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Old 3rd June 2009, 08:54 PM   #1
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Question Transformer winding resistances

Hi, I have recently finished building a 100w guitar amp and have tested psu and phase splitter successfully so moved on to the output stage.

I took some resistance measurements across the coils of the output transformer to refer back to in case anything went wrong. I expected to read 1700ohm across the primary to agree with the manufacturer but instead I obtained 16ohms either side of the centre tap and 33ohms across the whole winding. While those three readings appear to agree with each other, they don't agree with the 1700ohms.

My question is, is the 1700ohms likely to be taking AC signals into account thus being and impedance rather than a resistance? What i measured was a DC ohmic value with the amplifier switched off. Do my values appear to acceptable or is something wildly askew?

Thanks for any responses!
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Old 3rd June 2009, 09:19 PM   #2
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1700 Ohms is the dynamic (ac) impedance from anode to anode. It's the typical value for a Marshall (and related ) 100W design using four EL34s. It's much more than the dc value due to the inductance of the trafo windings.

16Ohm dc is a very typical Marshall-type value for DCR in a 100W trafo. Other designs show more.
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Old 3rd June 2009, 09:34 PM   #3
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Ah thank you for that clarification - it agrees with what I have which is a mercury magnetics version of a marshall transformer.
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Old 3rd June 2009, 09:39 PM   #4
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Transformers are not really complex, but they are not as simple as some may assume. You have measured the DC resistance of the primary winding, but this is not the reflected load impedance. 1700 ohms is the reflected load impedance of an 8 ohm load.
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Old 4th June 2009, 01:55 AM   #5
Enzo is offline Enzo  United States
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In fact, the transformer has no inherent impedance. It only shows the tubes that 1700 ohms when the specified load impedance in on the secondary. If I put a 2 ohm load or a 100 ohm load on the secondary, your tubes would no longer see the 1700 ohms.

Transformers are like a set of gears. There is a ratio between input and output, but no inherent speed. In fact even input and output are relative. Just like gears that can be driven from either end, a transformer will work "backwards"
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Old 4th June 2009, 12:16 PM   #6
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Nice analogy there - makes sense now what I was measuring and agrees with everyone here and with with manufacturer.
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