Long Tailed Pair Feedback

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.

The calculator below computes the feedback resistor value R_FB based on the desired reduction in voltage gain due to feedback. Long Tailed Pair Voltage Gain takes into account LTP loading caused by the grid resistors in the power amp. Power tube voltage gain is simply the change in plate voltage divided by the change in grid voltage, which can be obtained directly from the load line plotted on the composite characteristic curves.

The calculator assumes that feedback is 180 degrees out of phase with the input signal, which is exactly true only at midrange frequencies. The assumption is justified for classic guitar amps, however, because they use only moderate amounts of negative feedback and have a limited signal passband. High-fidelity amplifiers, on the other hand, need to be concerned with phase shifts at the extremes of bass and treble to avoid instability problems. The calculator also assumes that the presence control is at minimum. For higher control settings at treble frequencies the bypass capacitor C_P kicks in to reduce negative feedback and increase closed-loop gain.

¹Richard Kuehnel, Vacuum-Tube Circuit Design: Guitar Amplifier Power Amps, (Seattle: Pentode Press, 2008).