This Chapter 1
tion. Plate modulation might be added later to give full 'phone output, employing the grid modulator in a driver unit for the Class-B modulator. High-level plate and suppressor-grid modulation systems are the most tolerant in adjustment and maintenance. Grid-bias modulation is somewhat more complicated, as is also the Class-B linear stage with low-level modulation. The Class-B linear is used by amateurs to only a limited extent.
General Construction Practice
# Audio units for simple transmitters can be built up bread-board style, although a metal
FIG. 1202 —CIRCUIT OF A SIMPLE SPEECH-AMPLIFIER AND MODULATOR SUITABLE FOR LOW-POWER PLATE MODULATION, OR FOR SUPPRESSOR- OR CONTROL-GRID MODULATION Ci — .1 rfd.
C2, C3, C5 — 2-ixfd. electrolytic, 450-volt rating.
Rl — 500,000-ohm potentiometer.
T—-Class-B input transformer, ratio approximately 1:1.
A power supply furnishing 2.5 volts at 3 amperes and ISO to 250 volts at 40 milliamperes is required.
chassis foundation is preferable for a permanent job. Present practice tends toward unit construction on metal chassis, with rack mounting. Foundation bases of the type used for modern receivers are admirably suited. Shielding is important where high-gain audio systems are used, it being especially important to keep r.f. from overloading the low-level grid circuits. When two or more stages of speech amplification are used, particular care must be taken to prevent motorboating and distortion resulting from inter-stage feed-back. Coupling transformers should be isolated from each other or placed for minimum reaction between their magnetic fields. Proper positions can be determined by turning the transformers, one with respect to the others, until minimum hum or instability is obtained with the unit in operation at full gain. It is advisable to keep modulation chokes and transformers well away from other audio equipment because the strong magnetic field about the high-level audio unit is likely to cause trouble. Transformer cases should be grounded to the negative side of the circuit.
Microphone cables should be shielded and the shield connected to ground. It is generally good practice to shield the high-outpui SaiQ inPut circuit separately and keep it away from the high-level audio and r.f. sections of the transmitter. It is well to couple a speech-input amplifier by a step-down transformer (tube-to-line) in its output, through a twisted-pair to a line-to-tube step-up transformer into the higher-level audio circuit. Such an impedance matching combination is especially recommended with high-imped-ance microphones which require short leads to the first audio stage or pre-amplifier. Interconnecting leads and cables should be thoroughly shielded and the shields grounded. Radio-frequency chokes may be necessary between modulator and modulated amplifier in high-voltage supply leads.
A.c. filament and power-pack high-voltage supplies may be used for all stages, although more than ordinary filtering should be used for high-gain amplifiers. Filtering or decoupling in individual plate- and grid-feed circuits is advisable, as illustrated in the high-gain circuits which will be described.
Modulator Combinations
# Class-A and Class-AB modulators using receiving type tubes for grid-bias and suppressor-grid modulation in medium- and high-power transmitters, and for plate modulation in low-power transmitters, are essentially the same as the drivers for Class-B modulators which are described in the following pages and therefore require no special description. (See tables in Chapter Five for ratings.) They will differ only in the output transformer impedance ratio, which should be chosen to match the load impedance requirement of the particular audio tubes used to the modulating impedance of the r.f. stage. In the case of grid-bias or suppressor-grid modulation, the output may have a "dummy" load resistance connected across it to maintain the modulator load resistance practically constant, as shown later in several examples. Higher-power Class-À and Class-AB modulators are planned readily for the larger audio output tubes from the data given in the Class-A modulator table.
One of the simplest low-power modulator or driver arrangements, " capable of approximately 3-watt maximum output, is diagrammed in Fig. 1202. This unit is especially intended for grid-bias modulation of the gen-eral-purpose transmitter described in Chapter Nine, with the final stage bias circuit revised as shown later in Fig. 1219. It is also readily adaptable to suppressor-grid modulation of pentode stages using RK-20's, 803's, and similar tubes. It can be used as well to plate-
modulate a low-power Class-C stage operating at about 5-watt or so input, in which case the loading resistor jRs would be omitted.
