Bias Measurement and Adjustment Instructions

Accessing the bias adjustment

1. 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. As you look down the tubes, look at the deck to locate a round metal circle with a tiny hole inside it. This is the bias measurement point. where you insert the voltmeter probe to measure the voltage.
2. 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. 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.
3. 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. 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.  Pin 1 and pin 8 are keyed with either a flat line or a knotch. To measure the bias, place the probe on the resistor lead connecting to the tube.
4. To locate the bias adjustment point on the PC board, locate the trim pot with the blue casing. Refer to the circuit diagram also for the bias adjustment points.

Warning: 

Never attempt the above form of measurement or adjustment yourself, as voltages inside the tube amplifiers can be lethal.

Procedures to measure and adjust the bias setting:

1. Make sure that the amplifier is powered off. Remove the protective metal cage of the amplifier, and make sure that there is sufficient room around the amplifier for you to access the bias measurement points going from the front to the back of the amplifier. Make sure that the connection between the amplifier and the loudspeaker is intact. Set the multimeter for resistance measurement (ohms). Consult the owner’s manual of the multimeter to make sure that you have used the correct settings.
2. Connect the negative (black, "-") probe from the multimeter to the negative("-") speaker binding post of the amplifier.
3. Identify and locate the bias measurement points on the amplifier.. Insert the positive (red, "+") probe of the multimeter into each bias measurement point. The resistance reading on each point should be 10 ohms, within a 10% range.
4. Remove the multimeter probes from the amplifier. Check to make sure that the amplifier is connected to your loudspeakers. Either put your preamplifier in the mute state or turn the volume down to make sure that no signal is coming from the preamplifier. Power on the amplifier and let it warm up for about 10-15 minutes so that the tubes become stabilized.
5. Change the meter setting to measure DC voltage. Insert the negative probe (black) from the multimeter into the negative speaker binding post of the amplifier fitting it into the post where the speaker cable is connected. Insert the positive probe into the bias measurement point of each tube. The reading on the millimeter should stabilize after a little while and indicate a reading between 250to 300 millivolts (0.250 to 0.300 Volts.) The reading on the millimeter may fluctuate a little due to variations in the AC lines. In general, if there is a change of one volt of AC on your line, the bias measurement could be affected by 10 millivolts. It is not unusual for a 120V rated AC line to vary between 115V and 122 V.
6. Note: The formula for determining the current draw according to Ohm’s Law is R = V/I, where R is resistance in Ohms, V is voltage in volts, and I is current in amps. Changing the subject of the formula: I = V/R. The current draw for each tube is therefore the voltage reading divided by the resistance measured in Step 3 (either 5 or 10 Ohms, depending on the age of your amplifier). Therefore, to get a result of the required 27 - 30 milliamps the voltage reading should be 270 - 300 millivolts for the 10 Ohm resistance readings, and 125 - 150 millivolts for the 5 Ohms resistance measurements.

For routine measurements it is not necessary to make adjustments if the voltage is within 10 to 15% of the required reading.

7. Caution: If the voltage reading is 50 millivolts below or above the acceptable range, turn off your amplifier immediately, and contact your VTL authorized dealer or the VTL factory service department immediately.
8. In general a 10 - 15% variation around the 250-300 millivolts range is an acceptable bias measurement. When the measurement is outside this range, you will need to adjust the bias setting by using the following procedures:

1. Keep the positive probe from the millimeter in the same measurement point where you found the out of range bias reading. Locate the bias adjustment point whose number corresponds to the bias measurement point.
2. Insert a 1/8" flat tipped screwdriver (with a plastic shaft or a properly insulated handle) into the bias adjustment trimpot which corresponds to the out of range measurement point. Rotate the screwdriver slowly to see the changes on the multimeter reading. Try rotating the screw in both directions to see the effect of the change. Adjust the setting of the bias to the proper range, i.e. 250 to 300 millivolts.
3. After you’ve completed the bias adjustment changes, it is a good idea to go back and check the bias measurements for all the other tubes since changes made to one tube could possibly affect the setting of all the others.
4. Place the amplifier’s protective cage firmly back onto the amplifier after you are satisfied with the results of your measurements.