B+ Power Supply
The 760 B+ power supply started at the HT winding on the power transformer (see Figure 2). The HT winding stepped up the local mains voltage to 450 VAC. A bridge of four BY100 diodes mounted to the power supply tag strip converted the output from the HT winding to pulsed DC (direct current).
Following the diode bridge was a device called a "Brimistor" (see Figure 1)." A Brimistor was a thermally sensitive resistor encased in a heating element. It was designed to guard against a potentially harmful "switch-on" current surge in the amp.
The electrical resistance of the Brimistor was highest when the amp was cold. This allowed the Brimistor to have the greatest effect of limiting "in-rush" current as the amp was first turned on. As the amp warmed, the heating element inside the Brimistor caused the resistance of the thermally sensitive resistor to drop. After 30 seconds, this drop in resistance allowed the Brimistor to pass the full operating B+ voltage to the circuit.
The 760 power supply also had two 200 uf 350 volt capacitors (C56 and C57). These were wired to each other in series, allowing them to act as a single 100uf primary filter capacitor rated at 700 volts. This combined 700 volt rating was more than adequate to accommodate the ~550VDC from the rectifier bridge.
Each of the 200 uf filter capacitors had a 220k one watt resistor (R100 - R101) straddling their positive and negative terminals. These resistors ensured that the voltage from the 760 power transformer was shared by both filter caps. They also served as "bleeder" resistors, discharging potentially lethal stored voltage when the amp was off.
AC Filament Heater Supply
A 6.3 VAC winding powered the tube heaters supplied voltage to the power, standby and front panel illumination lamps.
DC Supply for the Solid State Preamp
A 20VAC winding in the secondary of the 760 power transformer, four OA200 diodes and a 500 uf capacitor created the 25VDC power supply for the solid state preamp circuit.
KT88 Screen Bias Supply
A 107VAC winding in the secondary of the 760 power supply, a BY100 diode, capacitors C54 and C55 and a 3.3k ohm resistor created the -143VDC screen bias supply. Bias was adjusted by VR13 and VR14, located just behind the tubes.