Figure 1. Simple zero to 500 volt adjustable power supply.
A 3 prong line plug has its ground, round, prong connected to the chassis. The neutral prong connects to the bottom end of a variable transformer, commonly known as a Variac. The neutral prong also connects to the bottom end of the primary of T1 and also to the bottom end of the primary of T2. T1 is a plate transformer with a 350, zero, 350, volt secondary rated at 200 m eh.
T2 is a filament transformer with a 12.6 volt center tapped secondary rated at 6 amps. The hot prong of the plug goes through a 2 amp fuse then through the main power on off switch to the top of the variable transformer. Note: The connection is to the top not the tap on the Variac. The junction of the switch and Variac goes to the top of the primary of T2. The wiper of the Variac connects through another switch to the top of the primary of T1. This switch is the high voltage on off switch sometimes called the standby switch. When this switch is turned off the high voltage is off but the heater voltages for the tubes remains on. The secondary of T1 has a 0.005 u f 3 k v capacitor connected across it. One side of the secondary connects to the anode of a diode 1N4007. The other end of the winding connects to the anode of another diode of the same type. The cathodes of the two diodes are connected together and connect to the positive side of a 100 u f 500 V capacitor. The negative end connects to the center tap of the secondary. The positive side of this capacitor connects through a filter choke to the positive side of another 100 u f 500 V cap. The negative side of this capacitor connects to the negative side of the other 100 u f cap. The filter choke is labeled the largest you can find or afford rated at 200 m eh. The negative side of both capacitors connects to an output binding post labeled simply with a minus sign. The positive side of the second capacitor goes to another binding post labeled with a plus sign. A third binding post is placed so as to form a triangle with the other two. This one is labeled GND and connects to the chassis. The output is labeled zero to 500 volts at 200 m eh. The top side of the secondary of T2 connects to a binding post. The center tap connects to another post just below the first. The low side of the secondary connects to a third post just below the second. A fourth post is positioned just below the third and is connected to chassis. Lines and arrows indicate that the voltage between posts 1 and 2 is 6.3 volts AC. The voltage between posts 2 and 3 is also 6.3 volts AC. The voltage between posts 1 and 3 is indicated as 12.6 volts AC. This completes the verbal description.