A General-Purpose 18-Watt Audio Unit
• Fig. 1203 gives the circuit and specifications of a general-purpose audio unit having many applications. It can be used as a grid-bias or suppressor modulator for any size amateur transmitter, or as the driver for Class-B modulators using tubes including 203-A's, 838's and other types in the 100-watt classification. The input circuit is adapted to either crystal or double-button carbon microphones, a resistance-coupling network being used with the
crystal mike
FIG. 1203 — CIRCUIT OF THE GENERAL-PURPOSE 18-WATT AUDIO UNIT This general-purpose unity with its audio output of 18 watts and high overall gain, is capable of driving a high-power Class-B modulator or can be used for public-address work.
crystal mike
FIG. 1203 — CIRCUIT OF THE GENERAL-PURPOSE 18-WATT AUDIO UNIT This general-purpose unity with its audio output of 18 watts and high overall gain, is capable of driving a high-power Class-B modulator or can be used for public-address work.
Ci — 0.1-fjfd. 400-volt metal-cased. C2, C& — 25-nfd. 25-volt electrolytic. C4 — 8-ufd. 200-volt electrolytic. C5 — 25-ijfd. 25-voll electrolytic. Ce, Cr — 0.1-nfd. 400-volt tubular. Cs — .003-fifd. 400-volt tubular. C9 — ,001-nfd. 600-volt tubular. Cío, C11 — 8-tifd. 200-volt electrolytic. C12, C13, Ci4 — 8-pfd. 450-volt electrolytic. Rl — 200 ohms, 1-watt. «9 — 20,000 ohms, 1-watt. i?2 — 10,000 ohms, 1-watt. Rio — 0.25-megohm vol-Rz, R4— 200 ohms, 1-watt. ume control.
R5 — 5 megohms, 1-watt. Ru — 2200 ohms, 1-watt. Rq— 2200 ohms, 1-watt. R12 — 50,000 ohms, 1-watt Rj — 0.1 megohm, 1-watt. R13 — 0.25-megohm vari-JÍ8 — 0.25 megohm, 1-watt. able (tone control).
Important — The power supply components shown in this list will furnish 25 watts of field power for dynamic speakers. In case no speakers are used, a 5000-ohm, 50-watt resistor must be connected across the high-voltage output.
Riq — 1500-ohm semi-variable, 30-watt.
L2 — Push-pull input transformer connected as shown (Thordarson T-7431).
L3 —10—30 henry swinging choke, 150 ma. (Thordarson T-7429).
T1 — Push-pull input transformer, 800-ohm secondary, ratio 1.5 to 1, total primary to total secondary (Thordarson T-7432).
T2 — Power transformer, 450 volts at 150 ma. with tap at 75 volts, 6.3-volt, 5-volt and 2.5-volt filament windings (Thordarson T-7428).
T% — Output transformer, depending on load to which amplifier is to be coupled.
latter to avoid hum pick-up and, at the same time, to bring the amplifier input down to approximately the same level as that obtained with the less sensitive crystal type. A dual power supply, integral with the amplifier, takes care of amplifier plate requirements
50-Watt 6L6 Modulator Or High-Power Class-B Driver
0 The 6L6 speech-amplifier unit shown in Fig. 1204 is also a general purpose affair, in that substitution of a suitable output transformer
FIG. 1205 —WIRING DIAGRAM OF THE 61.6 SPEECH-AMPLIFIER-MODULATOR
Ci — 10-pfd., 25-volt electrolytic. ('■2. Cs— 2-nfd., 200-volt electrolytic. Ci — 0.1-pfd. paper, 400-volt. Cs — 0.5-nfd. paper (or larger). Cq, C7 — 4-fifd., 400-volt electrolytic. Cs — 0.25-ufd. paper, 400-volt. C9 — 25-ufd. electrolytic, 50-volt. Rl — 5 megohms, V2 watt. R$,R3 — 3500 ohms, V2 watt.
Ri, IÎ5, Re — 50,000 ohms, V2 watt. R7, Rs — 0.25 megohm, V2 watt. J?9 — 0.5-megohm volume control. Rio — 100,000 ohms, V2 watt. Rl 1 — 10,000 ohms, Vl watt. R12 — 500 ohms, V2 watt. W13 — 2500 ohms, I watt. Rli — 15,000 ohms, 10-wall.. Rl5 — 1000 ohms, 10-watt.
Tl — Audiotransformer,singleplate to push-pull grids, ratio 3:1 (Thordarson T-574I).
Tz — Input transformer for coupling push-pull 6C5's to 6L6 grids (Thordarson T-8459).
Ts — Output transformer,3800-ohm load plate to plate, see text (Thordarson T-8470).
