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Neil Young's "Magic" 5E3

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  • #61
    Yes, resonant circuit. But perhaps what I was referring to was simply measuring the inductance, not the total impedance.

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    • #62
      Originally posted by Mike K View Post
      Yes, resonant circuit. But perhaps what I was referring to was simply measuring the inductance, not the total impedance.
      There's no relation between OT primary inductance and loaded impedance.
      - Own Opinions Only -

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      • #63
        Originally posted by Helmholtz View Post

        There's no relation between OT primary inductance and loaded impedance.
        What changes the AC impedance vs the reflected impedance based on turn ratio?

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        • #64
          Originally posted by Mike K View Post

          What changes the AC impedance vs the reflected impedance based on turn ratio?
          AC resistance is reflected secondary load resistance + primary and reflected secondary windings DCR.

          It helps to study the equivalent circuit of a transformer.
          - Own Opinions Only -

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          • #65
            Originally posted by Helmholtz View Post

            AC resistance is reflected secondary load resistance + primary and reflected secondary windings DCR.

            It helps to study the equivalent circuit of a transformer.
            But if that's all resistive load i.e. no reactive load like a speaker, isn't it constant as a function of frequency?

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            • #66
              Originally posted by Mike K View Post

              But if that's all resistive load i.e. no reactive load like a speaker, isn't it constant as a function of frequency?
              Yes, it's resistive (i.e. zero phase angle) and constant except maybe for frequency extremes.
              Nevertheless it can't be measured with DC because a transformer only works with AC.
              - Own Opinions Only -

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              • #67
                Originally posted by Helmholtz View Post

                Yes, it's resistive (i.e. zero phase angle) and constant except maybe for frequency extremes.
                Nevertheless it can't be measured with DC because a transformer only works with AC.
                I get that. But couldn't we just calculate it based on primary primary DCR + (turn ratio)^2*(secondary DCR + Load) and simply calculating the turn ratio by measuring the voltage ratio between primary and secondary? So no need for a LCR meter.

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                • #68
                  Originally posted by Mike K View Post

                  I get that. But couldn't we just calculate it based on primary primary DCR + (turn ratio)^2*(secondary DCR + Load) and simply calculating the turn ratio by measuring the voltage ratio between primary and secondary? So no need for a LCR meter.
                  Sure and I've done.
                  You'll need a good DCR meter to accurately measure secondary resistance.
                  - Own Opinions Only -

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                  • #69
                    Originally posted by Helmholtz View Post

                    Sure and I've done.
                    You'll need a good DCR meter to accurately measure secondary resistance.
                    OK - this is the same process I do for every PT. I've never done it for an OT, but it makes sense. Generally the primary DCR, even if high, is a low proportion of the reflected impedance. I have no idea about the secondary and if that was high, I assume could make a difference.

                    Sorry we got here in such a circuitous route, but when you mentioned AC impedance, I thought you were measuring something reactive.

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                    • #70
                      Originally posted by Mike K View Post

                      OK - this is the same process I do for every PT. I've never done it for an OT, but it makes sense. Generally the primary DCR, even if high, is a low proportion of the reflected impedance. I have no idea about the secondary and if that was high, I assume could make a difference.

                      Sorry we got here in such a circuitous route, but when you mentioned AC impedance, I thought you were measuring something reactive.
                      I tried to always use the term AC resistance (not AC impedance).

                      With a small OT having a high impedance, primary resistance may be higher than 500R. And 1R secondary resistance would reflect to 1250R with a 10k/8 OT.
                      I only raised the topic because a "10K OT" could actually be a "12K OT" depending on measuring method.
                      - Own Opinions Only -

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                      • #71
                        The LCR @1khz is the method i use anymore, since it's easy to switch 4,8, 16 and get a direct reading. As to 5E3's, I've probably built at least half a dozen (stuffed in a room upstairs)and have another i started a while ago on the bench, trying to clean up unfinished projects. I don't remember what the power output and choke were from , but taken from a amp or phono or reel. I've toyed with different caps, .033uf/.047uf but this one has 0.1 mustards and 0.1 quality caps from a Tektronix scope. As to the choke, i read that if you leave the 0.1's in place, and add a choke, a lot of the flubbiness will be gone. I shall find out. 22UF for all 3 power supply caps.

                        Most common complaint is high voltage on the output tubes when buying transformers. I try to shoot for 350-360. This one i have no idea, power trans is a bit bigger than a stock one and i didn't write down the loaded voltage.

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                        • #72
                          Originally posted by Helmholtz View Post

                          I tried to always use the term AC resistance (not AC impedance).

                          With a small OT having a high impedance, primary resistance may be higher than 500R. And 1R secondary resistance would reflect to 1250R with a 10k/8 OT.
                          I only raised the topic because a "10K OT" could actually be a "12K OT" depending on measuring method.
                          I'm quite interested in the one I have in the "10W PP" thread. That one has a very high DCR, it might be a significant deviation. I'll measure it when I get a chance, and maybe change the design a bit...

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                          • #73
                            Originally posted by Helmholtz View Post

                            Regarding (dynamic) plate dissipation, looking at the loadline in a chart showing the plate limit parabola easily shows that dissipation increases at lower load impedance.
                            Not a parabola, inverse relationship i.e. I = (constant)/V but mentioning that seems obvious to me now. Excessively low load will damage the plates.

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                            • #74
                              Originally posted by Mike K View Post

                              Not a parabola, inverse relationship i.e. I = (constant)/V but mentioning that seems obvious to me now. Excessively low load will damage the plates.
                              You're right. Should read hyperbola.
                              - Own Opinions Only -

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                              • #75
                                Curiously enough when I measure 5E3 OT I have (Mercury FTDO-59), I get the exact same turn ratio as the 18W (n=34:1 @ 8 ohm). Accounting for the primary and reflected secondary DCR, I get a 10.3k effective primary for 8R (this OT only has 8R).

                                Primary DCR is 655R and secondary is 0.7R.

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