Just found this if you wanted to try this formula. Like Mr. Gillespie had mentioned the OPT would still be under the voltage it would normally be taking using your 125v wall socket. Turns Ratio = V in (to the primary) / V out ( measured in the secondary) e.g. if we apply 5 Volts AC and we measure 0.21 Volts AC to the secondary the Turns Ratio of the transformer is : 5 / 0.21 = 23.8 That means a turns ratio 24:1 Knowing the Turns Ratio we can calculate the impedance ratio of the unknown transformer and the impedance in a given load to the secondary. The impedance Ratio is the square of the turns ratio: 24 X 24 = 576 that means an impedance ratio 576:1 So in a given load impedance of 8 ohms the transformer impedance is 8 X 576 = 4608 ohm ( it is about 4.5K)
I thought about that - not sure if the transformer maintains it's efficiency at that low of a voltage, but I'll know if my known value OTs test right.
You can use a low voltage source of say 10 Volts or less on the Secondary side, then measure the Primary side. Do the calculations the same way.
OK - testing completed. First I tested a Voice Of Music #VM12611 OT. The schematic shows it to have a primary impedance of 10K and single secondary of 4ohms. 7.67vac applied to the secondary produced 324.5vac on primary. This calculates to a primary impedance of 7.15K. Not what I was expecting. Second I tested an Altec-Lansing Peerless #16458 OT from a Heathkit W-5M. I always thought this to have 10K primaries, but I don't have documentation. 7.72vac applied to the 16ohm secondary produced 180.6vac. This calculates to a primary impedance of 8.75K. Again, not what I was expecting. Finally I tested the left OT of my 400. 7.76vac applied to the 16ohm secondary produced 193.6vac. This calculates to a primary impedance of 9.96K. Pretty close to what I was expecting. I disconnected the primary center tap and also the NFB circuit from the secondary to ensure it did not skew the results. All were lower than expected. Is it possible the transformers lost some efficiency operating "backwards" for lack of a better term? Either way, I think it's safe to say I have the later model OT's and can install the EFB circuit as drawn? Tom Kuhn
I started a thread for my 400 project, so I guess it makes sense for me to keep my future posts over there in one place. Thanks for sticking with me..... http://audiokarma.org/forums/index.php?posts/11509292/