FIG. 1205 —WIRING DIAGRAM OF THE 61.6 SPEECH-AMPLIFIER-MODULATOR
Ci — 10-pfd., 25-volt electrolytic. ('■2. Cs— 2-nfd., 200-volt electrolytic. Ci — 0.1-pfd. paper, 400-volt. Cs — 0.5-nfd. paper (or larger). Cq, C7 — 4-fifd., 400-volt electrolytic. Cs — 0.25-ufd. paper, 400-volt. C9 — 25-ufd. electrolytic, 50-volt. Rl — 5 megohms, V2 watt. R$,R3 — 3500 ohms, V2 watt.
Ri, IÎ5, Re — 50,000 ohms, V2 watt. R7, Rs — 0.25 megohm, V2 watt. J?9 — 0.5-megohm volume control. Rio — 100,000 ohms, V2 watt. Rl 1 — 10,000 ohms, Vl watt. R12 — 500 ohms, V2 watt. W13 — 2500 ohms, I watt. Rli — 15,000 ohms, 10-wall.. Rl5 — 1000 ohms, 10-watt.
Tl — Audiotransformer,singleplate to push-pull grids, ratio 3:1 (Thordarson T-574I).
Tz — Input transformer for coupling push-pull 6C5's to 6L6 grids (Thordarson T-8459).
Ts — Output transformer,3800-ohm load plate to plate, see text (Thordarson T-8470).
and furnishes bias to the Class-AB final stage. The design of this unit was described in detail in Jan. 1936 QST by W9GSA and W9UVP.
FIG. 1204 — METAL-TUBE SPEECH UNIT WITH PUSH-PULL 6L6 OUTPUT
This four-stage amplifier will deliver an audio output of approximately 50 watts with negligible distortion. The gain is sufficient for the popular diaphragm-type crystal microphone.
FIG. 1204 — METAL-TUBE SPEECH UNIT WITH PUSH-PULL 6L6 OUTPUT
This four-stage amplifier will deliver an audio output of approximately 50 watts with negligible distortion. The gain is sufficient for the popular diaphragm-type crystal microphone.
makes it adaptable either as a complete modulator or as a driver for Class-B units employing anything up to a pair of 354's or 204-A's. The voltage gain to the grids of the 6L6's is more than sufficient for crystal microphones of the diaphragm type, a peak input of about 0.005 volt being sufficient to drive the final tubes to full output. As shown in Fig. 1205, the input stage uses a 6J7 (equivalent to the 57 or 6C6) pentode; this tube is resistance-coupled to a 6C5 triode intermediate amplifier. The driver consists of a pair of 6C5's in push-pull, transformer-coupled to the preceding stage. The 6C5's are capable of delivering sufficient power for excitation of the 6L6 grids. The input transformer, Ti, is specially designed for the purpose. The 6L6 output transformer, Ti, also is a special job, arranged with a tapped secondary to work into loads of 2500, 5000 or 7500 ohms for modulation purposes; its turns ratio is such that the plate-to-plate load on the 6L6's is 3800 ohms.
The low-level speech-amplifier section occupies the left-hand section of the chassis. The design of the whole unit is perfectly straightforward. The microphone jack is on the back of the chassis near the 6J7 tube; the first 6C5 is at the front left-hand corner, with the gain control conveniently situated. To its right is the single-tube to push-pull coupling transformer; back of the coupling transformer are two
FIG. 1206 —DIAGRAM OF THE 6L6 POWER-SUPPLY UNIT Tl — Receiver power transformer; high-voltage winding to deliver app. 325 volts d.c. at 50 ma.; 5-volt, 2-amp. rectifier winding; 6.3-volt, 1.5-amp. filament winding (Thordarson T-7078). 7*2 — Filament transformer, 5 volts at 3 amps., 6.3 volts at 2 amps.
(Thordarson T-7984). Ts — Plate transformer, to deliver 400 volts at 100 ma. through choke-input filter (Thordarson T-5503). Ll — 50-ma. filter choke, 30-henry commercial rating. L.2— Input choke, 26 to 12 henrys, 250 ma. (Thordarson T-7551). Cl, C2— Double 8-pfd. dry electrolytics, 450-volt. Sif 1, .S ft 2 — S.p.s.t. toggle switch.
FIG. 1206 —DIAGRAM OF THE 6L6 POWER-SUPPLY UNIT Tl — Receiver power transformer; high-voltage winding to deliver app. 325 volts d.c. at 50 ma.; 5-volt, 2-amp. rectifier winding; 6.3-volt, 1.5-amp. filament winding (Thordarson T-7078). 7*2 — Filament transformer, 5 volts at 3 amps., 6.3 volts at 2 amps.
