Measuring Output Transformer Primary Inductance
Measuring output transformer primary inductance is critical to the device’s functioning. An output transformer is a device designed to transfer electric energy from one electric circuit to another circuit using the principle of mutual induction. The electrical transformer transfers voltage or electric current using its coils, which are defined as inductively coupled conductors. A varying current in the transformer’s first winding results to a varying magnetic flux in the core and a varying magnetic field in the secondary winding. This magnetic field creates the voltage in the secondary winding. When measuring output transformer primary inductance, the basic concepts should first be considered.
There are two main types of output transformers: the single ended and push-pull output transformers. Single ended output transformers are used for single ended amplifiers. Also, the quiescent current of the power triode is not compensated in the transformer core. Push-pull output transformers are used for push-pull amplifiers. The quiescent currents of the push-pull power tubes cancel each other out in the core of the output transformer. A single ended output transformer features a gap in the core to deal with the quiescent current. The push-pull type has a closed core with almost no gap. A push-pull output transformer cannot be used on a single ended amplifier.
All transformers, including three phase, step up, isolation and step down transformers, follow two basic principles: electromagnetism and electromagnetic induction. In electromagnetism, a current generates a magnetic field. In the principle of electromagnetic induction, a varying magnetic field within the transformer’s coil of wire induces a voltage across the ends of the coil. Modifying the electric current in the transformer’s primary coil will change the magnetic flux that is generated.
When measuring output transformer primary inductance, the basic concepts should be understood. If the primary wires of the transformer are the ones only connected and the inductance is measured, no energy leaves through any secondary windings. The amount of inductance measured is the primary inductance. It is the consequence of the iron and air in the magnetic field path. It has a nonlinear property.
When working with tube amplifiers, the output transformer impedance should be calculated. This is necessary especially if the transformer is old or if it has unknown values. The turns ratio of the transformer is calculated using an alternating current (AC) voltage source and an AC voltmeter. An AC voltage is applied to the primary of the transformer and the output voltage to the secondary is measured. The formula is Turns Ratio = V in (to the primary) / V out (measured in the secondary). By knowing the turns ratio, the impedance ratio of the unknown transformer and the impedance in a given load to the secondary are measured.
Before measuring output transformer primary inductance, the products of several manufacturers of output transformers should be analyzed. Mojo Musical Supply’s 15 watt single ended output transformer is available with 3.4K, 4K and 5K primary. This transformer is part of an amplifier for musical instruments. TEMCo’s output transformers are designed for various electrical products. One common application of TEMCo’s transformers is an audiophile amplifier. Hammond Manufacturing’s audiophile tube output transformers are built and tested at a Hi-Pot rating of 2,000 VAC root mean squared voltage. The company does not offer toroidal designs due to their inability to handle tube imbalance and high manufacturing cost.
