A valve or tube amplifier output transformer matches the relatively high impedance required by the output-valve circuit to the much lower impedance of a loudspeaker cabinet.
Use our Output Transformer Impedance Ratio Calculator for quick calculations.
Turns ratio
The turns ratio can be estimated by applying a known safe low AC voltage to one winding of an isolated transformer and measuring the voltage appearing on the other winding. The voltage ratio represents the turns ratio.
For example, if the measured primary-to-secondary voltage ratio is 100 V to 4 V, the turns ratio is 25:1.
Impedance ratio is the square of turns ratio
Transformer impedance does not change in the same 25:1 proportion. The impedance ratio is the square of the turns ratio. A 25:1 turns ratio therefore gives an impedance ratio of 625:1.
Reflected primary load
If an 8 Ω speaker load is connected to that transformer, the reflected primary impedance is 8 × 625 = 5000 Ω, or 5 kΩ.
This is why changing the load connected to a valve amplifier output transformer also changes the impedance reflected back to the output valves.
Speaker taps
Many guitar amplifiers provide 4 Ω, 8 Ω and 16 Ω output taps. Each tap uses a different portion of the secondary winding so that the intended speaker load reflects the designed primary impedance.
Use the correct output tap for the connected cabinet unless the amplifier manufacturer specifically permits another arrangement. Our Speaker Impedance Calculator can help determine the cabinet’s nominal load.
Testing safety
Do not use mains voltage as a casual transformer test source. Transformer-ratio testing should be performed with an appropriate low-voltage isolated AC source and with the transformer safely isolated from the amplifier circuitry.
Valve amplifiers can contain lethal stored voltages. If you are not experienced with high-voltage electronics, do not probe an amplifier internally.
Electronic Repairs UK provides amplifier fault-finding and component-level repair from our Telford workshop. Request a repair quote if you need help with an amplifier or output-stage fault.