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Mesa Boogie Tube Amp Bias

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Q: My Mesa/Boogie guitar amp has the Switchable Bias feature, so I can run either 6L6 or EL-34 power tubes with it. I’m just curious; what would happen if I set the switch the wrong way?

A: Make sure that this stays a hypothetical question, please! It is very important to follow Boogie’s rule on this one: the bias setting must match the tubes in use! Tube and/or amp failure can occur quickly if this switch is not set properly.

EL-34 tubes run hotter than 6L6 tubes. If you install 6L6s and set the switch to “EL-34,” you’ll cook the tubes and possibly other components. If you install EL-34s and set the switch to “6L6,” the EL-34s will be underpowered, wimpy, and thin.

Don’t do it!

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