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  • power supply choke

    Regarding a class AB push-pull amp.....
    I searched the word CHOKE, but could not really find an answer to my question.
    It seems like the screen grids are what get the "benefit" of the choke. The power tube plates do not seem to grab their B+ supply from the choke, always before.
    So I am wondering why. Do the plates get help from the out of phase, push-pull aspect of the circuit.?
    I suppose that is my question.
    Why do the plates always take their B+ before the choke.? (or do they. maybe I have just not looked at enough schematics)
    Thank You
    https://www.youtube.com/watch?v=7zquNjKjsfw
    https://www.youtube.com/watch?v=XMl-ddFbSF0
    https://www.youtube.com/watch?v=KiE-DBtWC5I
    https://www.youtube.com/watch?v=472E...0OYTnWIkoj8Sna

  • #2
    Because first, if they drew through the choke, then the ENTIRE current of the amp would have to flow through the choke, necessitating a much heavier and more expensive choke.

    The nature of a push pull amp is that they tend to cancel out some hum in the final stage. SO you can get away with a little ripple there.

    The screen tap come off the choke, yes, the screens do not draw much current, but don't forget, the entire preamp circuit and PI also draw through the choke and benefit from the filtering.
    Education is what you're left with after you have forgotten what you have learned.

    Comment


    • #3
      The power tube plates don't always take there power before the choke, in the old days they didn't have big power supply filter caps and the choke usually came first in the line. LCRC or even LCLC. A push pull output stage isn't affected by hum or ripple in the power supply as much as a single ended amp and you get a more low impedance source and higher voltage by taking the power earlier in the chain. So yeah, the push pull helps with the power supply noise.

      Comment


      • #4
        Enzo...
        Austin...

        10-4
        Thank You
        https://www.youtube.com/watch?v=7zquNjKjsfw
        https://www.youtube.com/watch?v=XMl-ddFbSF0
        https://www.youtube.com/watch?v=KiE-DBtWC5I
        https://www.youtube.com/watch?v=472E...0OYTnWIkoj8Sna

        Comment


        • #5
          Take a look at vintage Sunn schematics....say a 200S or Spectrum or Sorado. They had a choke input power supply and the plate was supplied after the choke.

          Greg

          Comment


          • #6
            Originally posted by soundmasterg View Post
            Take a look at vintage Sunn schematics....say a 200S or Spectrum or Sorado. They had a choke input power supply and the plate was supplied after the choke.

            Greg
            Hey -
            Thanks for taking the time.
            Looked at those, and yes Choke Input. You are correct.
            Did some basic reading, and now have a basic understanding of why some type of Capacitor Input, or Pi filter is usually preferred.
            Thanks Again
            I really do appreciate your guys time, expertise, and education.
            best
            https://www.youtube.com/watch?v=7zquNjKjsfw
            https://www.youtube.com/watch?v=XMl-ddFbSF0
            https://www.youtube.com/watch?v=KiE-DBtWC5I
            https://www.youtube.com/watch?v=472E...0OYTnWIkoj8Sna

            Comment


            • #7
              Technically speaking, those are still cap input filters. But they are examples of taking the main power after the choke.
              Education is what you're left with after you have forgotten what you have learned.

              Comment


              • #8
                Enzo -
                Yeah, you are correct.
                I guess I did not scroll down far enough to see the first cap in the schem. I was mostly paying attention to where the B+ was going. I thought I saw the choke was first in line on one of those, but as you say, they all run into a cap before they see the choke.
                You guys do not miss much
                Thanks Again
                https://www.youtube.com/watch?v=7zquNjKjsfw
                https://www.youtube.com/watch?v=XMl-ddFbSF0
                https://www.youtube.com/watch?v=KiE-DBtWC5I
                https://www.youtube.com/watch?v=472E...0OYTnWIkoj8Sna

                Comment


                • #9
                  All has been said. The last reason why usually cokes are not used for the whole supply is obvious : a BIG and expensive choke would be required, for little benefit.

                  Comment


                  • #10
                    I think caps used to be a lot more expensive, compared to everything else, than they are today. That is why Fender might use little 8uf caps in places they'd use 20uf or more today. And I think chokes were relatively a lot cheaper then than they are now. Adding a choke to the power filter was the equivalent of adding a cap stage. In my experience, by the time we get to the screen node, there is precious little ripple with the plain old resistor between caps, so replacing the resistor with a choke becomes an unecessary expense. And the choke doesn't provide the voltage drop a resistor does, for B+ scaling.
                    Education is what you're left with after you have forgotten what you have learned.

                    Comment


                    • #11
                      Originally posted by Enzo View Post
                      I think caps used to be a lot more expensive, compared to everything else, than they are today. That is why Fender might use little 8uf caps in places they'd use 20uf or more today. And I think chokes were relatively a lot cheaper then than they are now. Adding a choke to the power filter was the equivalent of adding a cap stage. In my experience, by the time we get to the screen node, there is precious little ripple with the plain old resistor between caps, so replacing the resistor with a choke becomes an unecessary expense. And the choke doesn't provide the voltage drop a resistor does, for B+ scaling.
                      I am glad you wrote this.
                      It brings up two questions I have had for a while:

                      1. When you get past the screens, what uF value cap do you REALLY need in the power supply.?
                      2. Are the last few caps in the B+ line more of a "de-coupling" component, than they are a "filter".?
                      That really brings me back to question #1. How many mics does a cap really need to be for the phase inverter and pre amp.?
                      Thank You
                      https://www.youtube.com/watch?v=7zquNjKjsfw
                      https://www.youtube.com/watch?v=XMl-ddFbSF0
                      https://www.youtube.com/watch?v=KiE-DBtWC5I
                      https://www.youtube.com/watch?v=472E...0OYTnWIkoj8Sna

                      Comment


                      • #12
                        Get Merlin's book on power supplies.

                        I am a troubleshooter, not a designer, so the guys who do load lines and stuff a lot are going to give better answers.

                        1. I never really think about it. Too much at the first filer and it clamps the B+ more tightly, robbing the amp of some dynamics (IMHO). Too little and you have increased hum, distortion on peaks, or just more distortion in general.

                        2. Yes, the filter caps after the screen node are largely ther for decoupling. Unless something is seriously wrong, there will no tbe any ripple left there. I have seen old amps from the caps-are-expensive era, where the B+ node for the input stage was tiny, like less than 1uf. Fender got away with 8uf on some models.
                        Education is what you're left with after you have forgotten what you have learned.

                        Comment

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