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Four stages in a compact unit.

Type 6L6G tubes in push-pull Class AB2 operation require a driver of low impedance and good regulation, because the tubes draw grid current on the positive signal peaks. When a transformer-coupled driver is employed, a fixed-bias source is needed which must have such excellent regulation that battery bias often is used. An excellent coupling transformer having fairly high inductance and low-resistance windings is required. Since it was desired to build a 40-watt modulator as compactly as possible, the use of a cathode-follower driver was investigated and found to be quite practical.(1)

The circuit shown in Fig. 1 was set up in the laboratory and measurements of power output and distortion were made. The results are shown by the curves plotted in Fig. 2. It will be seen that an output of 40 watts was obtained with about 3-per-cent total distortion. The total "B" current drawn by the sections of the 6SN7GT cathode followers remains constant at about 13 milliamperes for all signal voltages applied, which indicates that the regulation of the negative supply voltage is unimportant. This was verified by using a negative-voltage source applied through a 10,000-ohm resistor without by-passing to obtain - 67.5 volts at the cathode return of the 6SN7GT. The power output and distortion readings were the same as shown in Fig. 2.

Fig 1
Fig. 1. Circuit used for measuring power output and distortion in the cathode-coupled Class AB2 modulator.

Fig 2
Fig. 2. Curves showing power output and distortion from a 6L6 Class AB2 amplifier with cathode-coupled driver.

A graphical analysis of the cathode follower which was made in order to estimate the values of cathode resistors and operating voltages may be of interest. This is shown in Fig. 3. The solid-line curves are plate-current va. grid-cathode voltage (Eg) characteristics obtained from a tube handbook. An axis of Rklp, which shows the cathode-follower output voltage, is shown for Rk = 5000 ohms. The operating characteristic of the cathode follower is shown by the dash-line curve (only for Rk = 5000 ohms). This operating curve may be drawn easily because when Ip = 0, Ek = 0 and Epk = 300; when Ip = 10 ma., Ek = 50 and Epk = 250, etc.

Fig 3
Fig. 3. Curves used in 6SN7GT analysis. Grid-cathode voltage is shown across bottom. When upper 5000ohm line is extended, it crosses the Ea axis at +20 volts.

A line having a slope of 5000 ohms (for Rk) is drawn from Ip = 0 and Eg = 0 through the point at -10 volts Eg and 2 milliamperes, since -10 volts divided by 2 milliamperes is 5000 ohms. Now with no signal, Ek will be about 13.5 volts as shown by the arrow drawn horizontally from the intersection of the 5000-ohm solid line with the operating characteristic to the Ek axis. Since Ek cannot be made negative but can go only to zero on the negative swing of a sine-wave signal applied to the grid, about 13.5 volts peak is the largest sinusoidal output that can be obtained before clipping on the negative half-cycle starts. It may be seen that clipping on the positive side will not start until the peak grid voltage is much higher. This may be determined by sliding a ruler parallel to the 5000-ohm line along the E6 and Ek axes and extending the line to the right until it again crosses the Eg axis. To obtain higher voltage from the cathode follower the grid may be returned to a positive point. For instance if the grid-to-ground voltage is + 20 and Ek = +30, a peak output of about 30 volts can be obtained without clipping on the negative side.

This driver circuit was used in the 40-watt modulator shown in Fig. 4. The negative supply voltage was obtained by means of a half-wave rectifier, using the same transformer as the main "B" supply. It was found that the 10-rafd. bypass on the screen supply for the 6L6Gs, C11, was quite worth while since it resulted in about 10 per cent more power output than is normally obtained with an unby-passed screen-voltage divider.

Fig 4
Fig. 4. Circuit diagram of the 40-watt modulator using a cathode-coupled driver.

