      I keep the 1/2 watt sized resistors in 47k, 82k, 100k, 220k, and 1 watt in 10k and 27k. I also like to have 470 ohm and 1k in the 10-watt cement block style, and sometimes I like the Dale wirewound ones in gold coloured cases, for screen resistors.


      Random things to remember:

      Metal film resistors in the size that "1/4 watt" carbon resistors used to be, are actually 0.6 watt.

      They have a voltage rating of 350V, if you go much above this they break down internally and short out.

      For the rectifier tube filament problem, the answer seems to be "About half an ohm".

      Or if you can get a wire round your PT's existing windings, you could add a bucking winding in series with the 6.3V to remove 1.3 volts. Probably about 3 to 5 turns would do it.
      "Enzo, I see that you replied parasitic oscillations. Is that a hypothesis? Or is that your amazing metal band I should check out?"

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      • #18
        well everyone, I have to say that I appreciate the answers. Some good information here.

        Since the voltage drop will vary based on current, I guess now I need to understand the functioning of current within a tube circuit. I am assuming that current draw is pretty consistent throughout a tube circuit like a guitar amp? Or does it swing based on the signal like voltages do? Time to start googling around here.
        In the future I invented time travel.

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        • #19
          Originally posted by cminor9 View Post
          well everyone, I have to say that I appreciate the answers. Some good information here.

          Since the voltage drop will vary based on current, I guess now I need to understand the functioning of current within a tube circuit. I am assuming that current draw is pretty consistent throughout a tube circuit like a guitar amp? Or does it swing based on the signal like voltages do? Time to start googling around here.
          the resistance of the heater filament should remail pretty constant once heated up, hence keeping the current pretty constant. understanding the current needs of gain stages and such is more complex though, but does give some insight into how circuits behave and how power supplies have alot to do with the sound of an amp.

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