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Parallel 6146s in the mobile or fixed-station r.f. assembly

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Increasing power input to 180 watts c.w. or 135 watts phone.

The addition of a second 6146 to the "R.F. assembly for mobile or fixed-station work"(2) converts the unit into a full-fledged fixed-station transmitter. As modified, the rig retains the original features of appearance, compactness and operating convenience found usually only in more expensive commercial gear. And, of course, if you're interested in using it for that purpose, the rig can still be operated mobile.

February, 1955, QST(1) made reference to the addition of a second 6146 to the mobile of fixed-station transmitter.(2) The modification, now completed, should interest those who wish to operate with parallel 6146s in the final.

When converted, the transmitter loses none of the original features except for the addition of one tuning control. The new dimensions are 6¼ × 7_7/8 × 9_1/16 inches and the permissible power input level has been increased to the full ICAS ratings of a pair of 6146s. One very important consideration is that in spite of its small size, the unit is not difficult to construct - even when starting from scratch - nor does it require any special constructional aids or practices. Modification is not expensive. Very few of the original parts need be discarded or routed to the junk box and the cost of new parts - other than another 6146 - is a minor item.

In the text to follow, frequent reference will be made to the original schematic diagram of the transmitter. Therefore, Fig. 1, page 12 of QST, October, 1954, should be on hand as the following material is studied.

The basic problems in increasing the power were those of getting enough excitation for the 6146s without adding to the original 5763 tube line-up, and in redesigning the amplifier plate circuit for the higher power level.

Oscillator circuit modifications

Originally, the oscillator plate circuit, C2-L1, tuned no higher than 7 Mc. and, as a result, it was necessary to operate V2 as a quadrupler when driving the final at 28 Mc. This did not give adequate drive for two 6146s. Therefore, the arrangement shown here in Fig. 1 was devised. The inductance of L1 has been reduced to 2.2 µH, and will tune, with C2 and S3 adjusted, to either 7 or 14 Mc. Thus, with the tank resonated at 14 Mc., it is only necessary for V2 to double frequency for output at 28 Mc.

The oscillator plate circuit works as follows: With S3 set at position 1, both C7 and L1 are out of the circuit and the circuit is the familiar choke-coupled arrangement. Then, with a 3.5 Mc. crystal in use, the 3.5 Mc. output from V1 may be used to drive V2 either as a straight amplifier or as a frequency doubler. Substitution of a 7-Mc. crystal gives 7-Mc. drive for V2. With crystals in the 3.4 - 3.5-Mc. range, S3 at position 2, and C2L1 resonated at 10.5, 13.5 or 14 Mc., adequate drive is supplied for doubling in V2 to 21. 27 or 28 Mc., respectively. A 6.8-Me. crystal and position 2 of S3 may also be used when driving V2 as a doubler to 27 Mc.

Photo 1
Front view of the modified r.f. assembly. The tuning control for the amplifier plate circuit is centered on the extension at the bottom of the panel. The oscillator plate switch, Sa and the pilot-lamp jewel are to the left and right of the tuning dial. Strips of perforated aluminum are used to increase the height and depth of the cabinet.

Fig 1
Fig. 1. Schematic diagram of the revised oscillator plate circuit for the 6146 transmitter. C7, RFC6, and S3 are additions to the original circuit.
Values for C8, R9 and L1 have been altered in the new arrangement. *Indicates a mica capacitor.

L112 turns No. 24, 3/8 inch long, 5/8 inch diam. (B & W 3008). See text.
RFCs0.5 mH
S31 pole 5 position (3 used) ceramic switch, wired for progressive shorting (Centralab PA-10 section mounted on PA-300 index assembly).

Position 3 of the oscillator switch and capacitor C7 are required because the C2L1 combination will not otherwise cover the complete 7- to 14.85-Mc. range required. The operating range of the circuit is shifted to include 7 Mc. when C7 is switched across L1.

Driver circuit revisions

In reworking the driver circuit, it proved desirable to reduce the grid lead (R9 of the original circuit) to 47K. With this change, it seemed advisable to employ the r.f. choke now shown as RFC6, to prevent loading of the circuit by the lower value of resistance.

Photo 2
Top view of the parallel-6146 transmitter. The new 6146 is located at the bottom right-hand corner as seen in this view. The metal shaft coupler originally used for ganging the split-stator capacitors now serves as a pulley for a string drive for the amplifier plate tuning capacitor. La is mounted on the plate caps of the 6146s.

