G3sek 4cx1000 Grounded Grid Amplifier

-(10 meters).

5 turns, 1/4" copper tubing, 2 7/4 " o.o., 3" long.

capacitor G and G, the grid circuit bypass capacitor.

Screen voltage may be removed for tune-up purposes by control switch Sa, section B. The screen circuit is grounded in the "off" and "fil" positions by means of switch section C.

Amplifier The complete amplifier is

Construction built upon an aluminum chassis measuring 13" x 17" x 3" and has a 14" standard relay rack panel. The

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Figure 40 GRID TANK CIRCUIT ASSEMBLY

Coils are mounted to the ceramic switch decks by their leads. A small aluminum plate attached to rear of the switch assembly rods supports grid tuning capacitor which projects out rear of shielded enclosure. Entire assembly may be pre-wired before placing in enclosure.

Figure 40 GRID TANK CIRCUIT ASSEMBLY

Coils are mounted to the ceramic switch decks by their leads. A small aluminum plate attached to rear of the switch assembly rods supports grid tuning capacitor which projects out rear of shielded enclosure. Entire assembly may be pre-wired before placing in enclosure.

grid circuit components are mounted within an aluminum box measuring 3" x 4" x 4". Plate loading capacitor G, r.f. choke RFC-1, and output connector are placed within an enclosure measuring 6" x 6" x 3", made up of aluminum angle sections and sheet material. The plate circuit shielding is made of Reynold's "Do-it-yourself" aluminum stock, available at most hardware stores.

Layout of the major components can be seen in figure 37. The two tube sockets are placed directly behind the panel opening, with the plate r.f. choke between them, and the variable vacuum capacitor is mounted vertically to the chassis directly behind the sockets, on the center line of the chassis. To the right of the sockets is neutralizing capacitor G. The high voltage ceramic coil switch S;A-B is placed directly behind the vacuum capacitor, mounted in a vertical position.

The variable vacuum capacitor is panel driven by a counter-type dial, through a miniature right angle gear drive, as seen in the under-chassis view (figure 38). The plate and grid band switches are ganged and switched in unison by means of a shaft acting through two right angle gear drives and two bevel gears. Both circuits are thus switched by the "Band-switch" control located in the lower left corner of the front panel.

It is necessary to apply forced air to the sockets of the amplifier tubes. A large 115 volt a.c. operated blower is therefore mounted in the center of the bottom shield plate. The under-chassis area is thus pressurized and the majority of the air escapes through the socket ventilation holes located near the pins of the tubes.

All wiring beneath the chassis (with the exception of the filament leads) is done with 5KV insulated wire, encased in metallic braid which is grounded to the chassis every inch or so. The B-plus wiring from the high voltage terminal to the plate current meter is done with a section of RG-8/U coaxial line from which the outer braid has been removed. A similar piece of line is run from the meter to the plate r.f. choke, RFG2.

The three meters are mounted upon a lu-cite sheet placed behind a second lucite sheet mounted behind a cut-out in the front panel. The meters are shielded from the plate circuit of the amplifier by an aluminum enclosure that covers the wiring and meters, running the full length of the chassis. The meter leads pass through the plate circuit area via a short length of J/2-inch aluminum conduit that is threaded

Figure 41

OPERATING DATA AND SCHEMATIC, SCREEN AND BIAS SUPPLY

T,—870-410-0-410-870 volts at ISO mo. and 60 ma. 5 volts, 2 a., 6.3 v. 3.5 a. Stancor P-8307 h—235-0-235 volts at 40 ma. Stancor PC-8401

CHj—7 henry at ISO ma. Stancor C-J7J0 CHz—7 henry at 50 ma. Stancor C-7707 RYi—Overload relay, adjustable 100-250 ma. Note: J is insulated from chassis.

at each end and bolted to the chassis and the meter shield. Plate circuit wiring above the chassis is done with V2-inch silver plated copper strap as seen in the introductory photograph at the beginning of this chapter.

