How to Bias a Spider Valve

Caution: If you aren't sure what you're doing, take the amp to a repair shop.

Historically, musicians have preferred the warm, natural sound of vacuum tube amplifiers to solid-state amplifiers. However, solid-state electronics can provide a much broader tonal menu to the amplifier. The Spider Valve amplifier was an attempt to replicate the vacuum tube sound but with built-in solid-state signal processing electronics. Bias, on an amplifier, controls the signal flowing through the amplifier when the instrument isn't playing.

Things You'll Need

• Volt-ohm meter
• AC power source
• Screwdriver

Instructions

1. Turn the amp off and unplug it from the wall. Turn the amplifier around so that you are looking at the back and the tubes are visible. Remove the back plate by unscrewing the screws and removing the back panel, so you can see clearly and access the tubes and tube mounting areas.

2. Locate the bias trim pots, or potentiometers. These are small knobs near each of the vacuum tubes. These potentiometers are variable resistors that allow you to adjust the bias on each individual tube.

3. Locate a small hole near each tube in the base of the head, where the tubes are fastened. This hole is for your volt-ohm meter.

4. Connect the negative (black) lead from your voltmeter to the negative terminal of the speaker binding post.

5. Connect the other end of the volt-ohm meter to the holes near each tube and measure the resistance. It should be 10 ohms, plus or minus 1 ohm.

6. Remove the volt-ohm meter. Plug in the amplifier, turn the volume down to zero, and let it warm up for 10 to 15 minutes.

7. Connect the volt-ohm meter just as you did before, only this time, set the voltmeter to measure voltage, not resistance. Check the voltage against the specifications provided in the technical manual for that specific amplifier. For most amplifiers, you are looking for a bias of 250mv.

8. Adjust the bias trim pot. Use a flat-head screwdriver with a non-conductive grip. Watch the needle on the volt-ohm meter and adjust the bias trim pot by turning it clockwise or counterclockwise until the needle reads even with the spec, plus or minus about 10 percent.

Tips & Warnings

• The amperage inside a modern guitar amplifier is potentially lethal. Do not attempt this repair unless you are a qualified amplifier technician.

References

• Line6.com: Spider Valve Amplifiers
• TNT-Audio.com: Adjusting the Bias of Your Amplifier
• VTL.com: Bias Instruction

Resources

• Mesa Boogie: Bias Adjust