International projectionist (Jan 1941-Dec 1942)

former, which of course is the positive B source, can be traced up to the first connection, right through a red wire to the terminals leading to the monitor speaker field, and thence back to this same "maroon" line. Obviously, that line in fact is the common B positive bus, the monitor speaker field serving as a filter choke. This is confirmed, if confirmation were needed, by noting that two 8-mfd capacitors (built in a single unit as a "double" condenser) are connected directly across the monitor field circuit, and assist the filtering action. With the chassis of the amplifier established as the negative B common, and this long, horizontal line as the positive B common, any of the individual B circuits can be traced without the slightest difficulty. Two source terminals have been established. Consider the B circuit of any tube, the 6 SF 5 for example. The load terminals are the plate and cathode contacts of the socket of that tube. From the plate terminal trace directly downward through two 200,000 ohm resistors to the positive source. From the cathode trace directly downward to ground. That is. all there is to the B circuit of the 6 SF 5. The 6 N 7 has two B circuits, the cathode being the common negative of both. From cathode trace down, and left through 2,000 ohms to ground. From the lower plate (the phase-inverter section) trace straight down through 200,000 ohms to the positive source. From the upper plate trace right, down, left through 200,000 ohms, and down through 200,000 ohms to the positive source. Tracing from the plates of the 6 L 6 tubes, through the center-tap of the primary winding of the output transformer, shows that these tubes do not derive their plate power from the line that has been filtered by the monitor speaker field, but directly from the ultimate positive source, the center-tap of the rectifier filament secondary of the power transformer. This is readily seen by tracing straight down from the center-tap of the output transformer. The negative side of this circuit may be traced from the line linking the two 6 L 6 cathodes, trace left down through 250 ohms, and left to ground. Grid Bias Circuits The screen grid circuits of the 6 L 6 tubes, however, have for their source the secondary distribution line, the common B positive bus of this drawing. From those screen grids, shown surrounding the bottom of each plate, trace straight down. The negative side of the screen grid circuit is, of course, the cathode circuit of the same tubes. The plate and screen grid circuits of the 6 F 6 monitor amplifier tube are wired wholly in parallel to the corresponding circuits of the 6 L 6 tubes. To trace the grid bias circuits of Fig. 1 it is necessary to look for the grid bias resistors in the cathode lines of the amplifier tubes. Regard the two ends of each such resistor as the source terminals of bias voltage, and the control grid and cathode of each tube as the load terminals for the bias voltage. The cathode line of the 6 FS 5 input tube shows no bias resistor. The cathode is connected directly to ground. Therefore there is no grid bias circuit for that tube; the grid operates at zero bias; but it will be noted that the grid leak of the 6 FS 5 is given the unusually high value of 15 megohms. From the cathode of the 6 N 7 trace to ground through 2,000 ohms. All the plate current flowing through both sections of this tube must complete its path to negative through that resistor. The corresponding voltage drop through that resistor is the source of grid bias. The upper grid is connected to the negative voltage source through the 250,000 ohm volume control; the lower grid is tied to the same source through 10,000 ohms. The cathode, which is the positive load terminal of the grid bias circuit, is joined to the positive side of the source. The grid bias circuit of the 6 F 6 monitor amplifier tube is wired wholly in parellel to the grid bias circuit of the two 6 L 6 power tubes. That the positive leg, the cathode line, is common to all three tubes, has already been noted. In that line is a 250 ohm resistor, through which flows the plate current of all three tubes. The voltage drop across that resistor is the source of grid bias voltage for all three tubes. The negative side of that resistor is grounded. And the grids of all three tubes are also grounded (through suitable grid leaks). From the grid of the lower 6 L 6 trace straight down through 100,000 ohms to ground. From the grid of the upper 6 L 6 trace left, down through 200,000 ohms, left through 10,000 ohms, and down to ground. There is no difficulty about tracing these lines. The negative side of the grid bias resistor in the cathode line is grounded; therefore the grids must connect to ground, and it is only necessary to start at each grid and trace back. The amount of resistance in the grid leaks makes no difference so far as grid bias is concerned, because there is no grid bias current. The grid, being kept negative by the grid bias, repels electrons, and the grid bias circuits are always open at the grids. Thus, there is bias voltage, but no such thing as bias current. Since there is no grid bias current flow in the grid leaks, there is no grid bias voltage drop across those resistors, and their value does not in any way change the amount of bias voltage on the grids of the tubes. That voltage is determined solely by the drop across the grid bias resistor in series with the cathode. Speech current, however, flows as a.c. in the grid bias resistors, and the values assigned to them govern speech volume, and to some extent, frequency response. Four speech input circuits are provided in this amplifier. No. 1 terminal, at the extreme top left of the drawing, is one side of the input from No. 1 photocell, which runs right through a .004 mfd. condenser, and down and right to the grid of the input tube. One side of the input from No. 2 photocell enters at No. 2 terminal in the upper left-hand corner of the drawing and runs straight right' through a condenser to the same grid. The other side of both circuits is common, the shields of the input cables which, of course, are grounded to the amplifier chassis, and therefore to the cathode of the 6 SF 5. Speech Circuits of Fig. 1 Taking the plate and cathode terminals of the 6 SF 5 as the source terminals of that tube's plate speech circuit, the 200,000 ohm plate resistor may be regarded as the load, with the .5 mfd. condenser in series with one leg of the line. A branch circuit exists, of course. From the plate trace right through .01 mfd., down through the volume control to ground, and from ground back to the 6 SF 5 cathode. That portion of the volume control which lies between the arrowhead and ground may be regarded as the source of the speech grid circuit of the upper section of the 6 N 7; the load being the cathode and upper grid of that tube. The arrowhead connects directly to the upper grid; the cathode ties to ground through a 2,000 ohm resistor which is in series with one leg of the line. Cathode and upper plate of the 6 N 7 may be taken as the source of the plate speech circuit of the upper section of that tube. The 200,000 ohm plate resistor may be considered the load. The plate connects (trace down and right) to the right hand end of that res;stor. Its lefthand end connects left, through .5 mfd. to ground; while the cathode of the tube connects down, left through 2,000 ohms, to ground. There are two branch circuits. One may be traced from the plate right through .01 mfd. to the upper end of the 250,000 ohm tone control rheostat; thence down through 0.1 mfd. to ground. The other may be traced right through .01 mfd., down through 200,000 ohms, and left to ground through 10.000 ohms. The voltage drop across that 10,000 ohm resistor may be regarded as the source S INTERNATIONAL PROJECTIONIST