Survival Tips for Your Guitar Tube Amplifier: Speaker Impedance

Speaker Impedance

The proper matching of the impedance between your tube amp and speaker is extremely important. Improper matching causes tube wear and is a common cause of tube failure. Some amplifiers are more sensitive to this than others, notably the Marshall amps. Pay attention that the Marshall’s impedance selector is on 16 ohms when you’re running a common 16-ohm Marshall cabinet, and reduce it to 8 ohms when adding a second cabinet. Always check your cabinets by measuring with a volt meter on the ohm scale (these readers should always read low).

Beware the dangers of using a power attenuator with your Marshall, as most power attenuators do not match impedances closely enough for these amps. Using a power attenuator might let your Marshall distort at lower levels, but at the expense of much more rapid output tube wear. Fender amps are not as sensitive power attenuators as Marshalls, because of differences in design in the output section. However, since the tubes are putting out full power into the attenuator, they will wear out quicker than if they were just coasting at a moderate output level.

Of course, if you like the sound you get with the attenuator, be prepared to spend a little bit more on power tubes.