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

Silly transformer question
Looking at output transformers, I see ones listed as pushpull, 8000 ohms center tapped. Is that 8000 in each leg, or 8000 from plate to plate?

11th February 2013, 04:52 PM  #2 
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Join Date: Apr 2009


11th February 2013, 04:58 PM  #3 
diyAudio Member
Join Date: May 2006
Location: Rural Nevada

This usually means 8000 ohms platetoplate (often written as 8000 PP). 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 platetoplate (double the primary windings, thus 2^2 or 4 times the impedance.)
 John 
11th February 2013, 06:13 PM  #4 
diyAudio Member
Join Date: Feb 2013

Thanks! I was pretty sure it meant platetoplate. 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 
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? 
12th February 2013, 12:52 AM  #6  
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Join Date: Nov 2006
Location: Munich, Bavaria

Quote:
Class A: 4000 Ohms Class B: 2000 Ohms
__________________
Терпенье и труд все перетрут 

12th February 2013, 11:29 AM  #7  
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Join Date: Nov 2009
Location: Dahlonega, GA

Quote:
Measuring the input impedance, bandwidth and efficiency of transformers 

12th February 2013, 05:30 PM  #8 
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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 
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 
12th February 2013, 11:59 PM  #10 
diyAudio Member
Join Date: Nov 2009
Location: Dahlonega, GA


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