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Measuring Output Transformers

Posted by Triode_Kingdom (A) on May 6, 2007 at 19:59:57

Most of the transformers I've used in the past were selected based on the manufacturers' specifications for impedance. Recently, I've been testing various used transformers that I don't have specs for. The results I'm getting raise questions about my methods, so I'm wondering how everyone else does this. Following are a few examples of my tests. As you can see, I'm not getting what one would necessarily expect.

Transformer Conditions Impedance based on Voltage Ratio

Stromberg PP 6V6 Loaded 11,988
Stromberg PP 6V6 Unloaded 10,579

HK TA7000X PP 7591 Loaded 6,853
HK TA7000X PP 7591 Unloaded 5,215

HK Nocturne PP 6BQ5 Loaded 8,889
HK Nocturne PP 6BQ5 Unloaded 7,200

All of these tests were performed by connecting a 500 Hz signal source across the entire primary of each transformer. Voltage was measured at the primary and eight ohm tap of the secondary. The "loaded" condition means voltages were measured with an eight ohm load. I used a well-calibrated scope with a 10:1 high-impedance probe for all measurements.

Regarding the transformers, the Stromberg is an inexpensive small open frame from a 6V6 console. I had thought it would show 10K ohms. The HK 7591 is a midsize upright transformer rated for 35W. It has a smallish core for this power level. I expected 6.6K ohms. The HK 6BQ5 transformer is a full-size transformer with a nice, hefty core. I thought it would measure 8K ohms.

Any thoughts on this from those of you who have performed similar tests?

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