International projectionist (Jan-Dec 1935)

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8 INTERNATIONAL PROJECTIONIST April 1935 ure 1, the starter relay contains a "timedelay element" of such construction. Current is passed through that element, heating it, and causing it to bend until it closes the same contacts that the plunger closes in Figure 1. Voltage Control Switch Immediately to the left of the timedelay relay is seen the "110-120 Volt Switch", S-l, a double-pole, double-throw switch with a fuse connected across its blades. As shown in the drawing, the switch is thrown to the low-voltage side. We may trace the circuits of this switch, and the remainder of the 110volt line, as follows: Beginning at the upper arrowhead of the line power input at the right of the drawing, left and then upward to the top of the primary winding of the filament power transformer, T-10. From the tap near the bottom of that winding right, down, right, down, left through the upper blade of the switch, up and right to the lower arrowhead. The bottom terminal of the primary winding of the filament power transformer, T-10, is then opencircuited at the left-hand side of the voltage control switch. The plate power transformer circuit can be traced similarly: from the upper arrowhead of the power input down, left and up to the right-hand contact of the time-delay relay. Through the cross-arm of the plunger to the left-hand relay contact. Thence left, down and left through S-2, the "safety interlock switch" that open-circuits the high-voltage transformer when the cover of this amplifier is removed. (Remember that this switch does not afford complete protection; a high-voltage charge still is stored in the filter condensers and must be removed by short-circuiting them.) From the left-hand side of S-2 left, up and left to the bottom terminal of the primary of T-12, the plate power transformer. From the tap connection near the top of that transformer right, down and right to the lower blade of the voltage control switch, S-l. Left through this blade, up through the fuse and right to the lower arrowhead. The top terminal of the plate transformer primary is then open-circuited at the left-hand side of the voltage control switch. Filament Circuits When switch S-l is thrown to the left, for 120-volt input, the blades that now extend rightward from either side of the fuse run leftward instead, the tap-connections of both primaries are open-circuited, a larger number of turns are operative in each primary, and consequently a lower voltage is developed in all the secondaries, compensating for high line voltage. T-10, the upper of the two power transformers, supplies all four filaments of this amplifier. The lowest of the three secondaries of T-10 provides current for the two rectifier tubes, 8 and 9, which are RCA 866's, mercury-vapor rectifiers. The middle secondary of T-10 lights the filament of Tube 7 through its arrowhead terminals, which connect to the arrowheads of that filament. This method of drawing filament circuits reduces the number of wires shown in the drawing and makes it easier to follow. The right-hand arrow-point of the filament of Tube 7 does not make electrical contact with the wire drawn just above it. The filament circuit of this tube is exactly the same as that of its push-pull partner, Tube 6, and the arrowheads of each connect only to the appropriate arrowheads of their power transformer secondaries. The top secondary of transformer T-10 lights the Tube 6 filament. Plate Power Circuits Plate power for this amplifier is derived from the secondary of T-12, through the full-wave mercury-vapor rectifier composed of Tubes 8 and 9. The outer ends of this secondary winding go to the plates of the two rectifying tubes, the return being to the center-tap. Tracing from whichever end of the secondary of T-12 may be positive at the moment, the circuit runs to the plate of the corresponding tube. Thence (from positive to negative) across the mercuryvapor to the filament of that tube. Thence to the filament transformer secondary, the bottom secondary of T-10. From the center-tap of that secondary left about two inches to join a line running upward. (Just below and left of this junction are the two filter condensers, C-29 and C-30, bridging across to the negative side of this circuit.) The line we are now tracing runs upward, right, up, and right to the centertap of the amplifier plate transformer, T-8. From the top terminal of this transformer to the plate of Tube 6, and from its bottom terminal left, down, left, down, left and up to the plate of Tube 7. Thence from the center-tap of the top secondary of T-10 left, up and left to the right-hand side of R-36. Through R-36 and R-34, and downward to the ground connection, just left of the jack, J-4. Through the other ground connection seen just right of J-4 and a bit below it, right, down, left past filter-condenser C-29, down through filter-choke L-20, down past filter-condenser C-30 and through filter-choke L-21; down, right, up and right to the negative terminal of the rectifier which is the center-tap of T-12 secondary. The rectifier filter chokes are in the negative side of the line, the filter condensers, as usual, bridging across the line from positive to negative. The line is not grounded either at negative or at positive, but between, at the point between resistor R-34 and filter-choke L-20. The return from the other tube, Tube 7, is identical, and rejoins the circuit we have just discussed at the ground connection, having separated from it in the primary of T-8. We may trace it from the filament of Tube 7 to the middle secondary of T-10; from the center-tap of that secondary left, down a joggle, left to R-37, through R-37 and R-35 to the ground connection drawn just left and above J-4. In at the ground connection right and a trifle below J-4, right, down, left and down through the filter chokes, as already traced, to the mid-tap of the secondary of T-12. Jack J-4 is normally closed, "shorting" the 2-ohm resistor, R-35. A millivoltmeter, wired to a suitable plug, is plugged into this jack for the purpose of reading the plate current of Tube 7. When this is done the millivoltmeter forms a parallel path around R-35. Directly above R-34 is a similar jack, J-3, which is used in the same way to read the plate current through Tube 6. Resistor R-34 is marked as having 2 ohms resistance; R-35, although not described by the drawing, is of the same value. Consequently, by Ohm's Law every millivolt shown by the meter plugged into these jacks indicates % milliampere of current in the resistor. I = E/R: therefore with a one millivolt reading the current would be E (1/1,000) divided by 2 (2 ohms), or 1/2,000 amperes, or % milliampere. Since normal space current for these tubes is between 60 and 75 milliamperes each, correct reading on the millivoltmeter plugged into these jacks is from 120 to 150 millivolts, or .12 to .15 volts. Grid Bias Circuits The grid bias of the amplifying tubes of Figure 1 is obtained by means of the voltage drop in the two 2,000-ohm resistors, R-36 and R-37, together with the unimportant fraction of a volt drop in R-34 and R-35. The grid bias is therefore derived from a voltage drop in the plate circuit return. Tracing the grid bias part of that circuit in Tube 6, from positive to negative: from the filament of Tube 6 to the upper secondary of T-10; from the center-tap of that secondary left, up and left through R-36 and R-34; from R-34 left and downward to the ground connection; then in at the ground connection shown just to the left of Tube 6, upward through R-31 and through the upper half of the secondary of T-7, and right to the grid of the tube. The grid is then negative with respect to filament, by virtue of the drop in R-36. Assuming 60 milliamperes space current in R-36, the grid bias of this I