Steve Deckert
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On the new UFO output transformers that feature the switch, it is possible to run a 4 ohm speaker on the 8 ohm tap or visa versa. Impedance is reflected, so when you show an 8 ohm tap a 4 ohm load, that load is reflected through to the primary winding causing it to drop. When it drops, the output tube sees a lower primary transformer which causes it to put out more power with less headroom. Doing the opposite, running an 8 ohm speaker on the 4 ohm tap would raise the primary and thus cause the tubes RMS power to drop, but the headroom would increase. In addition to power, tone is also effected. The lower the primary impedance, the more midrange tone and bass weight. The higher the primary impedance the better the top end transparency and the tighter the bass. This is why the best way to adjust the switch is to not know which way it works, and simply choose the best sound for the particular listening volume your at. I say it that way, because when your back is up against the wall, and you are pushing the amp into the beginning stages of clipping, one of the two positions is always going to hold together longer and it may not be your preferred setting during more relaxed volumes.
Steve
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