Shop Talk: Speaker Solidarity

January 28, 2008

I have several Fender tube amps with extension speaker jacks. What should I be aware of when connecting extension speaker cabinets to these jacks?
—Paul Belvedere, Novato, CA

With Fender amplifiers, an external speaker cabinet can be used if its impedance is the same or greater than the impedance of the on-board speaker load. For example, with a Fender amp such as a Pro Reverb, Bandmaster, or Twin Reverb—all of which have a 4O speaker load—either a 4O, 8O, or 16O cabinet could be safely connected. With a 4O extension cabinet, the power will be split evenly between the internal speaker and the extension speaker cabinet. With an 8O extension cabinet, the internal speaker(s) would get 2/3 the power, and the extension would only receive 1/3 of the power. Similarly, when you connect a 16O speaker cabinet, the onboard speaker gets 4/5 of the power, leaving only 1/5 of the power for the external cabinet. Don’t connect any extension speaker to a Fender amplifier with a 2O impedance—such as a Super Reverb—as it will cause the total impedance to drop dangerously low, and possibly lead to premature tube failure.

Also, when using multiple speakers, you need to make sure that every speaker cone is moving the same direction at the same time. If one speaker cone moves backwards at the same time the others are moving forward, then the speakers will be out of phase with one another. This causes phase cancellation of the low frequencies, and the sound will not project.

An easy way to determine if your speakers are in phase is to connect a 9-volt battery to a standard 1/4" plug. The (+) terminal of the battery connects to the tip of the plug and the (–) terminal connects to the sleeve. As you insert the plug into your extension cab’s input jack you will hear a distinct pop, and you should be able to observe that the speaker cones in the cabinet are all moving forward together. If one of the cones moves backwards, simply reverse the two wires that feed that particular speaker, and it will move forward when you test it again. When you are sure all of the speakers in the cabinet are moving the same way at the same time, then you need to check to make sure that your amp’s speakers are in phase with the extension cabinet. Disconnect the speaker plug from the amp’s output jack, and connect the plug to the 9-volt battery the same way you did for the extension cabinet. Make sure the amp’s speakers all move forward when the battery is connected, and if necessary, reverse the leads feeding the out-of-phase speaker. All that’s left to do now is hook up your extension cabinet and wail.
—Gerald Weber, Kendrick Amplifiers

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