Partlist fig. 4
C1,C525 µF 50 volt electrolytic.
C20.1 µF paper.
C30.02 µF paper.
C48 µF 450 volt electrolytic.
C6,C7,C80.05 µF paper.
C9,C10,C11,C12,C1310 µF 450 volt electrolytic.
C1420 µF 150 volt electrolytic.
C150.01 µF mica, 1000 volts.
R1,R3,R81 MΩ, ½ watt.
R2680 Ω, ½ watt.
R4,R9,R10220 kΩ, 1 watt.
R547 kΩ, 1 watt.
R61 MΩ variable.
R71500 Ω, 1 watt.
R11,R12330 kΩ, ½ watt.
R13100 kΩ, ½ watt.
R14,R1522 kΩ, 1 watt.
R16,R176800 Ω, 1 watt.
R182500 Ω, 10 watts.
R1925 kΩ, 10 watts.
R205 kΩ variable.
R212 kΩ, 10 watts.
L110 H 200 mA filter choke.
F3 amp. fuse.
S1,S2S.p.s.t. toggle switch.
T1Modulation output transformer.
T2Power transformer 375-0-375 V r.m.s., 200 mA; 6.3 V, 4 A; 6.3 V, 0.5 A; 5 V, 3 A

The possibility of returning the cathode-follower resistors to ground and using self-bias on the 6L6Gs with a VR-75 voltage regulator across the self-bias resistor was considered. However, this would have required a "B" supply 75 volts higher, and also several VR-75 tubes in parallel would probably have been necessary, because the plate-and-screen current of the 6L6Gs rises to over 200 ma. under full drive. Since the negative voltage supply was so readily obtained, it was used in preference. If a selenium rectifier with a high-enough peak-inverse-voltage rating were available, it would further simplify the negative-voltage supply, since the use of a separate filament winding is advisable with the 6X4 because of the high a.c. voltage from cathode to ground.

Photo 1
The cathode-coupled modulator is built into a compact unit which includes power supply. The output transformer is mounted externally in the r.f. unit.

In the cathode-follower driver, the grid return was made to a point 34 volts positive with respect to the cathode return, resulting in a value of Ek of about 45 volts. Since the return for Rk is made to -68 volts, the potential at the cathodes of the 6SN7GT, which is applied to the grids of the output tubes, is -23 volts with respect to ground. This provides the correct bias on the output tubes as well as a large margin of safety in the signal voltage which may be applied to the cathode-follower grids before clipping can occur.

A Type 6SJ7 is used in the first amplifier stage which is followed by a 6SC7 phase inverter. The only requirement for these two stages is that they provide sufficient gain to obtain about 55 volts r.m.s., from grid to grid, at the 6SN7GT.

Construction

The photographs show most of the constructional details of the modulator unit. One small chassis contains all of the components except the modulation transformer, T1, which is mounted in the unit containing the r.f. amplifier, and is not shown.

The power supply occupies the rear half of the chassis with the filter choke, L1, to the left, the power transformer, T2, to the right and the 5U4G rectifier tube and filter condenser in between. Grouped along the front edge of the chassis, from left to right, are the 6SJ7, the 6SC7, the 6SN7GT, the pair of 6L6Gs and the 6X4 rectifier. The microphone input connector is set in the left-hand edge of the chassis and along the front edge are the gain control, R6 and the two toggle switches, S1 and S2.

Underneath, the various by-pass condensers are grouped around the tube sockets close to the points to be by-passed. The short lead between the microphone connector and the grid terminal of the 6SJ7 is covered with shield braid to prevent hum pick-up and instability.

This modulator has been in use for several months and has proved to be capable of modulating one hundred per cent an 829 running at about 80 watts input, as checked on an oscilloscope.

Photo 2
Bottom view of the cathode-coupled modulator showing the general location of small parts.

Notes

  1. Greenwood, "Cathode-follower circuits," QST, June, 1945, p. 11;
    Henry, "Improved driver stages for class-B amplifiers," QST, Nov., 1945, p. 45.

William J. Lattin, W4JRW.