Mystery SE OPT

[alexmoose]

hey guys, I bought a Hi-Fi at a thrift store a couple of months ago that is El84 SE I am pretty sure that they are triode strapped, however I am not certain. I want to know what primary resistance is on this OPT, is there anyway to find this out short of an educated guess?

these are small transformers, so probably 2-3 watt power handling

I know that the secondary resistance is 8 ohms

any help would be wonderful

-Alex

[Yvesm]

Disconnect all wires from the amp,
Tie the primary on a 117Vac mains wall socket,
Measure the secondary voltage, so you'll know the voltage ratio.
Square it to obtain the impedance ratio and multiply by 8 to have the reflected primary impedance.

Put hands aways when using mains !

Yves.

[alexmoose]

The amplifier is in no working conidition right now, could I just apply a small AC voltage to one side to figure out the ratio?

[Tom Bavis]

5K : 8 Ohms is 625 :1 square root is 25 : 1 voltage ratio. 7K : 3.2 Ohms is 2200 : 1 , or 46 : 1 voltage ratio.

[wrenchone]

This is essentially what the previous poster was saying, but I would not be so daring as to apply the full mains voltage across the transformer primary. A safer approach would be to unsolder all the transformer leads, noting (and labeling) which are primary and which are secondary.

You can then carefully apply AC voltage to the primary using a variac (10-20V p-p), and measure the input and output voltages using an AC voltmenter. For complete safety, one could do the same thing using a function generator.

With a function generator, one could also safely use an oscilloscope to look at both the amplitude and phase relationship of the input and output signals so that you can determine the winding polarities, an important thing to know if you're planning to use feedback with your amplifier (though you'd find out soon enough if the transformer phasing was wrong when the amplifier bursts into oscillation).