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Post subject: HRD Output Transformer DC Resistance ??
Posted: Mon Feb 08, 2010 10:05 am
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I have two HRDs that I measured the DC resistance of the primaries on the output transformer (center tap (red) to legs) to be 103 ohms on one side (blue lead) and 80 ohms on the other side (brown lead). The transformer leads were disconnected form the PCB so the measurement shouldn't be effected by any loading.

Is this normal ? I would expect both sides of the primaries to be the same for balanced operation. Wouldn't this cause a balance problem with the bias ?


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Posted: Mon Feb 08, 2010 9:08 pm
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Yeah you would expect the two sides to be closer in comparison. A discrepancy of 1% to 10% is the norm, depending on the manufacturer and the amp designers specs. But a perfect balance would be Hi-Fi and guitar amps are designed with that in mind. Meaning, that a certain amount of imbalance imparts a tonal characteristic that is intended. Would this imbalance affect the tube bias? Yes ! It would be interesting to know what the current draw is on the individual tubes. Art

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Posted: Tue Feb 09, 2010 7:33 am
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Thanks Art ! A 20% imbalance seems high but since both read the same, maybe Fender did that intentionally for the reasons you mentioned.

I'm building a bias meter that'l let me measure both tubes individually and will get back to you with the results. :?


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Posted: Tue Feb 09, 2010 8:56 am
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Art, why doesn't this surprise me?LOL :D


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Posted: Tue Feb 09, 2010 8:16 pm
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Yeah man, it's hard to say if that is intentional or just sloppy quality control. Wonder what the date codes are on those two transformers? Maybe some other HRD owners could measure their output transformer's primary DC resistance and we could get an idea about the consistency. They can be measured in the amp for this purpose. Art

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Posted: Tue Feb 09, 2010 8:27 pm
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Usually the taps are not the same. My BD is 74 and 85 ohms. That's quite a spread on the OP's though.


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Posted: Tue Feb 09, 2010 9:09 pm
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Thanks Rjake, I wonder if we can correlate the dates of these trannys. I mean, it would be interesting to see if there is any change over time of manufacture or if it's just a random anomaly. Art

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