3 weeks ago (edited)
With a transformer turns ratio of 1:2, that will indeed double the voltage driving the amp, which is also 6 dB. However, that comes at a price. The load impedance seen by the guitar pickups decreases 4-fold. If the amplifier has a 1 Meg input impedance, that now looks like 250k which will dampen the resonance of the pickups. Further, the cable capacitance will be multiplied by 4. For example, a 10 foot cable with 300 pF of capacitance will look like 1200 pF to the pickups with the boost on. Both of these factors will darken the tone by suppressing high frequencies.
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