KW-1 Transmitter

THE COLLINS KW-1 TRANSMITTER

GENERAL

The Collins KW-1 amateur transmitter is a vfo-controlled, bandswitching, gang-tuned, high-power am and cw transmitter. Provision is made for external frequency - shift keying. Power amplifier input is 1000 watts on the 80, 40, 20, 15, 11 and 10 meter bands, and 500 watts on the 160 meter band. Other features include TVI reduction, cw muting for a 75A receiver, a cw sidetone oscillator, a blower, door interlock switches, fuses and an overload relay.

Exciter tuning is ganged to one control. With the exception of the power amplifier output circuit, the entire rf section is tuned by the frequency selector control. The accurately calibrated exciter tuning dial indicates the exact frequency in kilocycles for each of seven amateur bands. Only the scale for the band in use is visible.

A stable, hermetically - sealed oscillator is followed by buffer and multiplier stages; the oscillator and multiplier stages are permeability tuned by powdered - iron cores. The rf driver stage is tuned by a variable air capacitor. The final amplifier plate-tank circuit is tuned by a variable vacuum - capacitor. The pi-L output circuit is designed to work into an unbalanced resistive load of 52 ohms with maximum standing wave ratio of 2.5 to 1.

Either crystal or high impedance dynamic microphones may be used. A 600-ohm phone patch is incorporated in the speech amplifier. Increased sideband power without over-modulation is made possible by a speech clipper followed by low-level low-pass and high-level low pass filters. The push-pull 810 modulator tubes fully modulate the 1000-watt input to the power amplifier.

The KW-1 transmitter is self-contained in a heavy gauge cabinet 28 inches wide, 18 inches deep and 66-1/2 inches high. All that is needed to place the transmitter in operation is a 52-ohm antenna system, a power source, and a microphone or a telegraph key.

KW-1 SPECIFICATIONS


Power Amplifier Input  1000 watts
                       (500 watts on 160 meters)
R-F Output Impedance   52 ohms
Maximum Permissible
Standing Wave Ratio    2.5 to 1

Amateur Bands Covered  160, 80, 40, 20, 15, 11, 10 meters
Frequency Range        1800-2000 kc
                       3500-4000 kc
                       7000-7300 kc
                       14000-14400 kc
                       21000-21450 kc
                       26960-29700 kc

Emission               Voice or cw
Frequency Control      70E-14 Master Oscillator, 1675 to 2050 kc
Microphone             High impedance crystal or dynamic
Phone Patch Impedance  600 ohms, unbalanced to ground

Weight                 600 pounds
Dimensions             66-1/2" high, 28" wide, 18" deep

Circuit Protection     Overload relay, fuses, high voltage arc gaps
Tuning Controls        Bandswitching, frequency selector, PA tuning, PA
                       loading
Other Controls         Filament switch, filament voltage adjustment, 
                       plate switch, overload reset switch, overload relay
                       adjustment, send-standby-calibrate switch, emission
                       selector switch, tune-operate switch, meter switch,
                       power amplifier excitation control, modulator bias
                       control, audio driver bias control, clipping level,
                       audio gain control, bandspread adjustment.
Accessories Required   High impedance microphone, telegraph key, 52 ohm
                       antenna, wiring to power source.

Power Source           230 v, 3 wire, 50/60 cycle, single phase, grounded
                       neutral; or 115 v, 2 wire 50/60 cycle, single phase.
Typical Power Demand,  Key closed             2000 w
CW                     Key open                800 w
                       Calibrate, key closed   660 w
                       Standby                 500 w
Typical Power Demand,  100% sine wave mod.    3100 w
Phone                  No modulation          2280 w
                       Calibrate               780 w
                       Standby                 600 w

Tube Complement        Oscillator - 2 6BA6
                       Exciter    - 1 6BA6, 4 6AQ5, 1 807W, 2 VR105, 1 6A10
                       Power Amp  - 2 4-250A
                       Speech Amp - 1 12AX7, 1 6AL5, 2 12AU7, 2 6B4G, 2 810
                       Rectifiers - 2 872A, 1 5R4GY, 3 5V4

Under the hood...

Block Diagram...

KW-1 CIRCUITRY

RF SECTION

The 70E-14 oscillator unit, shown in figure 6-11 consists of a 6BA6 oscillator, V-001, and a 6BA6 isolation stage, V- 002. The oscillator was baked dry before sealing, and should not be opened. If service other than tube replacement is required, the oscillator should be removed from the transmitter and returned to the factory as outlined in the maintenance section of this book. Good oscillator stability is made possible by regulation of the filament voltage as well as the plate voltage. OC3/VR105 gas type regulator tubes, V-202 and V-203, control the oscillator plate supply voltage. A 6A10 current regulator tube, V-201, is used in series with the filaments.

