- Turn off the amplifier, and let it to cool off for 20 minutes. The tubes heat up while you use the amplifier. Moving the amp when the tubes are hot can cause them to shatter.
- Unscrew the back panel with a Phillips screwdriver to expose the amplifier chassis. Gently slide the chassis toward you. The 5150 is a fixed-bias amplifier, meaning that there isn't a bias adjustment dial on the exterior. Open the chassis to access the circuit board.
- Put on a pair of latex gloves, and remove the tubes from their sockets on top of the chassis. The latex gloves protect the tube surface from the oils on your hand. Vertically lift each tube, and set them aside in a safe place.
- Unscrew the top of the amplifier chassis. Place it to the side, face up, to access the tube sockets. The wires between the tube sockets and the circuit board are long enough to remain connected with the top off of the chassis.
- Connect the first bias meter probe to a vacant tube socket. Connect the second probe to the connector pins on the tube that you removed from the socket, and connect the third probe to the volt meter. Set the volt meter to "Resistance." Turn on the amplifier, and monitor the reading on the volt meter. Write down the meter reading. Repeat this test on each tube and corresponding tube base before disconnecting the bias meter. Calculate the average bias of the tubes by adding the individual readings and dividing by the amount of tubes.
- Connect the volt meter probe to the resistor on the bias trim pot. On a Peavey 5150, this is 15k ohm resistor, denoted as "r68" in the wiring schematic. Observe the reading. Gently adjust the bias trim pot dial. If the reading is lower than the average bias of the tubes, turn the trim pot to the right. If the reading is greater than the average reading, turn it to the left to reduce the bias. This equalizes the bias.
Source : http://www.ehow.com/how_8361777_change-bias-peavey-5150.html
