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In Reply to: RE: Push/Pull OPT question... posted by drlowmu on May 10, 2008 at 06:13:07
This is a transformer specification. In Jeff's example, if you terminate the secondary with 8 ohms on the 8 ohm winding, you would measure a total impedance (with an AC impedance meter) of 5000 ohms from one plate connection to the other one. If you measure the impedance from one plate connection to the center tap (B+ connection), you would be using only 1/2 the primary winding, so the measured impedance would be 1/4 the first value, or 1250 ohms. Understanding how to choose transformer impedance for p-p circuits is a little complex, so I refer you to the standard textbooks, such as H.J. Reich "Theory and Applications of Electron Tubes" (2nd Edition, 1943), chapter 7, pp. 246 to 258. I have found it easier to keep track by using an ideal three-winding transformer (equal windings), and seeing what load on the third winding gives the correct I-V activity on each of the two individual plate windings. That load resistance is therefore 1/4 the plate-plate impedance of the transformer primary.
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