The long-tailed-pair phase inverter with negative feedback from the output transformer is one of the most ubiquitous constructs in guitar amp design. A signal from a tap in the output transformer secondary drives the feedback input to the phase inverter to counteract the amplified signal. The resulting closed-loop gain, measured from the phase inverter signal input to the speaker, is less than the forward, open-loop gain without feedback. At less than full power, feedback reduces nonlinear distortion and flattens the amplifier's frequency response. More important, however, are the dynamic effects when the power amp is overdriven.
Overdriving the power amp causes the output transformer secondary to produce a clipped output signal, which represents a lack of output response to a changing input signal. The output transformer provides the source voltage for negative feedback to the phase inverter, so clipping reduces negative feedback. This creates more closed-loop gain, which drives the amp further into an overdriven state, producing even more clipping. The net result is that negative feedback from the output transformer to the phase inverter accelerates the power amp's transition to an overdriven state.
1Richard Kuehnel, Vacuum-Tube Circuit Design: Guitar Amplifier Power Amps, (Seattle: Pentode Press, 2008).
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