UNITY COUPLED CIRCUIT. The MC2102 contains two identical channels. Each channel has three amplification stages: input/phase inverter, driver, and output. It uses the famous Unity
Coupled Output Circuit with eight KT88 or 6550 output tubes (four per channel) in a pushpull parallel configuration. Large transformers with grain-oriented silicon steel cores allow full power output down to 17Hz.
OUTPUT STAGE. The KT88/6550s operate with fixed bias. Because the tubes are additionally loaded in their cathodes they require a large drive signal (approximately 170V) for full output.
This signal is provided by the 12AT7 driver. Boot-strapping the driver to the plate winding of the output transformer results in greater amplification than otherwise possible.
DRIVER STAGE. The driver stage is fed by the 12AX7A input/phase inverter. A resistancecapacitancestep network couples these stages and minimizes low-frequency phase shift. The phase inverter has no coupling capacitors so no additional low-frequency phase shift is introduced.