Primary Inductance of a Marshall Output Transformer

Written by joel colby
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Primary Inductance of a Marshall Output Transformer
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Marshall amplifiers have been a standard in the music industry since 1962, and Marshall can list several illustrious music stars as clients. Pete Townshend, Eric Clapton and Jimi Hendrix are a few that have relied on Marshall equipment to help produce their distinctive sound. Amplifier performance owes its quality to several design and other factors, not the least of which is the output transformer. Magnetic Components Corporation of Chicago, Illinois manufactures an output transformer for use in the Marshall amplifier.

Transformer Dynamics

Transformer dynamics are very complex. An AC voltage applied to the primary coil creates magnetic flux in the core, which induces AC voltage in the secondary coil that is in-phase with the source voltage. Current drawn through the secondary coil to power a load induces a corresponding current in the primary coil, and draws current from the source. The primary coil serves as a load to the source and the secondary coil becomes a source to the ultimate load. An additional current in the primary coil lags the voltage source by 90 degrees. This magnetises the core producing the voltage to create the current required to energise the transformer to a rated voltage. Electromagnetic induction drives this device.

Importance of Impedance

The transformer turns ratio determines the impedance, reflected to the primary side of the transformer by the load impedance of the secondary side. Primary impedance influences power output and frequency response. Higher impedances yield narrower bandwidth, less distortion, and less output. A load impedance that is too low increases the current in the transformer windings, thereby reducing the primary impedance seen by the output device -- which, in turn, increases current flow. A load impedance that is too high will reflect a higher than normal impedance in the primary coil for the output device. Either of these conditions can cause poor sound quality and lack of volume.

Primary Inductance of Marshall Transformers

Magnetic Components Corporation of Chicago, Illinois manufactures output transformers for use in the Marshall amplifier. Marshall uses several differently-rated transformers in their final product, based upon the desired output wattage. It follows that the transformers' primary inductance will be different for different wattage amplifiers. For instance, the fifty Watt output transformer (40-18025) has a minimum rated primary inductance of 45Hy, as measured at 100V, 60Cy. Typically, it measures 76Hy nominal, and primary inductance of the eighteen Watt output transformer (40-18037) typically measures over 600Hy @ 50V, 60Cy. This is a calculated value based on the measurement of the open circuit impedance of 243K Ohms.

Output Impedance of Marshall Transformers

The inductance of the amplifier's output transformer is fixed by the design specifications and is not adjustable by the end user. The amplifier should never be operated without speakers attached, and all Marshall output transformers have a selectable output impedance of four, eight or sixteen Ohms. Choose the proper value to match the speaker load. An incorrect choice may result in a degraded output signal and may even cause damage to the speakers or the Marshall cabinet.

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