# Silly transformer question

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#### sparkydave

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?

#### rmyauck

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?

Not silly! Plate to plate is the usual way they are spec'd.

Randy

#### JohnAtwood

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

#### sparkydave

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.

#### ronaldw441

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?

#### the_manta

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.

Depends on the conduction angle.
Class A: 4000 Ohms
Class B: 2000 Ohms

#### ronaldw441

N\$BBQ,

Thank you. That's just what I was looking for.

Ron

#### JohnAtwood

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

#### N4BBQ

N\$BBQ,

Thank you. That's just what I was looking for.

Ron

You're very welcome. I've used that method numerous times to identify scrap transformers.

#### Cubdriver

N4BBQ - great link - thanks!

-Pat

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