(Thordarson T-7984). Ts — Plate transformer, to deliver 400 volts at 100 ma. through choke-input filter (Thordarson T-5503). Ll — 50-ma. filter choke, 30-henry commercial rating. L.2— Input choke, 26 to 12 henrys, 250 ma. (Thordarson T-7551). Cl, C2— Double 8-pfd. dry electrolytics, 450-volt. Sif 1, .S ft 2 — S.p.s.t. toggle switch.
electrolytic by-passes, and C7, followed by the push-pull 6C5's. The input and output transformers, as well as the 6L6's, are readily identified. The jack for measuring 6L6 plate current is mounted on the back of the chassis, along with a stock two-terminal strip for the output.
Two power supplies are used, one furnishing 400 volts at 100 to 200 ma. for the 6L6 plates and the other (rated to deliver 300 volts) furnishing plate power to the low-level stages as well as screen voltage and grid bias for the 6L6's. The circuit is shown in Pig. 1206. All tubes except the 6L6's get filament power from the first transformer, T\. A separate filament transformer, Ti, takes care of the 6L6's and the 83 rectifier of the 400-volt supply. An ordinary condenser-input filter with one choke (this choke is mounted underneath the power-supply chassis) is used on the 300-volt supply. The 400-volt supply has choke input, with the two sections of a double-8 electrolytic condenser in parallel across the output.
The fixed bias for the 6L6's is obtained from the 300-volt supply. Reference to Fig. 1206 will show that there is no ground on the negative side of the 300-volt supply (outlet A). The total current from this supply is made to flow through the right hand section of #15 (Fig.
1205) to ground; by means of the adjustable tap on #15 the bias voltage is set at 25 volts. is a bleeder resistor to load the 300-volt transformer to full capacity. It is desirable to do this so that the current through i?i5 will be as heavy as possible, thus maintaining the bias fairly constant even though grid current flows. #13 drops the voltage to the proper value for the speech-amplifier plates.
The power terminals on both speech and power-supply units are four-prong tube sockets. Connections are made by means of four-wire cables with plugs at each end.
A few words about operation: Provided the values given are followed, the only adjustment to be made is that of the bias on the 6L6's. Preferably, this should be done with the aid of a high-resistance voltmeter, with everything except the 400-volt plate transformer turned on. However, if no such voltmeter is available, a method which works about as well is to set the tap on #15 so that the plate current to the 6L6's is slightly over lOOma. It is essential that the screen voltage be exactly 300 volts, since the plate current is quite sensitive to changes in screen voltage.
With the values given in the circuit diagrams, the whole system is perfectly stable (a ground connection must be used, of course) and the hum level is negligible. Should the hum increase perceptibly when the microphone plug is inserted, it will be necessary to shield the grid cap of the 6J7. (See June 1936 QST for a more detailed description.)
The same circuit is used in the unit at the left in Fig. 1201, in which both amplifier and power supply are built on the same chassis for 19-inch rack mounting. All components are as given in Fig. 1205 except for the transformers T<i and 7V To minimize power-supply hum in the unit assembly a balanced type coupling transformer is used for Ti (Thordarson T-9004), and a Class-B input transformer for coupling 6L6's to 354's (UTC Type 18126) is used for T3.
Economy Modulator for 50-Watt Transmitters
# The modulator illustrated in Fig. 1207 and diagrammed in Fig. 1208 is intended especially for speech modulation of a Class-C amplifier operating with a plate input of 140 ma. at 600 volts (approximately 85 watts). Although the modulator tubes have a normal output rating of only 20 or 25 watts, 100 per cent speech modulation of this Class-C input is obtained with negligible distortion, following the principles outlined in the preceding chapter. This
FIG. 1207 —THE ECONOMY MODULATOR UNIT CONTAINS A CRYSTAL-MICROPHONE SPEECH AMPLIFIER, DRIVER AND CLASS-B MODULATOR, AS WELL AS A POWER SUPPLY FOR THE LOW-POWER STAGES While the 469s in the Class-B stage normally would be considered to have an audio output in the vicinity of 20 watts, for speech work they can readily be made to modulate a Class-C input of 80 watts, as explained in the text.
is accomplished by operating the modulator at slightly higher than usual plate voltage and by
FIG. 1207 —THE ECONOMY MODULATOR UNIT CONTAINS A CRYSTAL-MICROPHONE SPEECH AMPLIFIER, DRIVER AND CLASS-B MODULATOR, AS WELL AS A POWER SUPPLY FOR THE LOW-POWER STAGES While the 469s in the Class-B stage normally would be considered to have an audio output in the vicinity of 20 watts, for speech work they can readily be made to modulate a Class-C input of 80 watts, as explained in the text.
