Figure 18, simplified schematic of HP 712B rack mount regulated power supply. This power supply has two outputs. One is fixed minus 300 volts and the other is zero to 500 volts adjustable by a large knob on the front panel. The current rating of the minus 300 volt output is 50 mA while the 0 to 500 volt output has a rating of 200 mA. In this simplified diagram the rectifier filter circuits are represented by batteries. Further, the error amplifiers are represented by rectangular blocks that can be thought of as vacuum tube operational amplifiers. The two blocks are similar but not quite identical. This will not become an issue until the next diagram. For now, each block has two signal inputs which are located on the sides. These are the two grids of a 12 eh X7. Each block has two signal outputs which are located on top. These are the plate connections of a second 12 eh X 7. Each block contains two 12 eh X 7s in a differential or push pull DC coupled configuration. The plates of the first 12 eh X 7 are connected directly to the grids of the second one. At the top in the center, between the two outputs is the B plus input. At the bottom center is the minus 300 volt connection. The minus 300 volt regulator will be described first. The battery labeled E2 has its negative terminal connected to the minus 300 volt line. Its positive terminal connects to the plate of the pass tube 6W6GT. The positive terminal of E2 also connects through a resistor to the screen grid of the pass tube. The cathode of the pass tube goes to the common line. The grid of the pass tube connects through a resistor to the right hand output of the minus 300 volt error amplifier. The left hand output and B+ connection go to common. A voltage reference tube, 5651, has its cathode connected to minus 300 volts. Its anode connects through a resistor to common. The anode also connects to the right hand input of the error amplifier. The left hand input goes to the wiper of a pot labeled minus 300 volt adjust. Screwdriver adjustable. The bottom of the pot connects to minus 300 volts and the top connects to common. The minus 300 volt connection of the error amplifier goes to minus 300 volts. The minus 300 volt line connects to the minus 300 volt output. The common line connects to the common output.
Now for the 0 to 500 volt regulator. The minus 300 volt connection of the zero to 500 volt error amplifier connects to minus 300 volts. The left hand input connects to the anode of the 5651 reference tube. The B plus connection goes to common. The left hand output goes to the grid of the pass tube which is actually 4 6L6GCs in parallel. The right hand output goes to the cathode of the pass tube. The screen grid of the pass tube goes to the positive of E g 2. The negative of E g 2 connects to the cathode of the pass tube. The plate of the pass tube goes to the positive of E1. The negative goes to common. E1 is indicated as being a variable supply. More about this in a moment. The right hand input of the error amplifier goes to one side of a pot connected as a rheostat. The other side goes to minus 300 volts. This is a screwdriver adjustment and is indicated as 500 volt adjust. The right hand input also goes to another pot connected as a rheostat which is a user control and is labeled output voltage. A dashed line indicates that the pot is ganged with E1. The other end of this pot goes to another pot connected as a rheostat. The other end goes to the cathode of the pass tube which is also the positive output. This last rheostat is indicated as zero volts adjust. This completes the simplified diagram.