500 kW HF Broadcasting Transmitter Type B 6128

photo Marconi B 6128 ADVANTAGES



The Marconi B 6128 high power hf transmitter combines the merits of high efficiency operation with a fast frequency-follow tuning system, low initial cost and a low maintenance and running cost.

The transmitter is equipped, by the addition of optional units, for:
  1. Single sideband operation in accordance with CCIR recommendations for broadcasting.
  1. Programme controlled carrier techniques.


The B 6128 is a Pulse Width Modulated Transmitter having an inherently high efficiency.
Many factors affect the actual efficiency of an hf transmitter. The component losses in the transmitter, especially in the final rf stage, will vary across the band of frequencies used, and alter yet again when replacement tubes are fitted. Probably the biggest influence on the basic efficiency of a given installation is that of the ambient temperature range at the transmitter site. This will in turn determine the size of the associated heat exchanger and therefore the power consumption associated with it. The efficiency quoted in the data summary is therefore a mean figure for all bands and under specified climatic conditions.

Radio Frequency Stages

The Radio Frequency stages of the transmitter are based on a concept which has been born as a result of over four decades of high power short-wave transmitter design.
This concept is based on high power rf contacts which:
  1. do not move on power,
  2. are water cooled on both sides of the contact,
  3. are held together under pressure.

The concept was proved in the first of our modern range of high power short wave transmitters, the 300 kW B 6124, of which 17 are in service world wide.
The B 6128 is one of several transmitters evolved from the B 6124 using the same basic radio frequency design but having a pulse width modulator to replace the Class B modulator.

Pulse Width Modulation

This very efficient form of modulation depends upon a switched modulator. The switching rate is high compared with the maximum audio frequencies involved and the duration of the 'on' periods is varied in sympathy with the amplitude of the incoming audio so that a high amplitude produces long on periods. A modulator of this type can be used in series (with a low-pass filter to convert the pulsed waveform to a varying dc) with the HT supply to the final rf amplifier. Since the losses in the modulator are low in both the 'on' and 'off' state the pulse width modulator reaches a very high level of efficiency.

Advanced Pulsam Modulator

In 1980 the Marconi Company patented a unique system of pulse width modulation (PWM) and gave it the name PULSAM.
The heart of the pulsam system is the low-loss, low-capacity grid-deck on which is mounted the final modulator tube.
This grid-deck is the main component in the ADVANCED PULSAM system used in the B 6128.
The Advanced Pulsam Modulator utilises a single tube whose size and cost is smaller than any other PWM series modulator of comparable output. This is due to the unique way the tube is operated which allows its full capacity to be used without the danger of overloading it in any way.
PWM signals are created in an encoder and fed to the grid-deck via fibre optic links.


R.F Circuits

The radio frequency drive for the transmitter is normally a synthesiser, type H1542, which can be mounted either at the front of the equipment or externally if preferred. Provision is made, in the H1542, for using an external frequency standard of 1 MHz. When this is used the internal standard will act as an automatic standby to the external standard.

The synthesiser output level of 100 milliwatts is raised to 100 watts by a wideband solid-state amplifier covering the frequency range 3.95 to 26.1 MHz.

The wideband amplifier is coupled to the penultimate rf stage by a circuit having fixed input and output capacitors, and a servo controlled variable inductor. The penultimate stage, utilising a water cooled tetrode type 4CW25000A, is situated immediately below the output stage. This allows the achievement of short connection paths to the final stage. The coupling circuit utilises a water cooled inductor and pneumatically actuated off-load rf switches similar in principle to the high power circuits. Naturally cooled servo-controlled variable-vacuum capacitors are used in the coupling circuit.

The radio frequency output stage uses a tetrode which, although physically small, has an anode dissipation rating far in excess of that required when the stage is fully modulated. This is achieved by 'Hypervapotron' cooling the anode. Dissipation per unit area of the grids can also be much higher than for conventional tubes by using pyrolytic graphite grids. These are formed by oriented graphite and provide an unusually high heat transfer to the support structure. Mechanically the grids are robust, improving their strength as the grids warm up. Their secondary emission is lower than that of wire grids and the relatively lower working temperature reduces primary emission due to deposition of the cathode material.

The tuning inductors are of novel construction and are formed from lengths of straight copper tubing folded into an ideal mechanical arrangement for each circuit. The fixed contacts of the off-load switches appear on one face of the folded inductor. The inductor section beyond the fold is only active for the lowest frequency range. This results in extremely small frame return path inductance between input and output circuit capacitors.

Cooling water is passed through the inductor copper pipes, which also serve as a distribution main for the water cooled capacitors. All joints are hard soldered.

A low pass filter is mounted on the rf cabinet roof and is connected in series with the outgoing 50 ohm feeder. It attenuates emissions in the tv, fm and communication bands in the range 40 to 300 MHz.

An external balun is available to convert the output to 300 ohm balanced line if required.

Output power and v.s.w.r are monitored by wideband couplers in the output circuit. Power is automatically reduced if the v.s.w.r exceeds a safe value. An auto matching facility will correct for slow variations in v.s.w.r resulting, for example, from antenna icing thereby maintaining the radiated power.

