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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 10,579HK TA7000X PP 7591 Loaded 6,853
HK TA7000X PP 7591 Unloaded 5,215HK Nocturne PP 6BQ5 Loaded 8,889
HK Nocturne PP 6BQ5 Unloaded 7,200All 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?
Follow Ups:
I'm contemplating a high power SET. So I've been collecting purported 'wide-band' power trannies to use as output trannies. How should I go about measuring these a reverse engineering excercise? As if I had an unlimited choice of output tube to suit the voltage and turns ratio as measured? I'm working in the range of less than 3KVDC (by choice, not by dielectric strength)and basically unlimited current. Think various AM transmitter supplies. Thanks in advance for any thoughts.
I *like* the loudness button!
I don't think you will be able to get high power out of a power transformer unless you choose a tube (or parallel combo of them) that gives you a really low Rp (think sub 500 ohms)
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you are measuring too close to the self resonant frequency. Use your line frequency and repeat the tests.dave
Thanks, Dave-man! I tried 62 Hz, and all the numbers fell into place. I forgot that audio OPTs all have a big peak around 800 Hz.
I have rewound a toroid as a pp O/P transformer and have tried to make some measurements on it using it as the reactive element in a simple oscillator circuit but the results don't make much sense. why do they have a big peak around 800Hz?
Boiss
> why do they have a big peak around 800Hz?
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