Not quite so obvious in the new Fig. 1 is the reason for reducing the by-pass capacitance (Cs) to 220 pF. This value allows the driver stage to function normally at frequencies above 14 Mc. On the other hand, it also makes V2 somewhat degenerative at the lower frequencies where instability may otherwise be a bit of a problem. The degeneration stabilizes the circuit when working V2 as a straight amplifier, and may be employed in this instance because there is an abundance of output from V2 at the lower frequencies.

On the plate side of the driver tube, it was necessary to use new values of inductance in the multiband tuner to compensate for the additional shunt capacitance introduced by the grid of the second tube. Heretofore, we had aimed at values that would result in low C at 28 Mc. However, with a tuning capacitor of reasonable size, this results in relatively little separation between land 21-Me. resonances. In other words, when operating at 7 Mc., there may be some danger of output also at 21 Mc. To make the separation between resonances as great as possible, the frequency ranges were shifted so that 14 Mc. comes at maximum capacitance, and 7 Mc. near minimum capacitance.

Just how well this system works out is shown by the dial calibration (see tuning chart) for C3. Notice that 7, 28 and 21 Mc., in that order, resonate at dial settings of 10, 28 and 51, respectively. In other words, there are at least 18 dial divisions between any two of the three adjustments. At the high-capacitance end of the tuning range there are 8 dial divisions between the 3.5 and 14 Mc settings.

The layout and the wiring of the driver plate circuit remains unchanged. L3 is now 6.8 µH, a 22-turn length of B & W type 3012 Miniductor. L4 now has an inductance of 1.8 µH.

Loading resistor R3 in the first model has been retained for the original reasons. However, a value of 7.5K, obtained by connecting two 1.5K 1-watt resistors in parallel, is now used.

To make initial adjustment less critical, the two multiband tuners are no longer ganged. A separate control is installed for the output tuner.

The modified amplifier circuit

The addition of another 6146 to the final amplifier made necessary several major, but not difficult, alterations.

The four inductors, L6 through 1.5, must be replaced. New values of inductance are required, as in the grid circuit. Dimensions are given in Fig. 2.

Fig 2
Fig. 2. Schematic diagram of the amplifier plate circuit for the parallel 6146 transmitter. Dashed lines indicate components located in the amplifier tube compartment. Terminals A and B, the feed-through insulators used originally to support the L8-L9 assembly, now support L8. C and D are ½ inch cone insulators and E is a metal post. The physical arrangement of the terminals is similar to that shown in the bottom view of the transmitter.

Partlist fig. 2
C9,C10Centralab DD16-302 or DD30-302.
L5Parasitic choke: 14 turns No. 18 enam. on 1 watt resistor (any high value) as form, tapped at center.
L66.6 µH 13 turns No. 14, 1% inches long.
L75.2 µH 10¾ turns No. 14, 1_3/8 inches long.
L81.7 µH 4¾ turns No. 14, 5/8 inch long.
L9Inductance and length adjustable; 1½ turns No. 14; see text.
RFC5National R-300S.

Note: L6 through L9 made from 2-inch diam. coil stock (B & W 3900). See text for additional data.

A high-frequency parasitic oscillation was suppressed by the center-tapped choke, L5. All attempts to utilize the original parasitic choke were ineffective.

A set of 0.001 µF disk ceramic capacitors for the cathode and the screen terminals of the 6146 should be installed directly at the new tube socket. The original capacitors will not serve both 6146s.

The control-grid, heater and screen terminals of the 6146s should be connected in parallel. Ground returns for the new tube are made directly to chassis as were those of the first tube. Connecting the cathodes together allows both 6146s to be keyed through the original keying jack, Jt. The variable padder capacitor, C5, has been removed from the plate tuner. No replacement is required.

The following is a list of component changes:

  1. Grid-leak resistor, Rrt: now 15K, 1 watt.
  2. Screen-dropping resistor, R4: now 20K, 25 watts.
  3. Plate r.f. choke, RFC5; must be rated for at least 300 ma. d.c.
  4. Plate-meter shunt, R5: resistance reduced to provide 50 times multiplication for the 10-ma. milliammeter.
  5. Plate blocking capacitor C9, and by-pass capacitor C19 in Fig. 2: now 0.003-µf. high-voltage disk ceramics.

General circuit changes

Not shown on any of the diagrams of the transmitter is a 6.3-volt pilot lamp now connected to the heater line, installed principally to balance the new panel arrangement.