Amplifier After the amplifier is wired

Neutralization and checked, it should be neutralized. This operation can be accomplished with no power leads attached to the unit. The tubes are placed in their sockets, and about 10 watts of 30 Mc. r.f. energy is fed into the plate circuit of the amplifier, via the coaxial output plug J=. The plate and grid circuits are resonated to the frequency of the exciting voltage with the aid of a grid-dip meter. Next, a sensitive r.f. voltmeter, such as a 0-1 d-c milliammeter in series with a 1N34 crystal diode is connected to the grid input receptacle (Ji) of the amplifier. The reading of this meter will indicate the degree of unbalance of the neutralizing circuit. Start with a minimum of applied r.f. excitation to avoid damaging the meter or the diode. Resonate the plate and grid circuits for maximum meter reading, then vary the setting of, neutralizing capacitor C4 until the reading of the meter is a minimum. Each change in C< should be accompanied by re-resonating the grid and plate tank circuits. When a point of minimum indication is found, the capacitor should be locked by means of the auxiliary set screw.

Complete neutralization is a function of the efficiency of the screen bypass system, and substitution of other capacitors for those noted in the parts list is not recommended. Mica, disc-type, or other form of bypass capacitor should not be substituted for the units specified, as the latter units have the lowest value of internal inductance of the many types tested in this circuit.

Bias and The amplifier requires -60 to

Screen Supply -110 volts of grid bias, and plus 300 to 600 volts of screen potential for optimum characteristics when working as a class AB1 linear amplifier. Screen voltage for class C operation (phone) is 400 volts. The voltage may be raised to 500 volts for c.w. operation, if desired, although the higher screen voltage does little to enhance operation. Approximately -120 volts cut-off bias is required for either phone or c-w operation. A suitable bias and screen power supply for all modes of operation is shown in figure 41, together with an operating chart for all operating voltages. The supply furnishes slightly higher than normal screen voltage which is dropped to the correct value by an adjustable series resistor, Ri. This series resistor is adjusted for 30 milliamperes of current as measured in meter jack Ji when the supply is disconnected from the amplifier. Series bias resistor R2 is adjusted for the same current in jack J? under the same conditions. The value of protective bias may now be set by adjusting potentiometer Rs.

Additional bias is required for class C operation which is developed across series resistor Ri. Switch Si is open for class C operation and closed for sideband operation.

It is imperative that the screens of the tet

4CX-1000A ALL BAND AMPLIFIER PROVIDES TWO KILOWATT P.E.P. FOR SIDEBAND, OR ONE KILOWATT FOR PHONE C.W. OPERATION

Grid, plate, and output meters are across top of panel. Counter dial at center drives variable vacuum tuning capacitor, with loading control directly beneath it. Meter switch is at left, with bandswitch at right. Entire amplifier is screened to reduce unwanted radiation.

rode amplifier tubes be protected from excessive current that could occur during tuning adjustments, or during improper operation of the amplifier. The safest way to accomplish this is to include an overload relay that will open the screen circuit whenever the maximum screen dissipation point is reached. Two 4-25OA tubes or 4-400A tubes have a total screen dissipation rating of 70 watts, therefore relay RY-1 should be adjusted to open the screen circuit whenever the screen current reaches approximately 100 milliamperes.

29-10 A 2 Kilowatt

P.E.P. All-band Amplifier

Described in this section is an efficient all-band linear amplifier suited for SSB, phone, and c.w. operation up to the maximum legal power input limit. A 4CX-1000A ceramic power tetrode is employed in the basic circuit shown earlier in this chapter in figure 11C.

The Eimac 4CX-1000A is a ceramic and

4cx1000a Amplifier Schematic

Figure 43

SCHEMATIC, 4CX-1000A LINEAR AMPLIFIER

C,—500 fd., I0KV variable vacuum capacitor. Jennings UCSL 4-500 C,— 7500 ßufd. Cardwell 8013

Li-Si—High-C, all-band turret. Barker & Williamson 852

RFCj—Heavy duty plate choke. Raypar RL- 700

Ti—6 volts, 12.5 amperes, with primary taps. Stancor P-6463

Ti—725 volts, 75 ma. Stancor PS-8415

Figure 43

SCHEMATIC, 4CX-1000A LINEAR AMPLIFIER

C,—500 fd., I0KV variable vacuum capacitor. Jennings UCSL 4-500 C,— 7500 ßufd. Cardwell 8013

Li-Si—High-C, all-band turret. Barker & Williamson 852

RFCj—Heavy duty plate choke. Raypar RL- 700

Ti—6 volts, 12.5 amperes, with primary taps. Stancor P-6463

Ti—725 volts, 75 ma. Stancor PS-8415

Rj—100 ohm, 20 watt non-inductive resistor. May be made of a number of 2 or 4 watt composition resistors connected in series parallel. Two Sprague type NIT, 10 watt "Cool-ohm" 200 ohm resistors in parallel used.

