The amplifier test bed. There are 7 basic sections to this diagram. They are, 1. Power supply. 2. Screen voltage regulator. 3. Phase inverter and amplifiers. 4. Cathode follower driver. 5. Metering circuit. 6, plate supply switch. And 7. Delay relay circuit. Section 1. Power supply. The wide prong of the line plug connects to the bottom end of the primary of a plate and filament transformer and also to the bottom end of the primary of a 6.3 volt filament transformer. The narrow prong of the plug goes to one end of the fuse. The other end of the fuse goes to one side of a single pole single throw switch. The other side of the switch goes to the tops of both transformer primaries. The plate winding is 425 dash 0 dash 425 volts at 100 milliamps. The center tap connects to chassis and common ground. There is a .002 u f 6 kV disc ceramic capacitor connected across the entire winding. Each of the ends of the winding goes to the stator of a switch. The switch is shown in this position and it is labeled B plus on. The other position is labeled stand by and there is nothing connected to these stators. The switch rotors are connected to the two AC inputs of a bridge rectifier. Each diode in the bridge actually consists of two 1N4007 diodes in series. There are 8 diodes in the bridge. There is another .002 u f 6 kV cap connected from one AC input to the other. The positive output of the bridge goes to the positive of a 100 u f 450 volt cap. There is a 100 k ohm 1 watt resistor in parallel with this cap. The negative end of this cap goes to the positive end of another 100 u f 450 v cap and there is another 100 k ohm 1 watt res in parallel with this cap. The negative end of the second 100 u f cap is grounded. The positive output of the bridge also connects to one end of a 10 Henry 90 m A filter choke. The other end goes to the positive end of another 100 u f 450 v cap. There are two of these capacitors and two 100 k ohm 1 watt resistors connected exactly the same as the other two. The end of the filter choke away from the diode bridge is labeled plus 500 volts. The negative terminal of the bridge, two anodes, connects through a 2 k ohm 5 watt resistor to the negative end of a 100 u f 450 v cap. The positive end is grounded. There is a 220 k ohm 1 watt resistor in parallel with this cap. The negative end of this capacitor connects through a 1 k ohm 1 watt resistor to the negative end of another 100 u f 450 v cap. The positive end is grounded. There is another 220 k ohm 1 watt resistor in parallel with this second cap. A 3 k ohm 5 watt resistor goes from the negative end of the second cap to pin 7, cathode, of a 0A2, 150 volt V R tube. The anode, pin 5, goes to pin 7 of another 0A2 and pin 5 of this second tube is grounded. Pin 2 of the first 0A2 goes to a point labeled minus 300 volts. A connection inside the tube connects pins 7 and 2 together. The power supply should never be operated without the V R tubes plugged into their sockets. The 6.3 volt winding of the plate and filament transformer connects to the heaters of two 6BH6s, two 12 A X 7s, two 6CL6s, and a 6SN7. There are also two 150 ohm ¼ watt resistors one from each side of the winding to ground. There is also a number 47 lamp connected across the winding. It is labeled green. There is another number 47 lamp labeled red and one side connects to one side of the 6.3 volt winding. The other side of this lamp connects to the B plus on stator of the switch. The stand by stator connects to nothing. The rotor connects to the other side of the 6.3 volt winding. The 5 volt winding on the plate and filament transformer is not used. The secondary winding of the stand alone filament transformer connects to the heaters of two 6CL6s. One side of this winding is connected to a point labeled screen voltage. This permits the heaters of the screen voltage regulator pass tubes to be placed at the same potential as the cathodes. Otherwise the heater to cathode rating of the 6CL6s would be exceeded. This 6.3 volt winding is not connected to ground in any way. This completes the power supply section. Section 2, screen voltage regulator. One end of a 50 k ohm 5 watt resistor connects to plus 500 volts. The other end connects to the cathode of a 100 volt 5 watt zener diode 1N5378A. The anode is grounded. The diode cathode connects to the screen grid, pin 6, of