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Electronics-World-1959-05

www.americanradiohistory.com WE HEAR a lot these days about the value of "talk power" especially from the gang now using SSB. Although not directly comparable to SSB, the modulation system to be described provides a signal with very high effective modulation percentage and unusually high "talk power." It is capable of excellent voice quality, is easy to adjust, and a single tube such as the 6L6 will fully modulate up to a kilowatt peak power input. The schematic of the modulator, Fig. lA will he recognized as basically the circuit of the original clamp modulator first introduced in 1950 as a simple and economical step from c.w. to phone operation. Minor modifications of the original circuit, as shown in Fig. IB, include use of an ordinary 6L6 in place of the 6Y6, elimination of the self -bias resistor which is not required in this system, and substitution of the vacuum tube diodes of a 6SQ7 for the crystal diode generally used. Similarities bet ween conventional clamp tube modulators and this system, however, end at the modulator. As shown in Fig. 2, modulation in this system is applied to the screen of the r.f. driver, or buffer, where the voltages and currents are of much smaller magnitude, rather than to the final amplifier of the transmitter as is usual with clamp tubes. Other important advantages in using this system to modu- A IeRG_INAL CLAMP CIRCUIT 6Y6 R AS SIS LOW -LEVEL CLAMP CIRCUIT CURRENT 9F 1HOR VOLTAGE) TO SCREEN OF FINAL A r POS B NEG Cr PARTS OF AUDO c CYCLE PRESENT IN DC la) Cw POS PARTS OF AuD.O CYCLE ONLY NO NEG PRESENT TO OF SCREEN BUFFER LOW CURRENT B+ (LOW VOLTAGE) NO BIAS RESISTOR (BLS MODULATOR AIDS IN SUPPRESSING POS CL.. PPE R AUDIO ON ITS GRID.) (NO BACK -TO -FRONT CONDUCTIVITY, LESS VARIAT ON IN CHARACTERISTICS) (GI Fig. I. Changes made in early clamp -tube circuit to allow low -level modulation, less power consumption, and lower voltages. r I I I II I 1 t!il late at low level are explained in the following paragraphs. Operation of Modulator Fig. 2 shows the method of clamping on the screen of the driver, which in this case is an 807. The d.c. is fed from a fixed 375 -volt source through the 25,000 -ohm. 4 -watt resistor, R,, to the screen of the 807 and also to the plate and screen of the triode -connected 6L6 modulator. With no audio signal at the control grid of the unbiased 6L6, the plate and screen of the tube draw approximately 13 ma. through RI, producing a voltage drop of 335 volts and thus placing the screen of the 807 at 40 volts. This, of course, causes the 807 to operate at reduced r.f. output which, in turn, reduces the transmitter carrier output. On the other hand, speaking into the microphone produces an audio signal which is rectified by the diodes of the 6SQ7 and applied as negative bias (45 v. peak) to the grid of the modulator. This causes the modulator plate /screen current to decrease and the 807 screen voltage to rise, reaching peaks of 250 volts in accordance with the amplitude of the audio signal. Because of the clipping action of the diode section of the 6SQ7, combined with the zero -bias operating point of the modulator, positive peaks of audio at the grid of the modulator are neg- By MARVIN L. GASKILL W2BCY Circuit provides high "talk power," good voice quality, and is easy to adjust. Permits single 6L6 to fully modulate a kw. of peak power input. Low -Level Clamp Modulator 807 RF DRIVER FOR PA. Fig. 2. Triode connected 6L6 modulator is clamped to r.f. driver screen. Modulator draws current of only 13 ma. on zero audio. Fig. 3. Preferred circuit for r.f. driver and final amplifier. .005 A F. 6SQ7 RECTIFIER 2015. r_+ 40-250 V. 6L6 M00. NEG. PART OF AF CYCLE CLIPPED BY DIODES 8 6L6 FROM EXCITER R F RFC 807 DRIVER ANT E 60 AUDIO FROM SPEECH AMP (45V) PHONE CW -4SV TO ZERO 25K 4 WATT RI +3 5V. (FIXED SOURCE) TOOK R2 50K GRID DRIVE -45 V FIXED BIAS TO MOD B+ LINK COUPLING PREFERABLE t FIXED BIAS REOUIRED FIKE SCREEN ROH SUPPLY VOLTAGE ELECTRONICS WORLD

