How to Measure a Power Transformer in a Fender Amplifier

Written by simon foden Google
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How to Measure a Power Transformer in a Fender Amplifier
Fender amplifiers have two transformers. (amplifier image by Darko Draskovic from

A guitar amplifier power transformer converts power from the mains into an electric current strong enough to power the rest of the amp. The guitar's input is a low-impedance signal, and the current generated by the power transformer turns it into a high-impedance signal. If the guitar amp is losing volume or the tone is weak, there may be a faulty power transformer. If the transformer is underperforming, the tubes will not be properly powered, and the amp will malfunction.

Skill level:

Things you need

  • User manual
  • Screwdriver
  • Soldering iron
  • Latex gloves
  • Voltmeter

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  1. 1

    Read the user manual and schematic for the amp. The schematic illustrates the layout and values of the various internal components. It also contains information on tube plate values and idle currents, which is key to measuring the power transformer. Write down the power handling values.

  2. 2

    Unscrew the back. Place the screws to one side in their original order to make replacing the back easier. Unfasten any clips that connect wires from the power section to the chassis.

  3. 3

    Pull the chassis gently towards forward. Most of the wires are long enough to completely remove the chassis from the cabinet. Disconnect the wire that connect the power section to the speaker, and place the chassis to one side. Remove all of the tubes from the housing, and put them in a safe place. Put on latex gloves to handle the tubes.

  4. 4

    Locate the correct transformer. There are two transformers located on top of the amp chassis. The power transformer is the one nearest to the mains input and normally is steel. The other one is the output transformer, which typically is painted black.

  5. 5

    Calibrate the voltmeter. Test it on a piece of equipment that is in good working order to make sure that the voltmeter is producing an accurate reading, .

  6. 6

    Unscrew the outer shielding of the power transformer to expose the primary winding. Clip one meter lead to each side of the primary winding. Keep wires from dangling onto the amplifier chassis.

  7. 7

    Turn on the amplifier. Do not touch the amplifier with any part of your body. There is a live current in the amplifier, so proceed with extreme caution. The voltmeter quickly produces a reading that is equivalent to the local wall current, which is 120 volts. A zero voltmeter reading indicates two possible problems: Either the wire running from the mains input to the transformer is damaged, or the transformer, itself, is dead.

  8. 8

    Turn off the amplifier off and disconnect the meter leads. Connect the meter leads to the filament that winds in the transformer. Turn on the amplifier, and write down the reading. The reading should be equivalent to the rating in the owner's manual. A marginally higher reading is fine. A lower or zero reading indicates that the transformer is damaged.

Tips and warnings

  • Test the output transformer and plate resistors while the chassis out of the cabinet.
  • Consult a professional if questions or problems arise. Use caution and safety measures when measuring the power transformer. Most amps carry potentially lethal currents.

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