International projectionist (Jan-Dec 1936)

20 INTERNATIONAL PROJECTIONIST November 1936 "B" (connections not shown in drawing) and from that center tap up, left, right through R-28 and R-21, and down to the center-tap of the plate secondary, which is the negative terminal. R-28 and R-21 are the grid bias resistors for V-3, and R-28 serves further, as will be seen, in the operation of the plate current meter that is calibrated in percentage. From the cathode of V-2 trace down through R-12 and R-13 to the negative bus which, as seen previously, returns to the center-tap of the power transformer plate secondary. These resistors provide control grid bias for V-2, and R-13 is further associated with the action of that same plate current meter. From the cathode of V-l trace down, left, and down to the negative bus. R-l is the grid bias resistor. The meter does not read the plate current of this tube. Follow the positive bus left as far as possible, and then down through R-20, left, and down to the top prong of the input jack, J-l. From that prong an external line runs to the two soundheads. The return is through ground to the right-hand terminals of J-l, which are joined to the amplifier negative bus. Plate Current Meter The plate current meter calibrated in percentage is shown as a circle surrounding the letter "A," and labelled M-l. This meter is close to the center of the drawing. A short distance to the left of it will be found the single-pole, double-throw switch, J-l, by means of which this meter reads the plate currents of either V-2 or V-3, as desired. Now, these tubes are not the same, but, as the drawing shows, one is a pentode and one a triode and their plate currents are not the same. Conventionally, meter M-l would have two scales, and the projectionist would be obliged to note down or remember the correct reading in milliamperes for each tube. In this case, however, the memory work is done by the circuits of the amplifier. The right-hand terminal of the meter may be traced to the right and up to the negative bus. The left-hand terminal may be traced left to the blade of the switch. If the meter is to read V-2 plate current, the switch is thrown upwara. The meter actually serves as a voltmeter reading the voltage-drop across R-13. which is 23.1 ohms, and part of the grid bias resistance of V-2. All the plate current of V-2 flows through R-13, and the value of that resistance in ohms is so chosen that the meter reads just 100% when the correct value of plate d.c. passes through it. If the switch is thrown downward, the meter reads the voltage-drop across R-21, 0.64 ohms. This unit is part of the grid bias resistor of V-3. The value of that resistor has been so chosen that when the correct plate current for V-3 is flow ing through it the meter needle shrws 100%. In other words, this plate current meter is actually a voltmeter, reading voltage-drop across resistors through which flow the plate currents to be measured. Since these resistors can be given any desired value in ohms (less than the total resistance desired for grid bias) it is readily possible to obtain the same meter deflection for two different plate currents, each of which is correct for its own tube, and to get the same percentage of change in meter reading if either current is more or less than it should be. Grid Bias Circuits The resistors that supply bias to the control grids have been inspected in the course of tracing the plate circuit. The plate d.c. of V-l returns to negative through R-l, 2,000 ohms, in series with the cathode of that tube. The cathode is then positive, with reference to ground, by the extent of the voltage-drop across R-l. The control grid of that tube connects to ground as follows: up, left, down through the secondary of the input transformer, T-l; right, down, left, and down to the negative bus. The cathode of V-l is therefore positive with reference to the control grid, or, which is the same thing, the control grid is negative with reference to cathode, by the extent of the voltage-drop in R-l. R-12 and R-13, in series, serve the same purpose in the case of V-2. The control grid of that tube may be traced to giound, up, left, and down through R-9. That grid is negative with reference to its cathode by the extent of the potential difference across R-12 and R-13. The fact that when the meter switch is thrown upward the meter shunts R-13, is of no importance, since the meter is of much higher resistance than the resistor and has no influence whatever on the bias of V-2. The plate d.c. of V-3 returns to negative through the filament, thence to the "B" filament winding of the power transformer, and from the center-tap of that winding up, left, up and right through R-28 and R-21. The grid of that tube is traced to negative through R-18 and R-19, and is negative with respect to filament by the extent of the voltage-drop in R-28 and R-29. The Speech Circuits The resistances in series with the grids of course have no effect on the grid bias, since no d.c. flows in them; as far as d.c. is concerned there is no potential difference across them, and from the point of view of grid bias they may be considered merely as wires connecting the grids to ground. The same is true of the secondary winding of T-l. Speech input enters Fig. 1 at the ex treme left, through the left-hand terminals of jack J-l. A pair of leads run upward from those terminals to the input transformer primary. A loop around those leads, just above J-l, shows that they are shielded and that the shielding (like the loop) is connected to ground. From the secondary of the transformer speech a.c. is impressed across the control grid and cathode of V-l, directly to the grid, and to the cathode through C-l condenser. Regarding the plate and cathode of V-l as the poles of a generator of amplified speech a.c, trace from the plate up and left, down through the lead, R-4; down, left and down through the left hand C-4 condenser; left as far as possible; up, left, up and left through C-l to cathode. There is of course a parallel return through the source of plate supply; but the resistors R-5, R-6 (10,000 ohms) and the other R-6 (15,000 ohms) act in association with the easy path for a.c. provided by C-8 to confine the plate a.c. of V-l to C-8, and thus effectively "decouple" the speech current of that tube from the speech currents of V-2 and V-3. Still another lead in parallel to R-4 may be traced as follows : from the upper end of R-4 up and right through C-7 to control grid of V-2. From the lower end of R-4 down through C-4 to the negative bus, right past R-25 and then up, and right through C-6 to the cathode of V-2. The amplified speech current derived from V-l is thus impressed across the control grid and cathode of V-2. The speech output of V-2 may be traced from the plate of that tube right and down through the plate load, R-14; down through C-5 to the negative bus, left along that bus to the point between R-13 and R-25, and up and right through C-6 to V-2 cathode. The easy path for a.c. provided by C-5, in association with the decoupling resistor R-6 shown to the right and above it, effectively decouples this circuit from the source of plate supply. From the upper end of R-14 trace up and right through C-3 to the grid of V-3. From the filament of V-3 trace to the center tap of the corresponding filament secondary of the power transformer, and thence up, left, up and right through C-5; up, left along the negative bus and up through C-5 to the lower end of R-14. The alternating voltage-drop developed across R-14 by the speech output of V-2 is impressed across the grid and filament of V-3. The speech output from V-3 flows through the primary of the output transformer, thence to the right along the positive bus, down through C-8 and C-9 to the negative bus ; left, down past R-22 ;