KWS-1 Transmitter



The most advanced design features ever offered in an Amateur transmitter are incorporated in the KWS-1. Unprecedented compactness is achieved without crowding; the exciter and RF power amplifier are housed in a single receiver-size cabinet which can be placed on the operating desk or mounted on top of the power supply cabinet.

Collins engineering plus extensive on-the-air operation account for the KWS-1's reliability and optimum performance in CW, AM, and SSB operation. Circuit applications and components which have been proved in preceding Collins equipment are retained in the design of the KWS-1 - 70E VFO, Pi-L output network, extremely accurate VFO dial, and the Collins Mechanical Filter, to mention a few.

The frequency generating system provides stable output on the desired frequencies with minimum low order mixer crossover products and spurious responses. VFO operation is provided in amateur bands from 3.5 to 30 megacycles, with a dial calibration of 1 kc per division on all bands. Single conversion is used on the 80 meter band and dual conversion is used on all higher bands. Maximum overall stability is obtained by using an extremely stable variable oscillator and crystal controlled high-frequency oscillators and BFO. A permeability tuned, hermetically sealed VFO is used to provide a stable and accurately calibrated signal source. A new PTO oscillator, Type 70E-23, was designed to give stability comparable to that of Collins 75A-4 Receiver.

By using the Mechanical Filter, the Single Sideband generator provides more than 50 db rejection of the unwanted sideband and limits the audio passband to 3000 cps. By use of the balanced modulator in conjunction with the Mechanical Filter, the carrier can be reduced more than 60 db. The third order distortion products are down approximately 35 db.


POWER AMPLIFIER INPUT - 1 kw peak envelope power on SSB, 1 kw CW operation.
     Equivalent to 1 kw on AM when using narrow bandwidth receiver.



AMATEUR BANDS COVERED - 80, 40, 20, 15, 11, 10 meters.


            80           3.0 -  4.0
            40           7.0 -  8.0
            20          14.0 - 15.0
            15          21.0 - 22.0
            11          26.4 - 27.4
            10          28.0 - 29.0
            10          29.0 - 30.0

EMISSION - SSB, AM carrier plus one sideband, CW.

FREQUENCY CONTROL - 70E-23 Master Oscillator.

HARMONIC AND SPURIOUS RADIATION - (Other than 3rd order distortion products.)
     Intra-channel radiation is at least 50 db down. All spurious radiation
     is at least 40 db down at the output of the exciter. The second harmonic
     is at least 40 db down and all other harmonics are at least 60 db down.

FREQUENCY STABILITY - After 15 minutes warm-up, within 300 cps of starting
     frequency. Dial Accuracy: 300 cps after calibration.

AUDIO CHARACTERISTICS - Response: +/-3 db, 200 to 3,000 cps.
     Noise and hum: 40 db or more below reference output level.
     Input: .01 volts for rated power output.

DISTORTION - SSB, 3rd order products approximately 35 db down
     at 1 kw PEP input.

MICROPHONE INPUT - Will match high impedance dynamic or crystal.

PHONE PATCH INPUT IMPEDANCE - 600 ohms, unbalanced to ground.

WEIGHT - 210 pounds (both units).

DIMENSIONS - 40-1/2" high, 17-1/4" wide, 15-1/2" deep (both units).

RACK MOUNTING - Angle bracket kits available for RF unit and power supply.

TUNING CONTROLS - Bandswitching, frequency selector, PA tuning, PA loading.

OTHER CONTROLS - Filament power, plate power, filament adjust, PA bias
     adjust, tune-operate, multimeter switch, VOX speaker gain, VOX speech
     gain, band change, audio gain, sideband select, emission selector,
     dial lock, zero set.

ACCESSORIES REQUIRED - High impedance microphone, telegraph key,
     52 ohm antenna.

POWER SOURCE - 230 v, 3 wire, 50/60 cycle, single phase, grounded neutral;
     or 115 v, 2 wire, 50/60 cycle, single phase. 1500 W 1 kw input CW.

Under the hood...

[Internal View]


While the KWS-1 Transmitter is capable of AM, SSB, FSK, or CW operation, its design emphasis on SSB perhaps warrants description in the SSB condition. Referring to the block diagram, the audio input is fed through a two-stage amplifier to a cathode follower (1/2 12AT7). This cathode follower matches the audio signal impedance to that of the balanced modulator. The output of the low frequency oscillator is also fed into the balanced modulator. The output of the balanced modulator will contain both upper and lower sideband signals with the carrier balanced out. By selecting the proper LFO frequency, either the upper or lower sideband signals can be placed in the passband of the 250 kc Mechanical Filter. At the output of the Filter, one of the two sidebands appears. The carrier and opposite, or undesired, sideband will be greatly suppressed. The output of the Filter is fed to the first mixer, (12AT7) and here it is mixed in a dual triode with the VFO output. The output of the mixer is fed through a two-stage linear amplifier with five Hi-Q tuned circuits. These two stages (6BA6's) and the five coils provide sufficient gain and selectivity to give an output signal in the 80 meter band essentially free of spurious signals. On 80 meters, the signal is then fed to the driver stage which is a pair of 6CL6's. The output of the 6CL6's is very conservatively rated at 3 watts PEP. This is sufficient output to drive the linear Class AB1, 1 kw PEP power amplifier.