a 6BH6. The cathode, pin 2, and suppressor grid, pin 7, are grounded. The control grid, pin 1, connects through a 470 k ohm resistor to minus 300 volts. The plate of the 6BH6 connects to the junction of three resistors, two 1 k ohm and a 100 k ohm. The other end of each 1 k ohm goes to the grid, pin 9, of its own 6CL6. The two cathodes of the two tubes, pin 1, and the two suppressors, pin 7, connect together and to the other end of the 100 k ohm resistor. This point is labeled screen voltage. This point connects through a 150 k ohm resistor, then a 500 k ohm pot connected as a rheostat, to the grid, pin 1, of the 6BH6. The pot is labeled screen voltage adjust. A .068 u f 630 v cap connects from the point labeled screen voltage to pin 1 of the 6BH6. The plates, pin 6 and the screens, pin 3, of the two 6CL6s all connect together and to plus 500 volts. End screen voltage regulator. Section 3, phase inverter and amplifiers. Input 1 goes to the top of a 100 k ohm audio taper pot. The bottom is grounded. The wiper goes through a .1 mike cap to the grid, pin 2, of a 12 A X 7. There is a jumper wire shorting out the capacitor. Input 2 goes to the top of another 100 k ohm audio taper pot. The bottom end is grounded and the wiper goes through another .1 mike cap to the grid, pin 7 of the 12 A X 7. There is also a jumper shorting out this cap. Each grid goes through its own 620 k ohm resistor to ground. The two cathodes, pins 3 and 8, connect together and to pin 5 of a 6BH6. The screen grid, pin 6, is grounded. The cathode, pin 2, and suppressor, pin 7 connect together and through a 270 ohm resistor to minus 82 volts. The anode of a 1N5375A, 82 volt 5 watt, zener diode connects to the minus 82 volt point. The cathode of the zener is grounded. The minus 82 v point connects through a 20 k ohm 5 watt resistor to minus 300 volts. Each plate of the 12 A X 7, pin 1 and pin 6, go through its own 100 k ohm res to a point labeled B plus 2. One plate goes through a 15 k ohm res then an 82 pf cap to the other plate. Each plate goes to one end of its own .1 mike cap. The other end of the cap from pin 1 goes to the grid, pin 2 of a second 12 A X 7. The other end of the cap from pin 6 goes to the grid, pin 7, of the second 12 A X 7. One of the cathodes of the second 12 A X 7, pin 3, goes through a 910 ohm res then a 22 k ohm res to ground. The other cathode, pin 8, goes through a 910 k ohm res, then a 12 k ohm res, then a 25 k ohm pot connected as a rheostat to ground. The junction of the 910 ohm and 22 k goes through a 470 k ohm res to the grid, pin 2, of the 12 A X 7. The junction of the 910 ohm and 12 k ohm goes through a 470 k ohm res to the grid, pin 7, of the 12 A X 7. The two plates, pins 1 and 6, go through 100 k ohm resistors to B plus 2. The plate, pin 1, connects through a .1 mike cap to the grid, pins 2 and 9, of a 6CL6. The plate of this 6CL6 connects through a 330 k ohm resistor to the cathode, pin 3, of the 12 A X 7. The other plate, pin 6, connects through another .1 mike cap to the grid, pins 2 and 9, of another 6CL6. The plate of this second 6CL6 goes through a 330 k ohm res to the cathode, pin 8, of the 12 A X 7. The grids of both 6CL6s go through their own 470 k ohm resistors to ground. The cathode, pin 1 and suppressor, pin 7, of each 6CL6 are tied together. Each cathode connects through its own parallel combination of a 430 ohm resistor and a 100 mike cap to ground. The screens of the 6CL6s are tied together and go through a 100 mike 450 v cap to ground. The screens also go through a 6.8 k ohm 1 watt resistor, then another 6.8 k then a third 6.8 k ohm 1 watt res and finally a fourth 6.8 k ohm 1 watt res to plus 500 volts. Each plate, pin 6, goes through its own 20 k ohm 5 watt resistor to plus 500 volts. Each plate also goes through a .47 mike cap to its own output terminal. Each plate also goes through a .015 mike cap to a second set of output terminals. The B plus 2 point goes through a 100 mike 450 v cap to ground. B plus 2 also goes through a 47 k ohm 1 watt res to plus 500 volts. End phase inverter and amplifiers. Section 4. Cathode follower driver. A 150 k ohm ½ watt resistor connects from the minus 300 volt point to the counter clockwise end of a 100 k ohm pot. The other end is grounded. The pot is labeled grid bias. The wiper of this pot connects to the wiper of another 100 k