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.

  1. A transformer ( 10 - 15 volts ) is used to apply the ac voltage to the primary of the Output Transformer (OT)
  2. An ac voltmeter is connected to the secondary of the OT to measure the output voltage.
  3. 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.

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