The plate-voltage input terminals of the transmitter have been by-passed for TVI. A 0.001 µF 1 kV. disk is connected between pin 4 of 14 (original circuit diagram) and ground, and a 500 pF 3 kV. disk is connected between pin 3 of 14 and ground.


Before starting construction - either a modification or new - the supplementary data(1) for the first model should be reviewed. In addition, a comparison of the photographs of both versions will clearly illustrate that most of the original construction remains intact.

The cabinet is 3 inches higher than before. Strips of perforated aluminum, attached to the sides and the rear of the box, provide the increase in height. These extensions and the original members should be overlapped on the inside by narrow strips of aluminum held in place by machine screws, lockwashers and nuts. If a 3-inch extension is added to the panel as shown in the front view of the transmitter, the crack between sections may be sealed by bolting a narrow strip of aluminum (inside the unit) in place. The bottom view shows how ½ × ½ inch angle is used at all outer edges of the cabinet and the panel extensions.

As shown in the top view of the transmitter, installation of the second 6146 requires no displacement of original components. The method of mounting the new tube socket is identical to that described for the first. A row of ¼ inch ventilating holes should be drilled in the chassis plate below the envelope of each 6146. The metal shaft coupler, used originally to gang the split-stator tuning capacitors, C3 and C4, is no longer fastened to the tail shaft of C3. Instead, only the setscrew at C4 is tightened; the setscrew at C3 is loosened so that it can turn freely on the shaft of C3. The coupling is now used as a pulley for a drive string for C4. Before mounting L4 on the terminals of C3, free the outside end turns of the coil from three of the four support bars so that they may be spread for adjustment.

The lengths of the three pieces of No. 34 enameled wire used as the plate-meter shunt, R6, should be shortened to approximately 18 inches. This gives a full-scale meter reading of 500 mA with the meter switch set at position E-E1 in the earlier circuit.

A bottom view of the transmitter shows the mounting of the amplifier plate coils, and Fig. 2 identifies the supports for these inductors. The L6-L7 assembly is made from a single length of coil stock clipped, without breaking the support bars, to provide windings of 13 turns (L6) and 1034 turns (L7). Start with a 27- or 28-turn length of material, to provide extra wire at each end that may be straightened out and used for the leads to Terminal B (Fig. 2) and C6.

Photo 3
As seen in this bottom view of the transmitter, L8 is located at the upper right-hand end of the chassis, just above the 1½-turn coupling coil, L9. The L6L7 assembly is directly below L9. The cold end of L7, located approximately at the center of L6L7, is grounded to a metal post to the right of the coils. R4 is mounted on the panel extension at the upper left-hand corner of the unit and Ss is below the drive shaft for C4. The oscillator plate coil, L1, is mounted on the side wall to the right of C2. A metal bracket, mounted between walls of the cabinet, 3½ inches to the rear of the panel, supports a bearing for the drive shaft. The oval slot to the rear of C6 provides through-chassis clearance for the drive string.

Tuning chart for the 6146 transmitter
Oscillator V1Driver V2Amplifier V3,V4Link Cir. C6, S1B
Xtal. Mc.Sw. Pos. S3Dial C2Ip mAOutput Mc.Ig mADial C3Ip mAOutput Mc.Ig mADial C4IpOutput Mc.Sw. Pos. SBDial C4
50 Ohm1Bulb250 Ohm1Bulb250 Ohm1Bulb2
7.0355147.0 2.75412.521.03.5545823023021.0HI8100

1 Bank of Ohmite type D-101 resistors used as dummy load.
2 150 watt lamp used as load.
3 C2 used as excitation control.

The L8L9 assembly is also made from a single piece of stock. Allow an extra half turn or so at each end of the winding for leads to Terminals B and C. Free the last % turn of L9 (the end connected to Terminal C) to make the link adjustable with respect to L8. In the final construction, the output ends of both L7 and L9 are connected to the link tuning capacitor, Cs, by a length of No. 12 tinned wire.

The 47K grid resistor for V2 is now connected, at the meter end, to the tie point that supported one end of the original R9. The other end of the resistor is used to support one side of RFC6. Either Pin 8 or 9 of the socket for V2 may be used to support the tube end of the choke.

The oscillator plate coil, L1, is the original inductor, reduced in size. A ½ inch cone insulator supports the inductor as in the previous model.

