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 11th February 2013, 04:37 PM #1 sparkydave   diyAudio Member   Join Date: Feb 2013 Silly transformer question 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
diyAudio Member

Join Date: Apr 2009
Quote:
 Originally Posted by 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?
Not silly! Plate to plate is the usual way they are spec'd.

Randy

 11th February 2013, 04:58 PM #3 JohnAtwood   diyAudio Member     Join Date: May 2006 Location: Rural Nevada 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
 11th February 2013, 06:13 PM #4 sparkydave   diyAudio Member   Join Date: Feb 2013 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.
 11th February 2013, 11:42 PM #5 ronaldw441   diyAudio Member   Join Date: Aug 2008 Location: Broomfield, CO 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
diyAudio Member

Join Date: Nov 2006
Location: Munich, Bavaria
Quote:
 Originally Posted by 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.
Depends on the conduction angle.
Class A: 4000 Ohms
Class B: 2000 Ohms
__________________
Терпенье и труд все перетрут

N4BBQ
diyAudio Member

Join Date: Nov 2009
Location: Dahlonega, GA
Quote:
 Originally Posted by 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?
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

 12th February 2013, 05:30 PM #8 ronaldw441   diyAudio Member   Join Date: Aug 2008 Location: Broomfield, CO N\$BBQ, Thank you. That's just what I was looking for. Ron
 12th February 2013, 06:37 PM #9 JohnAtwood   diyAudio Member     Join Date: May 2006 Location: Rural Nevada 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
diyAudio Member

Join Date: Nov 2009
Location: Dahlonega, GA
Quote:
 Originally Posted by ronaldw441 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.

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