ohm pot which is labeled DC Balance. Each end of this second pot goes through its own 1 meg ohm resistor to ground. Each end also goes to one end of its own 470 k ohm res. The other end of one of the 470 k ohms goes to the grid, pin1, of a 6SN7. The other 470 k goes to the other grid, pin 4, of the 6SN7. Each grid connects to its own input terminal. Each cathode, pins 3 and 6, goes through its own 100 k ohm 1 watt resistor to minus 300 volts. Each cathode also goes to its own output terminal. The two plates, pins 1 and 5, are tied together and go through a 22 mike 450 v cap to ground. The plates also go through a 15 k ohm 1 watt then another 15 k ohm 1 watt resistor to plus 500 volts. End cathode follower driver. Section 5, metering circuit. The meter switch is a 5 position two pole rotary switch. The plus side of the 1 milliamp meter is connected to the rotor of the right hand section. The negative side of the meter is connected to the left hand rotor. The five meter positions are, 1, plate voltage, 2, screen voltage, 3, average grid voltage, 4, screen current, and 5, plate current. There are 3 1N4148 diodes connected from the left hand section to the right hand section on positions 1, 2, and 3. The diode cathodes are connected to the left hand section of the switch. The voltmeter section has 5 connection points. They are labeled, cathode, grid 1 A, grid 1 B, screen grid, and plate. The cathode terminal goes to positions 1 and 2 on the left side and position 3 on the right side. The grid 1 A and grid 1 B terminals connect to separate 120 k ohm resistors. The far ends of these resistors connect together and to one end of a resistor marked star 1 k ohms. The star means adjust the value until the meter reads the same as a digital meter connected to read the same voltage or current. The other end of the star 1 k ohm connects to position 4 of the left hand section. The screen voltage terminal connects through a star 160 k ohm then a 360 k ohm resistor to position 2 of the right hand section. The plate voltage terminal connects through a star 160 k ohm then a 360 k ohm resistor to position 1 of the right hand section. The current metering circuits have four connections. They are screen current plus, screen current minus, plate current plus, and plate current minus. The screen current plus connects to the screen current minus through a 1.6 ohm ¼ watt resistor. The screen current minus connects to position 4 of the left hand section. The screen current plus connects through the parallel combination of two resistors to position 4 of the right hand section. The two resistors are labeled, 51 ohm and star 2 k ohm. The plate current plus connects to the plate current minus through the parallel combination of two 1 ohm ¼ watt resistors. The plate current minus connects to position 5 of the left hand section. The plate current plus connects to position 5 of the right hand section through the parallel combination of two resistors. They are labeled, 220 ohm and star 3.9 k ohm. End meter circuit. Section 6. Plate supply switch. The built in power supply will not provide enough current for the plates of most output tubes. This will come from an external 200 or 300 milliamp power supply. This external supply is switched by the same switch which turns the screen and bias sources on and off. It goes like this. The binding post for the positive connection to the external plate supply goes to the B plus on stator of The switch which has 4 poles. The stand by stator is not connected to anything. The rotor is connected to the point where the plate lead of the output transformer can be connected. Section 7. Delay relay. The heater of a 6C4 is connected in parallel with the red indicator lamp described earlier. Pin 3 to one side and pin 4 to the other. The plate, pin 1 or pin 5, goes to one end of two 220 k ohm `1 watt resistors in parallel to make a 110 k ohm 2 watt resistor. The other end of the paralleled combination goes to plus 500 volts. The grid, pin 6, and cathode, pin 7, are tied together and go to the top end of the relay coil. The bottom end of the coil is grounded. The relay is labeled 2.5 mA pull-in current. The contacts are STSP normally open. One contact is grounded. The other goes to a point labeled output cathodes. End verbal description.
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