The new control for C4 employs a panel-bearing extension shaft. A second bearing, mounted on an aluminum strip spanning the chassis (see bottom view) supports the rear portion of the shaft. A Millen 39003 shaft coupler is attached to the end of the shaft to serve as a pulley. Another shaft coupler of the same type is fastened to the tail shaft of C6. This provides a smooth surface for the dial cord to travel over on its route through the chassis. Two lengths of cord are used between the drive-shaft pulley and the pulley on the tail shaft of C6. When installing the cords, first tighten the coupler or pulley on C4 so that the top end of the setscrew points toward V2 when C4 is rotated for minimum capacitance. Now tighten the pulley on the drive shaft with the setscrew pointing toward the right wall of the cabinet (as seen from the front view). In stringing the drive, use the pulley setscrews to anchor the ends of the cords. Allow a full wrap around the pulleys at each end of the cords and make sure that one set of turns travels in a clockwise direction while the other rotates counterclockwise.

As shown in the front view, the oscillator switch knob and the pilot jewel are each 2¼ inches from the amplifier tuning dial. A 2¾ inch dial, E. F. Johnson type 116-262, is used as the amplifier tuning control. Note that the decal marking for C3 at the top right-hand corner of the panel has been changed from AMP to DRIVER.


The heater power requirements for the complete r.f. line-up are 6.3 volts at 4 amperes. A supply delivering 300 volts at approximately 50 ma. should be available for the 5763s. Maximum ICAS ratings permit 600 and 750 volts to the 6146s for 'phone and c.w. operation, respectively. The tubes may be loaded to 225 ma. plate current with plate modulation and may be loaded to 240 ma. for c.w. work. In either case, the supply should be capable of delivering an additional 30 ma. or so for the screens.

A 150-watt lamp bulb or a noninductive resistor should be used as a dummy load while testing the transmitter. A 150-watt bulb and a bank of series-parallel connected Ohmite dummy loads were used to obtain the data shown in the accompanying tuning chart.

Plate and screen voltages should be removed from the power amplifier while the exciter is undergoing initial tests. Tuning of the oscillator and the driver stages is completely conventional, and the tuning chart lists typical current readings for V1 and V2 as well as settings for C2, C3 and S3. Note that the oscillator plate tuning capacitor, C2, is to be adjusted for minimum capacitance when the circuit is operated at 3.5 Mc. When using a 7-Mc. crystal and straight-through amplification in the plate circuit, C2 should be used as an excitation control; increasing the capacitance of C2 reduces the drive to V2. In this case, the control may be properly adjusted only with the power amplifier in operation, of course.

If it is noticed that the dial settings for the driver plate circuit do not correspond with those listed in the tuning chart, it will be necessary to experiment with the inductance of L4. Adjust the inductance by varying the positions of the adjustable turns until the 14-Mc. setting of the dial coincides with that listed. The dial readings for all frequencies above 14 Mc. will fall into line with the 14-Mc. point properly located.

It is intended that the 6146 stage will be operated straight through at all amateur frequencies between 3.5 and 29.7 Mc. However, the circuit may be used as a frequency doubler with the usual decrease in efficiency.

The tuning chart lists tuning-dial and plate-current readings that may be expected when the amplifier is operated at 600 volts. Observe that nearly all readings depend to some degree on the type of dummy load in use. The spacing between L8 and the adjustable portion of L2 was approximately 6 inch while the readings were made.

The series-tuned output circuit is perfectly standard and is described in detail in Chapter Six of the '54 and '55 Radio Amateur's Handbook. The system is designed to work into a specific impedance - 50 ohms in this case. If the impedance of the antenna deviates from 50 ohms by any appreciable amount, it is necessary to employ a matching circuit immediately following the amplifier. The Handbook describes couplers designed for matching 50 ohms to higher values of antenna impedance and also explains how a s.w.r. bridge can be employed during the matching adjustments. Of course, antennas fed with 50-ohm coaxial cable may be coupled directly to the output circuit of the transmitter.

The new circuit may be used without further modification when a 300- or 400-volt plate supply is employed. The 6146s may be loaded to better than 100 ma. at the lower of the two plate voltages and with the 20K screen resistor in the amplifier. If the mobile supply has current to spare, and if 14 is lowered in value to approximately 7K, the amplifier may be loaded to approximately 150 ma.


  1. Chambers, "Supplementary data on the r.f. assembly for mobile or fixed-station work," QST, Feb., 1955.
  2. Chambers, "An r.f. assembly for mobile or fixed-station work," QST, Oct., 1954.

C. Vernon Chambers, W1JEQ.