The Presence control increases the guitar in the mix, by increasing the mids and high that is caused by reducing the negative feedback. The negative feedback circuit feeds a small part of an out-of-phase signal back into the amp to reduce the effect of certain frequencies. The negative feedback circuit consists primarily of the presence control potentiometer, feedback resistor and where the feedback resistor is connected. The negative feedback affects not only gain and frequency response but also dampening, a lot of tonal variation can be achieved, simply by changing out the value of this resistor or by changing which output transformer tap it is connected to. The output signal is fed back into the amp before the phase inverter and the presence knob controls the amount of negative feedback.