How to Bias Your Amp
Setting the bias on your tube amp is an essential part of maintenance and is critical to getting the best sound of the amplifier. Before biasing the power tubes on your PRS amplifier, please make sure you are comfortable and familiar with the process as there is the potential of damaging the tubes.
What is bias on a tube amplifier?
Bias is the means of controlling the amount of current flowing into the power tubes of an amplifier.
What happens if bias is set too high or low?
If your bias is set too high, your sound will get mushy, your tubes will get hot, and could cause damage to the tubes themselves.
If your bias is set too low, your tone will sound choked, and this can lead to the amp shutting down altogether.
You will need:
- A quality multi-meter with a DC mV setting
- Flat head or Phillips head screwdriver, depending on the model of amplifier
How to Bias your PRS Amplifier
- Plug the amplifier into the wall with a speaker connected to the speaker jack, and no instrument cable plugged into the front jack.
- Turn the amplifier power on, and off standby.
- Take the common probe and insert it into the common (COM) port in the back of the amplifier.
- Take the voltage probe and insert it into either the left or right bias port(s) in the back of the amplifier [left port(s) will represent the left tube(s), right port(s) will represent the right tube(s)]
- When biasing a PRS amplifier, we will bias the tubes in pairs and will use the recommended bias adjustment printed on the back of the amplifier near the bias port.
- PRS amplifiers will ship with matched tube sets, but after usage can drift apart.
- Bias to the average of the pair of tubes by adding the voltages together and dividing them by 2.
[For example, if the left tube reads 28.1 mV, and the right tube reads 25.3 mV, the average will be about 26.5 mV.]
- Clockwise adjustments on the bias adjustment screw will raise the bias of the tubes collectively.
- The bias adjustment is a very sensitive adjustment, so it’s important to make changes to the bias pot in small increments, making sure not to over-adjust.
- If your PRS amplifier has 4 power tubes, you will have 4 bias jacks in the back of the amplifier, each corresponding to 1 of 4 power tubes. When setting the bias on a 4-tube system, you will use the bias adjustment pot to adjust the collective average of all 4 tubes at once.
- If while biasing your amp, you find a port has a 0 mV reading, the amplifier could potentially have a bad tube in that position.
- If you have more than an 8 mV difference between two power tubes, we suggest replacing the tubes with a new matched set.
- If you plug the probes into the back of the amplifier, the tubes are lit, and you are getting no reading from the multi-meter, the amplifier could potentially have a blown fuse.
- If you need further assistance with maintaining or re-biasing your PRS amplifier, please contact PRS Customer Service.
To watch the associated video for this Help Center Article, please follow this link: How To Bias Your Amp: https://youtu.be/f_XW_xcaEeI