M,, M,—0 - 7 d.c. milliammeter M_—0 - 7 d.c. ammeter Eimac SK-800 air socket

Figure 44 END VIEW OF AMPLIFIER

4CX-1000A sub-chassis is shown, with plate circuit choke mounted at rear. External blower provides forced air to cool tube seals via entrance hole in rear of chassis. Bias adjustment potentiometer is in foreground, between tube and meter switch.

metal, forced air-cooled, radial beam tetrode with a rated maximum plate dissipation of 1000 watts. It is a low voltage, high current tube specifically designed for class AB1 r-f linear amplifier service where its high gain and low distortion characteristics may be used to advantage. At its maximum rated plate voltage of 3000, it is capable of producing 1680 watts of single-tone output power in SSB service. Maximum grid dissipation of the 4CX-1000A is zero watts. The design features which make the tube capable of maximum power operation without driving the grid into the positive region also make it necessary to avoid positive grid operation.

Circuit The circuit of the all-band am-

Description plifier is shown in figure 43.

A resistance loaded, untuned grid input circuit is employed in conjunction with a pi-network plate output circuit. Grid driving requirements are 60 volts peak across resistor Ri which has a value of 100 ohms. This corresponds to 36 watts p.e.p. all of which is dissipated in resistor Ri. For normal voice operation, a 20 watt non-inductive resistor may be used. A simple 1N66 diode voltmeter is incorporated in the grid circuit to monitor the excitation voltage. Grid and screen current are also measured. The d.c. grid bias voltage is obtained from a small selenium-type half-wave power supply contained within the amplifier.

The plate circuit is designed around the new Barker & Williamson type 852 bandswitching turret, especially designed for high current, low voltage tubes of this type. The input tuning capacitor of the network is a 500 /¿¡¿id. variable vacuum unit, and the output loading control is a 1500 fj.fi.fd. variable air capacitor. For operation into low impedance loads in the 80 meter band, an additional 1500 ¡ififd. fixed mica capacitor may be placed in parallel with the output loading capacitor, G. A second 1N66 diode voltmeter is placed across the output circuit as a tuning aid.

Screen Grid Tetrode tubes may exhibit re-Operation versed (negative) screen current to a greater or lesser degree depending upon individual tube design. This characteristic is prominent in the 4CX-1000A and, under some operating conditions, indicated negative screen current of the order of 25 milliamperes may be encountered. In general, high values of negative screen current will be measured at the lower values of plate potential.

It is difficult to measure screen dissipation in the presence of secondary emission of this type. The usual product of current and voltage may not provide an accurate picture of actual screen dissipation. Experience has shown that the screen will operate within the limits established for this tube if the indicated screen current, plate voltage, and drive voltage approximate the values given in the operating chart of figure 48.

The screen supply voltage must be maintained constant for any values of negative and positive screen current that may be encountered. Dangerously high plate currents may flow if the screen power supply exhibits a rising voltage characteristic with negative screen cur rent. Stabilization may be accomplished in several ways. A combination of voltage regulator tubes may be connected across the screen supply, or a heavy bleeder resistor may be placed across the supply that will draw approximately 70 milliamperes at the operating potential of the screen. Screen voltage is not particularly critical, especially at plate potentials of 2500 volts and higher. However, the screen volt&ge should be adjusted for best linearity of the stage when the tube is operated at 2000 volts plate potential. A 350 volt, 150 ma. supply utilizing choke input and bled to 70 milliamperes, in conjunction with a 100 watt primary "Variac" or variable voltage transformer is a satisfactory source of screen voltage. Three VR-105 regulator tubes passing 40 ma. may be employed for a fixed-voltage screen supply.