FIG. 1208 —CIRCUIT DIAGRAM OF THE SPEECH AMPLIFIER AND ECONOMY CLASS-B MODULATOR The power supply furnishes plate and filament power for the first three tubes only; the Class-B stage must be supplied from a separate source. If a power transformer having an additional 2.5-volt winding is used, filaments of the 46's may be heated from the second winding.
Tl, T2— Class-B input and output transformers; (National Type Bland BO respectively). The input transformer should have a turns ratio, total pri* mary to one-half secondary, of 2:1. Output transformer turns ratio should be be-tween 1.05:1 and 1.3:1, total primary to total secondary.
t3 — Midget power transformer» 275 volts each side center-tap with 5-volt and 2.5-volt windings. (Thordarson type T-5002.)
L — 22-henry, 35-ma. filter choke (Thordarson type T-1892).
J— Single closed-circuit jacks•
RFC— Short-wave choke (National type 100).
proper output transformer ratio (total primary to total secondary turns ratio of 1.15-1). The driver stage requirement of 50 volts peak grid swing is easily delivered by the two-stage speech amplifier with crystal microphone input. The total voltage amplification for the two stages is approximately 1400, adequate for close talking.
The complete assembly is built up on a chassis measuring 7 by 11 by 2 inches. The first-stage 57 is at the rear left corner; midway on the left-hand edge is the second 57 (triode connected) with the 45 driver at the left front. The Class-B input transformer is behind the meter panel, with the 46 modulator tube directly behind. The Class-B output transformer is in the rear right corner, the power transformer for the speech-amplifier and driver stages being at the front, beside the 80 rectifier. With the transformers placed as shown, hum pick-up at the input is imperceptible, although different orientation of the power transformer with respect to the output transformer brings the hum up considerably. In a compact as sembly of this type, the location of the respective transformers for minimum hum can be checked readily by listening with a pair of 'phones connected to one of the audio transformer windings while moving the transformers about slightly with the amplifier in operation. The small filter, in conjunction with the resistance-capacitance filtering of #12 and C5, is satisfactory. It is especially important in the first stage that the screen voltage be maintained at the proper value. The voltage divider Rvr-Rn serves this purpose. Particular care should be taken to see that the screen voltage is not too high, since this will reduce gain, cause distortion and may even give rise to supersonic oscillation.
With speech modulation of the Class-C amplifier input specified, the plate current should swing no higher than approximately 80 ma. on speech peaks. Class-B grid current should be between 10 and 20 ma. under corresponding conditions. The Class-C plate current should remain constant, of course. Because of the different voltages on the Class-B
- FIG. 1209 — CIRCUIT OF THE 100-WATT AUDIO SECTION
Ci, C2 — .001 fi/d. C3 — 25-jj.fd., 25-volt electrolytic. C4, C5, Ce — .1 ufd. C7 — 8-ufd. electrolytic.
C9, ClQ — 8-fjfd. electrolytic. Cn, C12, C13 — .1 tifd.
R13 — 1500 ohms, 2-watt. Ru — 3000 ohms, 5-watt. W15 — 40,000 ohms, 2-watt. Rid — 50,000 ohms, 2-watt.
Pi —250,000-ohm (each section) dual potentiometer.
P2 — 500-ohm, 10-watt potentiometer.
P3 — 1000-ohm, 2-watt potentiometer.
Jl to ./," , inc. — Single circuit-closing jacks.
Ta — Class-B output transformer.
Te — Power transformer, 250 watts, 600 volts c.t.
and Class-C stages, separate power packs of good regulation are advisable.
A 100-Watt AU Push-Pull Audio Section
9 The speech-amplifier and modulator unit diagrammed in Fig. 1209 has an audio output capability of 100 watts with double-button carbon microphone input. It employs a number of features to minimize distortion, including push-pull speech amplification. It is designed for use with a Class-C amplifier using 1000-volt tubes, such as a pair of 800's or RK-18's in parallel or push-pull, or a single 100-watt type. By careful adjustment, with speech modulation it also can be used to modulate still higher Class-C input than the usual 200 watts, having been used successfully by its designer (W4UP) to modulate a pair of 261-A tubes in push-pull operating at a plate input of 300 watts (230 ma. at 1300 volts).
As in the smaller unit just described, although a compact assembly is used hum and feed-back are eliminated by careful placement of the components and shielding. Tube shields are used on the input stage and copper braid shields the grid-leads, while resistors such as Ri and R2, R$ and Ri, R\i and B13, Bs and J?9,
Input
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