## Calculate Tube Amplifier Output Transformer Load Resistance To determine the turns ratio we apply an ac voltage to the primary, and measure the voltage in the secondary. A transformer ( 10 - 15 volts ) is used to apply the ac voltage to the primary of the Output Transformer (OT) An ac voltmeter is connected to the secondary of the OT to measure the output voltage. Now, measure then primary voltage of the OT For example, if the voltage on the primary measures 13 volts and the secondary measures 0.52 volts the turns ratio is 25:1 To determine the load resistance for the desired given speaker load, square the turns ratio you calculated and multiply that number by the desired speaker load (i.e. 4, 8, 16 ohms).

## Calculate Transformer Ratio & Load Resistance

 Measured Voltage across the secondary taps ( Vout ) = Volts Measured Voltage across the primary taps ( Vin ) = Volts Desired speaker load (i.e. 4, 8, 16 ohms) = Ohms Turn Ratio = Load resistance for the desired given speaker load = Ohms

## Replace Output Transformer

 Lets assume we have a set that has an output transformer with an open winding that must be replaced, and the output tube is a single 2A3 Triode. First we look at the 2A3 Tube Datasheet and see that a 2A3 should work into a load resistance of approximately 2500 ohms. With this information we can determine what the turns ratio should be. By dividing the secondary load of the loudspeaker ( 4, 8, 16 ohms ) into the 2500 ohms required by the 2A3, we get an impedance ratio of 312.5 : 1 If the impedance ratio is the square of the turns ratio, then the turns ratio is the square root of the impedance ratio. Taking the square root of 312.5 gives us a 17.7 : 1 ratio, so an output transformer with a turns ratio in this range should work.

## Tube Datasheet Links

 211 2A3 300B 5881 6550 6A3 6AS7 6BQ5 6CA7 6L6 6V6 EL34 EL509 EL84

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