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mystery single ended output transformer

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  • #16
    Nope. Again, that .4 ohms is DCR- not impedance. The heater voltage would, of course, need to be AC.
    "I took a photo of my ohm meter... It didn't help." Enzo 8/20/22

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    • #17
      Do you have a signal generator? I use one set at 1v across the secondary and measure voltage across the primary. Bit safer than higher voltage-higher current, easier to calculate, but you can still feel it if you touch the wires (by accident).

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      • #18
        measure the resistance of any 6v winding on a power transformer. You will find it very low.
        Education is what you're left with after you have forgotten what you have learned.

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        • #19
          "Nope. Again, that .4 ohms is DCR- not impedance. The heater voltage would, of course, need to be AC"

          Yes, but I with 6.3 vac and 0.4 ohms I get (E/R = I) 15.75 amps. Am I misunderstanding?
          It's weird, because it WAS working fine.....

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          • #20
            That formula applies to DC. We aren't talking about resistance and DC voltage. We are talking about impedance and AC voltage.
            "I took a photo of my ohm meter... It didn't help." Enzo 8/20/22

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            • #21
              Yes. You keep thinking of the transformer winding as a resistor.

              You can run that 6v into that 0.4 ohms because the current is transformed into a different voltage/current on the other side of the transformer. If you short across the other side of the transformer, THEN it turns into a 0.4 ohm resistor.


              Imagine a typical power transformer, 120v primary and 6v secondary. That 6v secondary has a super low resistance. GO check one. The primary may only have 100 ohms resistance, but the 120v doesn't care as long as it can turn that into 6v on the secondary. But short across that 6v winding, and BAM you blow the mains fuse, because now the transformer can't transform. SO the primary becomes a short circuit across the mains.


              I view a transformer as a set of gears.
              Education is what you're left with after you have forgotten what you have learned.

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              • #22
                Impedance Z is an AC quantity, given by ratio of AC voltage to AC current. There is no general relation between the impedance and DCR. The secondary of the OT just behaves like an inductor as long as there is no current flowing in the primary. The impedance of an inductor is always higher than its DCR. The 6.3V heater voltage corresponds to an output power of around 5W in the amp, i.e. the AC current in the secondary is the same as with an output of 5W into 8 Ohms.
                - Own Opinions Only -

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                • #23
                  i.e. the AC current in the secondary is the same as with an output of 5W into 8 Ohms.
                  Maybe you intended to say "5W into 8 ohms mean 6.3 Volts AC?"

                  We can be certain of Voltage ... not so much about current because we donīt know what load is applied at the other end of the transformer.

                  And if as usual itīs measured unloaded (the meter does not count as a load in this case) then current is nil.
                  Juan Manuel Fahey

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                  • #24
                    Originally posted by J M Fahey View Post
                    Maybe you intended to say "5W into 8 ohms mean 6.3 Volts AC?"

                    We can be certain of Voltage ... not so much about current because we donīt know what load is applied at the other end of the transformer.

                    And if as usual itīs measured unloaded (the meter does not count as a load in this case) then current is nil.
                    You're right, of course. My bad. With open primary the secondary current at 6.3V is only determined by the secondary magnetizing inductance and thus lower by a factor than with primary terminated by the tubes' Raa.
                    What I wanted to get across is that it is safe to use heater voltage (as I proposed above) and that the secondary is far from being a short.

                    My recommendation to beginners: Use your DMM's AC current measuring function and measure voltage and current to determine Z. (Note: The Z of a transformer winding gets reduced by the load on the other windings. Or in other words , the current in each winding is influenced by the current in the other windings.)
                    Last edited by Helmholtz; 07-11-2018, 09:06 PM.
                    - Own Opinions Only -

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