Warning

Amplifiers utilize levels of voltage that may cause serious injury or bodily harm. If you are unfamiliar with the maintenance of tube amplifiers, we recommend seeking assistance from a trained amplifier technician.

About Amplifier Power Tube Bias

There are two types of power tube bias used by PRS amps:

Cathode biasing (self-biasing)
Fixed biasing (adjustable biasing)

The following information concerns PRS amps with fixed (adjustable) biasing. PRS amps with cathode biasing, such as the DGT 15 and Custom 30, are not applicable.

Bias is the means of controlling the amount of current flowing through the power tubes of an amplifier.

If the bias is set too high, the amp's tone will get mushy and the tubes will overheat, potentially damaging the tubes.
If the bias is set too low, the amp’s tone will sound choked and the amp may shut down at extreme settings.

Bias Specifications

PRS amplifier power tubes are biased in pairs and will use the recommended bias adjustment printed on the back of the amplifier near the bias port. This bias specification can also be found in each model’s manual.

Any time power tubes are changed for a new set, the amp will need to be biased. Changing preamp tubes does not require biasing.

How to Bias Your Amp

Setting the bias on a tube amp is an essential part of maintenance and is critical to getting the best sound of the amplifier. All PRS amplifiers ship with manufacturer-matched tube sets, but after usage, even matched tubes can drift apart.

Before biasing the power tubes on a PRS amplifier, make sure you are familiar with the process. Improper biasing may damage the tubes or amplifier.

If you are not comfortable making this type of adjustment, we recommend seeking assistance from a trained amplifier technician.

You will need:

A quality multimeter with a DC mV setting
Flat head or #00 Phillips head screwdriver, depending on the model of amplifier

Steps:

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 ensure that standby (if applicable) is disengaged.
Be sure to set the meter to DC mV.
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)]
Average the readings of the tubes by adding all tube voltages together. If your amplifier has 2 power tubes, divide the sum by 2. If your amplifier has 4 power tubes, divide the sum by 4.
1. For example, in an amp with 2 power tubes, if the left tube reads 28.1 mV, and the right tube reads 25.3 mV, the average will be about 26.7 mV.
Adjust the tubes according to this average. Clockwise adjustments on the bias adjustment screw will raise the bias of all tubes collectively. Counterclockwise adjustments will lower the bias of all tubes collectively.
The bias adjustment is very sensitive. Adjust the bias pot in small increments, do not over-adjust.

Troubleshooting

If while biasing your amp, you find a port has a 0 mV reading, the amplifier could 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 the tubes are lit, and you are getting no readings from the multimeter in the bias ports, the amplifier could have a blown fuse.
If you notice any issues with one or more of the tubes, we recommend replacing them with a new matched set.
If you need further assistance with maintaining or re-biasing your PRS amplifier, please contact PRS Customer Service.