KW204 Transmitter

The KW204 transmitter was an HF amateur band transmitter capable of delivering around 100 W PEP output from two 6146 valves / tubes.

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The KW204 was the last separate HF SSB / CW transmitter designed by KW Electronics. It essentially adopted the transmitter elements from the KW2000B, but was contained within a G-Line case and had its own integral power supply.

Running two 6146 valves in the final, it had a final DC input power level of around 180 watts PEP, and this gave around 100 watts PEP output. This enabled the transmitter to provide global amateur radio communications using a good antenna.

The KW204 was a reliable transmitter that was used by many who already had a receiver that they liked and wanted the flexibility of having "separates."

As the transmitter was pre-WARC 79, it only included the traditional HF amateur radio bands from 160 metres through to 10 metres.

KW204 HF SSB / CW amateur radio band transmitter it was contained in the KW G-Line case and had its own internal power supply.
KW204 transmitter

KW204 transmitter basics

The KW204 was a convenient transmitter to use, having sufficient controls for successful operation without having a very cluttered front panel.

The circuit for the KW204 was quite straightforward and int he years since its launch in the early 1970s it has proved to be a reliable transmitter, and many are still in use to this day.

The transmitter did not over-stress components such as the RF PA final valves - some other designs used TV line output valves that could take short duration peaks, but cold not tolerate high duty cycle operation. Accordingly the PA valves tended to have a much longer life than those used in some other transmitters and transceiver.

KW204 HF SSB / CW amateur radio communications transmitter in a radio station with an Eddystone EA12.
KW204 transmitter in a radio station with an Eddystone EA12 as the companion receiver

The KW204 provided a good specification which compared well with those from other manufacturers and at the time the cost was quite competitive, especially for those in the UK where the transmitter was built.

KW204 transmiter specification

The specification for the KW204 was provided in the technical handbook as well as other marketing material from the company.

Brief Specification for the KW204 Vintage Radio Transmitter
Parameter Specification
Summary All band (pre-WARC '79) amateur radio transmitter.
Modulation types SSB, CW, AM
Frequency coverage The KW204 transmitter covered all HF amateur bands of the time from 160 metres up to 10 metres in 500 kHz segments.
160 metres:     -     1.8 - 2.3 MHz
80 metres:     -     3.5 - 4.0 MHz
40 metres:     -     7.0 - 7.5 MHz
20 metres:     -     14.0 - 14.5 MHz
15metres:     -     21.0 - 21.5 MHz
10 metres:     -     28.0 - 28.5 MHz
10 metres:     -     28.5 - 29.0 MHz
10 metres:     -     29.0 - 29.5 - 2.3 MHz
10 metres:     -     29.5 - 30.0 MHz
DC power input to final RF power amplifier SSB     -     180 watts PEP
CW     -     150 watts
AM     -     75 watts
Power output SSB     -     100 watts PEP nominal
Output impedance 52 / 75Ω with SWR 2:1
Microphone impedance High impedance
Audio frequency response 300 - 2500 Hz at -6dB
Carrier suppression 50dB
Unwanted sideband suppression 45dB
Third order distortion -30dB
Power input 117V or 234V ±5% at 45 - 65 Hz
Power consumption Approximately 320 watts on transmit
Weight 27 lbs

In addition to this the advertising material advertised that there was the capability for an additional VOX unit to be added, ALC was present to ensure splatter was minimised and that the set had "silky smooth tuning."

KW204 valve line-up

The valve line up within the KW204 consisted of 11 valves and there were also 19 semiconductor diodes.

The valves are the "miniature types" with B7G and B9A bases with the exception of the much larger final amplifiers that use a form of Octal base.

Valve Line-up for KW204 Vintage Radio Communications Transmitter
Valve Reference Type Use within the circuit
V1 12AX7 Microphone amp & tone osc.
V2 12AT7 Audio cathode follower & carrier oscillator
V3 EF183 455 kHz amplifier
V4 12AT7 1st mixer
V5 12AT7 2nd mixer
V6 6CH6 PA driver
V7 6146 RF power amplifier
V8 6146 RF power amplifier
V9 6U8 Variable frequency oscillator
V10 6AM6 HF crystal oscillator
V11 OA2 Voltage stabiliser

KW204 transmitter circuit description

The KW204 has a fairly traditional block diagram and the circuits are relatively standard - few surprises to be had when looking at the way it works. However this has meant that the circuit is reliable and works well, as the basic circuit has bene updated from previous designs including the KW2000, KW Vespa, etc.

Circuit block diagram of the KW204 vintage radio transmitter
Circuit block diagram of the KW204 vintage radio transmitter

The signal chain started with the audio input. This is stage used half of a 12AX7 double triode. It provided a high input impedance to the microphone allowing ceramic and other similar microphones to be used along with the lower output dynamic microphones.

The output from the second stage of audio amplification passed into a cathode follower audio buffer stage.

The carrier oscillator is provided by the triode of a 12AX7. This is a crystal controlled oscillator and two crystals are used, one for USB and the other for LSB.

The audio and carrier oscillator are applied to a single balanced modulator or mixer consisting of two semiconductor diodes. Balancing the mixer properly enables the carrier level to be suppressed sufficiently.

The output from the balanced modulator was passed into an EF183 pentode amplifier. This not only amplified the signal level but also acted as a buffer after the balanced modulator, providing a much more consistent load impedance for the balance modulator.

After the EF183 amplifier, the signal entered the 455 kHz sideband filter - a Kokusai mechanical filter. This removed the unwanted sideband as well as providing a little extra carrier attenuation.

The signal required converting from the fixed IF up to the final operating frequency. This was accomplished in two stages. The first stage mixed the IF single sideband signal with the variable frequency oscillator to convert the signal up to the broadband IF of between 2.655 and 3.155 MHz. A 12AT7 double triode was configured as a balanced mixer for this.

The second frequency conversion used a crystal oscillator fed into a second 12AT7 double triode balanced mixer circuit. One crystal was used for each band to give the correct output frequency.

Once through all the conversions, the signal was amplified, first to provide an intermediate power level using a 6CH6 to give sufficient power to drive the two 6146 output power valves.

To generate CW and introduce carrier for AM, a small amount of carrier at the base intermediate frequency was introduced into the grid of the first mixer. This gave a much better CW note than the audio oscillator that was used in the KW2000

The KW204 was a popular transmitter at the time and it provided excellent service in many amateur radio stations. Being around 50 years old now, it can certainly be termed a piece of vintage radio equipment, but it can nevertheless perform well, even on today's amateur radio bands.

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