The 6BA6 oscillator tube, V-001, the 6BA6 isolation stage, V-002, and the 6BA6 buffer. V-301, always operate on 160 meters. The 6AQ5 first multiplier operates either straight through on 160 meters or as a doubler to 80 meters. The 6AQ5 second multiplier, V-303. is used only as a 40-meter doubler. The 6AQ5 third multiplier doubles to 20 or triples to 15 meters. The 6AQ5 fourth multiplier doubles to 10 meters. Double tuned circuits are used in the frequency multiplier stages to provide the necessary sub-harmonic and higher-harmonic attenuation.

Output from the proper multiplier stage is selected by S- 301G to drive the 807 rf driver stage, V-204. This stage is always operated as a straight-through amplifier to drive the 4-250A power amplifier grids.

V-101 and V-102, the parallel V-250A power amplifier tubes, operate at inputs up to 1000 watts on all bands except 160 meters. On the 160-meter band the power amplifier is designed for inputs up to 500 watts. A pi section followed by an L section forms the power amplifier output circuit. Each section is provided with a high-voltage gap to ground. L-117 is a static drain choke. A variable, high-vacuum capacitor is used for plate tuning in the pi-network input circuit. The pi network matches the power amplifier plate impedance to the input impedance of the L matching section. The matching section output impedance is 52 ohms and is designed to feed standard 52-ohm transmission line such as RG-8/U. Use of the L section in addition to the pi-section helps attenuate the higher harmonics. A gear-driven band switch, ganged with the band switches of the exciter, selects the proper value of inductance in the L and pi sections. The pi-L network used in this application will match a wide range of load impedances. However, in order to attenuate harmonics and prevent excessive rf voltages. The standing-wave ratio must be held to 2-1/2 to 1 or less. Use of RG-8/U coaxial line is recommended for all transmission lines within the building in which the transmitter is located.

Power and control leads from the exciter and power amplifier TVI shield are brought out through low-pass filters, thus reducing radiation of undesired signals from these leads. All controls used during normal operating procedures are located on the front panel.

During key-up conditions with cw emission, a blocking bias is applied through a wave-shaping filter to overcome the excitation voltage and cut off the plate current of buffer V-301 and multipliers V-302, V-303, V-304 and V-305. Plate current to the rf driver and the power amplifier is reduced to a safe value by fixed bias voltage.

AUDIO SECTION

The speech amplifier may be used with either crystal or high-impedance dynamic microphones. Suitable filtering is used at the input of the speech amplifier to keep rf voltages out of the first audio grid. The microphone input, J-401, incorporates provisions for push-to-talk operation. A 600-ohm phone patch input, J-402, is included in the second audio stage input circuit. When a remote line is used, an isolation transformer must be used to separate the dc voltages involved.

A peak clipper consisting of a series-diode gate limits the amplitude of the input signal to a predetermined value to provide a high average level of modulation without danger of overmodulation. The output of the second audio stage is coupled to the series clipper, V-402, through C-407. R-408 and R-411 are the clipper input and output resistors, respectively. The clipper plates are connected together and tied to the clipping level control, R-410, through series resistor R-409. R-410 acts as a voltage divider between the B+ line and ground. The exact point at which clipping will occur is set by R-410, which controls the positive potential applied to the plates of V-402.

Under static conditions, a dc voltage is obtained from voltage divider R-410 and applied through R-409 to both plates of V-402. Current flows from the power supply through voltage divider R-410 and the 330,000-ohm series resistor R-409. The current then divides through the diode sections of V-402 and their 220,000-ohm load resistors R- 408 and R-411. Under static conditions, the dc voltage drop maintains all parts of the clipper circuit at a positive potential above ground. The voltage drop between the plate and cathode of each diode section of V-402 is very small compared to the drop across 330,000-ohm resistor R-409 in series with the plates. The plate and cathode of each diode are therefore maintained at approximately equal potentials as long as there is plate-current flow. Clipping does not occur until the peak audio input voltage reaches a value greater than the static voltage at the plates of the diodes.

Assuming that voltage divider R-410 has been set to a point that will give 4 volts at the plates of V-402, when the peak audio input voltage is less than 4 volts, both halves of V-402 conduct at all times. As long as V-402 conducts, its resistance is very low compared with the 330,000-ohm resistor, R-409, in series with the plates. Whenever a voltage change occurs across input resistor R-408, the voltage at all of the tube elements increases or decreases by the same amount as the input voltage change, and the voltage drop across R-409 changes by an equal amount. This action permits all of the tube elements to be at the same dc level above ground. As long as the peak input voltage does not exceed 4 volts (to which the plates were assumed to have been set by R-410), V-402 acts merely as a conductor, and the output cathode is permitted to follow all voltage changes at the input cathode.