Amplifier Layout of the amplifier may

Construction be seen in figures 42, 44, 45, 46, and 47. A 12i/4" x 19" rack panel is employed to form the front of an enclosure 14" deep. The enclosure is built of '/2-inch aluminum angle stock and the top, sides and back are covered with perforated aluminum sheet. The bottom of the enclosure is formed from a solid sheet of aluminum. Only a very small chassis is required to mount the 4CX-1000A tube socket, filament transformer, and grid circuit components. A chassis measuring 6" x 3" x 14" with one end cut off is employed (figure 47). This chassis is mounted at one end of the enclosure and supports a small aluminum angle bracket holding the two pi-network capacitors. The band-switching plate tank inductor is mounted di-

4cx1000a Amplifier

Figure 45 TOP VIEW OF 4CX-1000A AMPLIFIER

Tetrode tube fits in £/-mac air socket on small chassis at one end of enclosure. Pi-network capacitors are mounted on aluminum bracket attached to chassis, and are panel driven through flexible shaft couplers. Heavy duty, high-C band-switching inductor is at opposite side of assembly, with output circuit diode voltmeter mounted above it on rear of enclosure. Extremely low harmonic content of amplifier removes need of meter shields. Amplifier may be run without enclosure with no sign of TV I on any band in normal TV signal area.

Figure 46 REAR VIEW OF CHASSIS ASSEMBLY

Air inlet, power receptacle and coaxial input jack are located on rear of amplifier chassis. Pi-network capacitor assembly is at left, with Raypar wideband plate r-f choke in foreground. Plate blocking capacitor is made of four 500 fijifd. TV-type capacitors mounted in "sandwich" between stator plate of vacuum capacitor and round aluminum plate.

rectly to the aluminum bottom plate of the enclosure as seen in figure 45.

The low impedance plate tank circuit requires a higher than normal value of plate coupling capacitor. Four 500 ¡¡.y.fd., 20 KV TV-type capacitors are therefore mounted between two aluminum plates to form a 2000 /¿//id. "sandwich" which is bolted to the rear stator terminal of tuning capacitor G. The 10 meter inductor section of coil Li bolts directly to the aluminum plate, as does the plate lead of the 4CX-1000A.

Special breechlock terminal surfaces are used on the tube in place of the conventionrl base. Three tabs protrude through the ceramic envelope for each tube element, providing a low inductance termination. The new Eimac SK-800 socket having a built-in screen bypass capacitor and an air vent should be employed with this tube.

The rated heater voltage for the 4CX-1000A is 6.0 volts and the operating voltage, as measured at the socket, should be maintained within plus or minus 5% of this value if long life operation is to be obtained. Note that the cathode and one side of the heater are internally connected.

Sufficient cooling must be provided for the anode and ceramic-to-metal seals to maintain operating temperatures below 200 degrees, Centigrade. At sea level, with an inlet air temperature of 20 °C., a minimum of 35 cubic feet per minute of air flow is required. In this case, the chassis is pressurized and a 50 cubic foot/minute blower is connected to the air inlet on the rear of the chassis by means of a short length of automobile radiator hose. The cooling air must be maintained on the ceramic seals during standby periods when only heater voltage is applied to the tube.

Figure 47 UNDER-CHASSIS VIEW OF 4CX-1000A AMPLIFIER

Bias supply is placed at one end of chassis. Filament transformer is mounted next to air socket and short, heavy strap leads are used to eliminate voltage drop. Special air socket has built-in screen bypass capacitor to ensure low inductance ground path.

Amplifier Amplifier operating characteris-Operation tics are given in figure 48. The bias potentiometer R2 is adjusted to provide a zero-signal plate current of 250 milliamperes, At this value, the bias voltage will be approximately -55 volts. Resting plate current and screen voltage may both be varied over small limits to provide the greatest linearity range. Maximum single tone plate current is 1.0 ampere. Good linearity may be obtained at 2000 watts p.e.p. at a plate voltage of 2500 and maximum plate current of 800 ma.

Care should be taken not to apply excitation to the amplifier in the presence of screen voltage but with no plate voltage present. A relay interlock may be applied to the screen circuit to open it in the absence of plate voltage. Grid exciting voltage should be limited to about 60 volts, and grid current should be monitored to see that it does not exceed 0.5 milliamperes during tune-up.

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