How to bias a fender super-sonic amplifier

Written by dave n.
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How to bias a fender super-sonic amplifier
Changing the tubes of the Fender Super-Sonic also requires the amp to be biased. (old vacuum tubes image by Dmitry Rukhlenko from

The Fender Super-Sonic amplifier line has been updated multiple times since it was first introduced to the market. It is available as a combo amplifier with a reverb option or à la carte as a head with no reverb to be used with a guitar loud speaker cabinet. Like any tube amplifier, the Fender Super-Sonic amplifier needs new tubes from time to time and should be biased after new tubes are installed. Biasing helps maintain optimal voltages that power the tubes increasing tube life and creating the best tone possible from the amplifier.

Skill level:

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Things you need

  • Philips screwdriver
  • Digital multimeter
  • Small flathead screwdriver

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  1. 1

    Remove the chassis of the amplifier from the enclosure with the Philips screwdriver. Place the chassis on a well-lit, cleared off table with plenty of working space around the amplifier. Adjust all of the equaliser settings to 50 per cent. Set the reverb and presence settings to maximum, if available. Leave the volume setting at zero, and connect the chassis to its speaker or cabinet.

  2. 2

    Locate the trim pot marked R156 inside of the chassis. The trim pot may be marked differently, but it is the only trim pot inside of the chassis if the amplifier was manufactured before 2005.

  3. 3

    Clip the red test probe the lower lead of the resistor in the chassis marked R137. Connect the black test probe to the chassis itself for ground. Turn on the digital multimeter, and set it to the nearest rounded up value of 100 VDC.

  4. 4

    Turn the amplifier on, and read the measurement of the digital multimeter. Adjust the trim pot with a small flat-heat screwdriver until the reading shows -54.5VDC. Turn the amplifier off when this setting is confirmed.

  5. 5

    Remove the test probes, and return and the chassis to the enclosure with the Philips screwdriver.

Tips and warnings

  • There may be a positive reading if the test probes are connected backwards. R137 is a 1/4W 220k ohm resistor located to the right of the trim pot inside of the chassis.
  • Tube amplifiers are capable of carrying lethal levels of voltage even when turned off and removed from a power source.
  • Fully discharge filter capacitors prior to working inside of the chassis.
  • Consult a professional technician if unsure of capability.

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