To measure the bias, place the probe on the resistor lead connecting to the tube. Pin 1 and pin 8 are keyed with either a flat line or a knotch. The technician will need to remove the bottom cover of the amplifier, turn the amplifier so that the PC board faces out and locate the 10 ohm resistor near the tube socket, between pin 1 and pin 8 of the tube socket. If you cannot locate the bias measurement and bias adjustment points from the top deck of the amplifier, then you will need to ask a technician to locate these points on the PC board.This trim pot could be mounted on the PC board and accessible from the top of the deck via a cut hole on the deck. Locate the bias adjustment point by looking for a trim pot with a blue casing and a set screw in the middle of the pot.where you insert the voltmeter probe to measure the voltage. As you look down the tubes, look at the deck to locate a round metal circle with a tiny hole inside it. Remove the protective cage of the amplifier (if there is one) and look for the bias measurement and bias adjustment points on the top deck of the amplifier.Bias Measurement and Adjustment Instructions Accessing the bias adjustment

Bias guitar amp without probe