The input signal is routed directly to the grid of a 12AX7 tube through a 22k ohm grid-stop resistor. Input impedance is 1 megohm. The output of this section drives the clean tone stack, the output of which feeds the Volume control and then the second section of the 12AX7 tube. The output of the second stage is routed to the Clean Master control and then on to the output stage in the Clean mode, and to an internal Overdrive Trim control in the Overdrive mode.
The Overdrive Trim control feeds the first stage of a second 12AX7 tube, the output of which is routed to the Drive control and then to the second section of the tube. The output of the second section drives the overdrive tone stack and then the Overdrive Master control, the output of which goes to the output stage when the amp is in Overdrive. This means that the signal level input to each of the four preamp tube sections can beindependantly adjusted, resulting in complete control of the distortion introduced in each stage. This, together with the choice of different operating points for each preamp stage, is responsible for the incredible overdrive sound.
The output stage consists of a "long-tailed" phase splitter, implemented using a 12AX7 tube, driving a pair of output tubes in push-pull configuration. A Presence control is provided to control the amount of feedback supplied at high frequencies. A robust output transformer is used to provide full bandwidth at high power levels. The power supply provides separate filters for each preamp tube and utilizes a choke between its first and second sections to improve filtering. Series electrolytic capacitors are used to increase reliability and easily handle the high supply voltages.
All components are mounted on a heavy steel chassis. Ceramic tube sockets are used, together with tube shields for the three 12AX7 tubes. The signal path is implemented using point-to-point wiring, with components mounted on terminal strips. Parts are located as close as possible to tubes and controls to minimize wire lengths. The power supply components are mounted on a veroboard "brick," with interconnections made using wires and copper traces.