Output Transformer Turns Ratio / Impedance Calculator

Test an unknown output transformer with a low AC voltage and compare its measured turns ratio with common guitar-amplifier impedances.

AC voltage applied to full primary (V RMS) Plate-to-plate for push-pull; full winding for single-ended. AC voltage measured on secondary (V RMS) Measure the relevant secondary tap, with no speaker connected. Primary impedance matching tolerance (±%) Percentage difference in reflected impedance, not voltage.

Reference primary impedances (Ω) Edit this list to suit the valves or transformer being investigated. Separate values with commas.

Measured result

Primary : secondary turns ratio

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With a 4 Ω speaker

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With an 8 Ω speaker

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With a 16 Ω speaker

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Enter the measured secondary voltage to calculate the turns ratio and reflected impedances.

Reference combinations

Expected secondary AC voltages with 15 V applied to the primary

Target primary	Speaker	Turns ratio	Expected V	V range (±Z tolerance)	Actual Z difference

Green = nearest listed combination within tolerance. Pale yellow = other combinations within tolerance. Matching a nominal value does not identify the transformer's intended rating.

How the calculation works

The voltage ratio approximates the turns ratio when the secondary is unloaded:

Turns ratio (n) = V_primary ÷ V_secondary

The impedance seen by the output valves depends on which speaker is connected:

Reflected primary impedance (Z_p) = n² × speaker impedance (Z_s)

For reference combinations, the calculator works backwards to find the expected secondary voltage. The tolerance is applied to primary impedance, so the displayed voltage limits are not symmetrical around the nominal voltage.

Source: Merlin Blencowe, Valve Amplifiers, 4th ed., Chapter 1, p. 25. Measurements establish a turns ratio, not the transformer's power rating, insulation condition, frequency response or suitability for single-ended use.

Safety: Test the transformer disconnected from the amplifier and disconnected from mains power. Apply only a suitable, isolated, low-voltage AC test signal to the entire primary winding. Do not apply 240 V mains to a winding. An output transformer can produce dangerous voltages if driven backwards.