How to Set Bias on a Peavey 5150 II

Written by michael black
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How to Set Bias on a Peavey 5150 II
Changing the bias on a tube guitar amplifier can drastically alter the tone. (amplifier knobs image by Darko Draskovic from Fotolia.com)

The bias setting of a tube guitar amplifier like the Peavey 5150 II has a major effect on the tone of the amplifier. While amps generally come with a preset bias that usually sounds pretty good and is designed to optimise tube life, many guitarist adjust the bias on their amplifiers to change the characteristic of the overdrive that the amp produces. While this may shorten the life of the tubes, most guitarists agree that the sought-after tone makes up for the cost of replacing tubes.

Skill level:
Moderately Challenging

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Things you need

  • Screwdriver
  • 6.8K 1/4 watt resister
  • Solder
  • Soldering iron
  • Soldering wick
  • Tube amp bias probe

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Instructions

  1. 1

    Find the adjustable bias knob on the back of the amplifier and see if you can use it to get a sound that you like before actually modding the amp. The factory set available sweep of this adjustment knob is quite narrow, so you may have to mod the amp in order to get the sound you want.

  2. 2

    Remove the chassis from the back of the amplifier with a screwdriver and carefully place the screws in a safe place. Be warned, however, that by doing this, you are voiding the warranty that came with the amplifier.

  3. 3

    Find the small circuit board on the inside of the amplifier and carefully bring it to where you can work on it. It is possible to do this modification without removing the actual amplifier unit from the chassis.

  4. 4

    Find the bias trim pot mounted on the circuit board and remove the final bias resistor (12K 1/4 watt) with the soldering iron and soldering wick.

  5. 5

    Solder the 6.8K 1/4 watt resister in the place of the old resistor. What this will ultimately do is widen the available sweep of the bias adjustment knob, opening up several new sonic possibilities.

  6. 6

    Probe one of the tubes using a tube amp bias probe. The bias probe should read between 36 to 40mA of plate current per tube (you only need to probe one, however).

  7. 7

    Reattach the chassis and the circuit board.

  8. 8

    Adjust the bias adjustment knob until you find a tone that you like.

Tips and warnings

  • If you do not feel comfortable making these adjustments yourself, it is a good idea to take your amplifier to a qualified repair person to do the work for you.

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