How to Set the Impedance for Guitar Amps

Proper match of amp and speaker impedance is required for tube amps.

Impedance is a measure of the electrical resistance of a circuit and is measured in ohms. When connecting a tube-powered guitar amplifier to a speaker system, it is important to match the amplifier impedance setting to the impedance of the speaker system. Mismatched impedance can cause damage to the amplifier such as premature output tube failure. Tube amplifiers typically have either multiple speaker output jacks, each with different impedance settings, or an impedance switch to select the proper impedance.

Things You'll Need

• Guitar amplifier
• Guitar speaker cabinet(s)

Instructions

Determine the Impedance of Your Speaker System, and Set the Amplifier to Match

1. Look at the back of your speaker cabinet(s). Most cabinets have a label indicating the impedance (ohms) of the cabinet near the input jack. If you are using just one cabinet, select the impedance on your amplifier to match the cabinet either by selecting the corresponding output jack or setting the impedance switch.

2. Divide the cabinet impedance by two if you are using two identical cabinets in parallel (each plugged into a different output jack on the amp). Using this impedance value, select a matching impedance on the amplifier either by using the corresponding output jacks or by using the impedance selector switch.

3. Double the impedance value of each cabinet if you are using two identical cabinets in series (both powered from a single output jack on the amplifier). Using this impedance value, select a matching impedance on the amplifier either by using the corresponding output jacks or by using the impedance selector switch.

Tips & Warnings

• Older, worn cabinets may not have the impedance value indicated. In this case, you may be able to find in on the speaker itself. If not, don't use the cabinet as you may damage your amplifier.

References

• KBapps: Survival Tips for Tube Amps