On all bands except 80 meters, the output of the two-stage RF amplifier is fed to a second mixer. Here it is mixed with a crystal oscillator. The proper crystal is selected by the bandswitch to give the correct output frequency for the desired band. The output of the mixer is fed to an RF amplifier with three Hi-Q tuned circuits. Again sufficient gain and selectivity is provided to give an output signal on the desired band that is essentially free of spurious signals. The output of this stage is then fed to the pair of 6CL6's in the driver stage.

The Linear RF Power Amplifier uses two 4X250B's in Class AB1 operation. The 4X250B's are used because of their superior performance as linear amplifiers, their small size, and lower plate voltage requirements. Negative RF feed-back is employed to improve linearity. Third order distortion products are reduced to approximately 35 db below either tone in a two-tone test.

The speaker anti-trip circuitry is in the audio section of the exciter. Here a differential-type circuit is provided that compares the audio level from the speaker -as picked up by the microphone- with the audio level from the receiver output circuit. When this circuit is balanced, loudspeaker signals picked up by the microphone will not actuate the VOX relay. However, the operator speaking directly into the microphone will actuate the VOX relay.

When operating AM, the output of the LFO is fed around the Mechanical Filter and provides operation with full carrier and one sideband. It is possible in this position to vary the amount of reinserted carrier, thus providing Single Sideband reduced carrier operation.

CW operation is provided also by feeding the output of the LFO around the Mechanical Filter and by blocked-grid keying of the 6BA6 amplifier following the first mixer and the output stage.


High and low voltage power to the KWS-1 Transmitter is provided by the 428A-1 Power Supply, contained in a single cabinet 17-3/8" wide, 30" high, and 15-1/2" deep. The 428A-1 Power Supply provides filament power and filtered dc voltage to the plates and screens of the 4X250B Power Amplifier tubes. The 428A-1 also includes a plate contactor, fuses, primary switch, suitable interlocks, and a blower to provide cooling air for the Power Amplifier tubes.

The Low Voltage Supply consists of a single-phase full wave rectifier and filter supplying dc plate voltage to the exciter, a second full wave rectifier which furnishes negative dc voltage for bias and blocked grid keying of the transmitter, and filament power for the exciter and oscillator tubes. A fuse is provided in the primary power line for equipment protection.


The audio signal is fed from the microphone jack J101 on the front panel through a two-stage amplifier into a cathode follower audio output stage. The cathode follower matches the audio signal to the 1N67A diode balanced ring modulator. The output of a 250-kc oscillator is amplified and fed into the balanced modulator. The output of the modulator contains both upper and lower sidebands, but the carrier is attenuated by more than 50 decibels. The signal is fed through a mechanical filter, the input of which is series tuned to match the low impedance of the balanced modulator. The FL101 Mechanical Filter has an output containing either the upper or lower sideband, depending upon the selection of the operator. The carrier and undesired sideband are greatly suppressed, or, for all purposes of signal consideration. are eliminated.

The output of the FL101 Mechanical Filter is parallel tuned to match the high impedance of the first mixer grid. In addition to accepting output signals from the FL101 filter, the first mixer also accepts signals from the vfo. The mixer plate circuit contains a special feedback network which provides an additional 20 decibels of attenuation to undesired vfo signals appearing in the plate circuit. The desired output of the mixer is passed through two stages of linear amplification, including five high "Q" tuned circuits. The tuned circuits provide sufficient gain and selectivity to produce essentially spurious-free signals in the 80-meter band.

At this point, the 80-meter signals are fed directly into the paralleled driver stages, while the frequencies of the 40, 20, 15, 11, and 10-meter bands require further conversion. These signals are fed into a second mixer, which also receives an input from a crystal oscillator. The oscillator signal is selected by the BAND CHANGE control to provide correct output frequencies for the desired band. The conversion frequency output of the second mixer is fed through two stages of r-f amplification, including three high "Q" tuned circuits to produce a signal which is again, essentially spurious-free. The amplified signal is then fed into the 6CL6 driver stages. The output of the drivers is fed directly into the Power Amplifier.

The Power Amplifier is designed for linear class AB1 operation with individual tuning and loading controls in the final amplifier output circuit. Grid-plate neutralization is included to provide improved stability of operation. An r-f feedback system is used to permit operation with lower distortion.

An Automatic Load Control circuit is also included in the Power Amplifier, however, a complete analysis of this circuit is included in the detailed circuit description.


For AM signals, the output of the 250-kc oscillator is reinserted at the output of the mechanical filter to provide operation with carrier and one sideband. By adjustment of the CARRIER LEVEL control, it is possible to vary the amount of carrier reinserted and produce the proper relationship between the sideband and the carrier.


CW operation is produced by reinserting the 250-kc oscillator signal at the output of the mechanical filter, as in AM operation, and by grid-block keying of the first mixer and the driver stages. The EMISSION control eliminates exciter circuits unnecessary for cw signals. A wave shaping network is included to reduce transients in the cw output.


Although a 600-ohm audio input circuit is available for use with an external phone patch circuit, it is also possible to feed an external audio FSK oscillator into this circuit to provide for carrier shift FSK operation. With the EMISSION switch in the SSB position, the mark and space frequencies of the audio oscillator produce two r-f voltages in the exciter output, spaced by the correct amount for all bands.

75A-4/KWS-1 Accessories
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Copyright © 1996-WA3KEY & 1957-Collins Radio Company
This Home Page was created by wa3key, Thursday, February 22, 1996
Most recent revision Friday, March 15, 1996