ligible. Consequently, the voltage waveform at the output of the modulator and on the screen of the 807 consists of positive half -cycles of audio. This is an important requirement. See "Transmitter Adjustments." Tests have shown that the power output of the 807 buffer on modulation peaks is four times that of the stage under conditions of no modulation or "idling." Also, with proper adjustment of R, as well as buffer and final amplifier grid drive (which is quite easily obtained), the resultant carrier power is proportional to the voltage change on the screen of the 807. (Note: This does not mean that the plate current of the 807 is also proportional to screen voltage change.) Resistor R, provides excellent load for the output circuit of the modulator. Its value is fixed at 25,000 ohms and requires no initial or periodic adjustment. Also, it will be noted that the resistor handles a maximum current of only 10 to 13 ma., so that the power rating need be only 4 watts, and a minimum of source voltage is required. Design of R.F. Stages Conventional input and output circuitry is employed in buffer and final amplifier stages, as shown in Fig. 3. Important design requirements are: (a) sufficient drive for each stage to provide recommended operating grid current values for the type tubes used; (b) a fixed source of grid bias and screen supply voltage for the buffer and final amplifier; (c) complete isolation of stages to prevent both regenerative and degenerative feedback; and (d) provision for r.f. drive adjustment to both stages. Of course, the plate supply sources for both stages should have reasonably good regulation to prevent peak clipping. Also, the antenna system to be used with the transmitter should be capable of loading the transmitter to at least 75% full power output. Inadequate antenna loading results in a lower depth of modulation than is otherwise obtainable. The r.f. drive adjustment for the buffer, and to some extent for the final amplifier, is provided by the adjustable grid resistor R. Other methods can be employed for the purpose, of course, but if more than ample drive power is available from the preceding stage, the adjustable resistor method is particularly effective; moreover, it has the advantage of maintaining high efficiency in the buffer and thus high peak -power output from the stage. Optimum grid current values for the final amplifier can be obtained conveniently by readjustment of the tuned grid circuit. In fact, use of the tuned circuit in this manner is the preferred method of adjusting the grid current for the final stage. One more point should be mentioned in regard to r.f. design and that is the advisability of using a slightly higher "Q" in the plate circuit of the buffer and in the grid circuit of the final. This provides better load regulation for the N 150 LEGEND R.F DRIVE VARIATIONS UPWARD AT AUDIO FRED. I 1 Fig. 4. How r.f. drive to power amplifier pre. vents p.a. cut -off, and constantly changes grid operating point at a.f. frequency. 's r izo Slo Ñ T S U . ,el 11 5 45 50 55 60 ANTENNA CURRENT 65 CLASS A82 «.GLAS$ ADI I FOR Fig. 5. Actual antenna current values for several values of buffer screen voltage. large variations in r.f. current in these circuits. Final Amplifier Efficiency Fig. 6 is a diagram showing the pertinent elements and values of a 1 kw. input p.e.p. (peak- envelope -power) transmitter which is currently being used by the author on the 80 -, 40- and 20 -meter phone bands. It was originally a 500 -watt rig, designed and built by the author in 1950, and utilized a pair of 803's in the final. QSO reports on the transmitter were excellent. In 1951 the 803 tubes were replaced by the new and more efficient 4 -125A type tubes, Fig. 6. Shown at the right is the schematic diagram of the modulator, buffer, and the final power amplifier stage of the one -kilowatt (peak- envelopepower input) transmitter that is employed by the author. Refer to the text for a complete description. AUDIO FROM $PEECN AMP 445 V) 807 R F DRIVER which were operated from the original clamp tube exciter at the 500 -watt p.e.p. input. Recently, however, the author obtained a new plate supply which provides the power increase. Since modulation is applied to the driver stage, it might be assumed that the final amplifier is necessarily operated as a class B linear. This, however. is not the case. Those who are familiar with the 4 -125A tubes will note that the grid bias value used is that specified by the manufacturer for class C c.w. operation. Also, the transmitter is initially tuned up for class C operation in the same manner as for any c.w. transmitter. This is possible because the system described is a form of "efficiency modulation," that is, the grid operating point of the final amplifier is constantly changing as modulation is applied to the preceding driver and remains at a fixed point only when no modulation is applied. Another very important reason is that modulation is applied to the driver, only in the form of positive cycles of audio, as explained under modulator operation. Thus, the driver screen (and plate) voltages never go negative with respect to the fixed d.c. potential ( "B + ") on the stage. This of course results in a grid operating point for the final amplifier which, although constantly changing, is never so far down on the plate current curve as to cause cut -off. Observation of the r.f. modulated waveform on an oscilloscope shows the negative modulation peaks to be only a small percentage of the amplitude of the positive peaks. See Figs. 4 and 5. While it is practical to adjust this system so that the carrier will be near zero with no modulation, it is not considered desirable. It can be seen that the average carrier power will be lower. Experience with this system of modulation has indicated that it is preferable to maintain an unmodulated carrier (Continued on page 144) 4- 125A'S FA 375 V (TOTAL CJRRENT 15 MA) ANT May. 1959 61

Electronics-Illustrated-1963-07
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