If under static conditions 4 volts appears at the diode plates, then twice this voltage, or 8 volts, will appear if one of the diode circuits is opened, removing its dc load from the circuit. As long as one of the diode sections continues to conduct, as is always the case with a clipper of this type, the voltage at the diode plates cannot rise above twice the voltage to which it was set by R-410. In our example this voltage cannot rise above 8 volts. If the audio input voltage through C-407 is increased to any peak value between zero and plus 4 volts, the first cathode or V- 402 will increase in voltage by the same amount to the proper value between 4 and 8 volts. The remaining tube elements will assume the same potential as the first cathode. However, the plates of V-402 cannot increase more than 4 volts above their 4 volt static level. When the input voltage through C-407 increases to more than plus 4 volts, the input cathode potential increases to more than 8 volts. The plates and output cathode potentials increase to 8 volts and remain there until the input voltage through C-407 drops below 4 volts.

When the input voltage swings in a negative direction, it will subtract from the 4 volt drop across R-408 and decrease the voltage on the input cathode by an amount equal to the input voltage. The plates and output cathode will follow the voltage level at the input cathode as long as the input voltage does not swing more than 4 volts negative. If the input voltage changes more than 4 volts in a negative direction, the plates will also become negative. The potential at the output cathode will follow the voltage at the input cathode and decrease from its normal value of 4 volts positive until it reaches zero potential. As the input cathode voltage decreases to less than zero, the plates will follow. However, the output cathode, which is connected to ground through R-411, will stop at zero potential as the plate becomes negative. Conduction through the second diode is impossible under these conditions. The output cathode remains at zero potential until the voltage at the input cathode swings back to zero.

The voltage across output resistor R-411 follows the voltage variations across input resistor R-408 as long as the input voltage does not swing to a peak value greater than the static voltage at which the plates are set by voltage divider R-410. When the static plate voltage is set at 4 volts, input voltage peaks greater than 4 volts in either direction cause the output voltage to swing 4 volts in the direction of the peak and remain at that level during the time the peak is above 4 volts. Effective clipping may thus be obtained at any desired level.

The square-topped audio waves generated by the clipper are high in harmonic content, but these higher order harmonics are greatly reduced in amplitude by a low-level speech filter consisting of C-409, L-401, and C-410, which attenuates all audio frequencies above 3000 cycles. A 12AU7 phase inverter follows the speech filter and excites a pair of 6B4G tubes used as a driver stage. The driver provides sufficient grid swing for the push-pull 810 tubes in the class B modulator. A second low-pass filter, C-503, L-503, and C-504, at the output of the modulation transformer further attenuates the high frequencies caused by the speech clipper and eliminates distortion products generated in the speech amplifier subsequent to the low- level speech filter. The modulator provides sufficient power to fully modulate 1000 watts input to the power amplifier.

When the transmitter is keyed on cw, a sidetone oscillator, V-406, is also keyed to provide an audio sidetone signal that may be used for monitoring purposes as described in paragraph 2-15. Each time the key is depressed, the voltage drop across the cathode of V-406B is made available for receiver muting as described in paragraph 2-14.

POWER SUPPLIES

Bias and plate power required by the transmitter are furnished by a bias supply, two low voltage supplies, and a high voltage supply. Improved voltage regulation in the high voltage supply has been obtained by connecting a 0.15 mfd capacitor across the input choke. This capacitor resonates the choke at approximately 120 cycles under no- load conditions. The resulting parallel tuned circuit presents a high impedance to the lowest (120-cycle) ripple frequency and aids in limiting the no-load voltage. In the interest of safety, electrical interlocks are used to disable the 2500-volt and 500-volt supplies when the rear door, upper front panel, or lower front panel is opened. However, interlocks do not disable the 500-volt supply when the SEND-STANDBY-CALIBRATE switch is in CALIBRATE position. As shown in figures 6-2, 6-3, and 7-2, high voltage shorting switches, S-501 and S-502, are provided to mechanically short circuit the high-voltage filter capacitors after the electrical interlocks have operated. This protection is additional to that provided by the bleeder resistor. The transmitter can be operated from 115 or 230 volts. (See paragraph 2-6.) Overload protection is assured by the use of an overload relay and by fuses in the bias supply, the low voltage supply, and the ac line.

The winding of the overload relay, K401 connected in series with the P.A. filament return; the contacts are in series with the H.V. interlock circuit as shown in figure 7-2. Power for the control circuit is furnished by the bias supply, making operation of the transmitter impossible unless bias voltage is present. The bias supply rectifier, a slow-heating tube, acts as a time-delay device to prevent operation of the control circuit and application of plate voltage to the 872A mercury vapor rectifiers before they have reached operating temperature. When 872A tubes are first placed in the transmitter, they must be operated with only the filaments energized for 10 minutes to allow the condensed mercury within the tube to vaporize.