Looking at output transformers, I see ones listed as push-pull, 8000 ohms center tapped. Is that 8000 in each leg, or 8000 from plate to plate?
This usually means 8000 ohms plate-to-plate (often written as 8000 P-P). Don't forget that the impedance is proportional to the square of the turns ratio, so if the transformer primary was 8000 ohms each leg, then it would be 32000 ohms plate-to-plate (double the primary windings, thus 2^2 or 4 times the impedance.)
- John
- John
Thanks! I was pretty sure it meant plate-to-plate. Interestingly, I stumped one of the electrical engineers where I worked when I wondered if one leg of this 8000 ohm transformer would look like 2000 ohms or 4000 ohms, but when I did the math it would be 2000 ohms as John pointed out. Glad I remember something about transformers from school.
Sorry to temporarily hijack this thread but I have a question for JohnAtood.
John, what is your preferred method to determine the input impedance of an output transformer? Is there a more accurate method than using the turns ratio?
John, what is your preferred method to determine the input impedance of an output transformer? Is there a more accurate method than using the turns ratio?
I'm not John, but you can easily measure a transformer's impedance yourself. See the link below.
Measuring the input impedance, bandwidth and efficiency of transformers
Sorry for being away from this thread. N4BBQ's link describes the technique better than I could! One thing I might add is that the choice of RL may have to be an educated guess. For example, if you have a mystery output transformer that has two secondary taps, you have to decide whether they are 4 and 8 ohms or 8 and 16 ohms. Vintage amps often had only 8 and 16, whereas modern tube amps more often opt for 4 and 8. If there are three taps, it is almost always 4, 8, and 16.
- John
- John
You're very welcome. I've used that method numerous times to identify scrap transformers.
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