High Power Tuning Components

Tuning the transmitter involves two major features in the penultimate and final rf amplifiers. Coarse range switching is achieved by progressively reducing the active length of trombone type inductors by shorting bar switches as the frequency increases. These switches have replaceable, water cooled contacts and are pneumatically operated to provide a controlled rate of closure with a self cleaning action. The water cooling applies to both fixed and moving contacts and is optimised by directing the water at the rear of each contact. By this means sliding or rolling rf contacts have been eliminated from the final stage of the transmitter. In addition, the shorting bars are grounded when in the inoperative position.

Fine tuning is achieved by means of servo controlled vacuum capacitors, each of which is operated by a robust gearbox with a stepping motor. The gearboxes are activated by servo drives which are rack mounted modules situated at the front of the transmitter.

Changing Frequency

For any frequency not already stored in a memory a frequency change is accomplished by two operations: firstly, the 'keypad' is set to the required frequency, secondly, the 'Action' control is operated. Change of frequency then follows automatically. Remote control equipment to perform the same function must provide the keyboard for frequency selection and a fleeting contact for actioning frequency change.

Auto Tuning (Frequency-follow)

Having 'selected' and 'actioned' the desired frequency, output from the synthesiser is fed to ranging circuits.

The hf spectrum is sub-divided into a large number of bands to facilitate coarse tuning.

Receipt of frequency information causes the switches to take position necessary for the chosen frequency.

Similarly, each variable capacitor is set, on receipt of frequency information, to the approximate setting required for that frequency. If the balun is used its variable capacitor is also set to the value necessary for the desired frequency. Finally, the wideband driver stage output level is set. The preceeding actions are carried out with the transmitter in the 'Standby' mode.

Upon completion of the coarse tuning, full HT voltage is applied to the equipment, together with a reduced (tune) level of screen grid voltage.

The penultimate grid inductor is now tuned, indicated by minimum reflected power being fed back to the wideband amplifier.

Sequence switching then causes the penultimate stage anode to be tuned. The loading capacitor has been previously set to its approximate value.

The resulting final stage control grid current is then proved. This permits sequential tuning of the final stage output followed by the anode shunt and tuned circuits. They are indicated by minimum reflected power on the internal reflectometer and forward power level respectively.

A comparison is made of the forward power and dc input to the final stage anode. Drive level is adjusted to give the required final stage grid current.

Memorised frequencies

Whereas coarse frequency setting information is held in read only memory (ROM), much more detailed information can be stored in the random access memory (RAM) provided. Settings initially achieved by frequency-follow tune are fed into RAM. Selection of that frequency subsequently will cause the fine setting information to be accessed and used, in addition to the ROM data, to retrieve the required tune condition. This enables very fast frequency changes to be made to previous tuned frequencies, leaving only the optimisation to be achieved by the automatic process. Up to 4000 frequencies can be stored in this way tor fast subsequent retune.

Full screen voltage is then applied. When the forward power exceeds approximately 50 kW, programme is applied. The optimum operating condition is obtained by successive tuning of the final stage anode and coupling circuits using the 'hill climb' principle. During operation regular readings are taken of the output load match, and corresponding corrective action taken to maintain minimum internal reflected power. This sampling procedure causes automatic compensation for slow variations of v.s.w.r at the output feeder within prescribed safe limits. Should the operational v.s.w.r at full power exceed safe limits, the transmitter with automatically reduce power until the v.s.w.r improves, permitting full power to be safely restored, unless the v.s.w.r reaches the value of 4:1, when the transmitter locks out.

The Advanced Pulsam Modulator

Audio is processed by the solid state encoder located at the transmitter front panel. The resultant output is a string ot constant amplitude pulses at a high repetition rate, with the duration of the pulses varied in accordance with the audio amplitude.

The heart of the Advanced Pulsam System is the low loss, low capacity, grid-deck on which is mounted the final modulator tube. Because the modulator is in series with the final rf amplifier the grid-deck potential with respect to the transmitter frame varies between zero and the full HT voltage.

The pulse width modulation drive from the encoder is fed to the grid-deck through fibre optical circuits and there applied to both control and screen grids of the modulator tube. Circuitry in the grid-deck ensures that both grids are driven under constant current conditions. This enables the full capability of the tube to be exploited without overdissipating the grids, therefore allowing the use of a smaller tube than in the voltage driven case. This not only reduces replacement cost but also reduces the capacity of the tube to ground. As a result the switching loss due to stray capacity is reduced.

The other main source of loss is in the forward resistance of the tube when conducting. The pyrolytic graphite-gridded tube chosen for the modulator has a very low forward resistance in this state.

Filament, grid bias and screen supplies for the modulator tube are fed to the grid-deck through transformers with a large air gap between primary and secondary windings, providing low capacity to ground.

Feedback information, together with control and status information is also carried by the fibre optical circuits to the grid deck.


Silicon diodes are used, with the applied peak voltages limited by surge diverters, capacitor/resistor damping or other means of absorbing excess voltage. All rectifiers have adequate thermal capacity and protection to survive direct short circuits across their output terminals.

Diodes were chosen for the main HT rectifier in preference to thyristors because of their simple circuit requirements, proven reliability and cost effectiveness.

The transmitter tetrodes are rated to accept full HT voltage applied at the correct point in the starting sequence. Under fault conditions ac to the rectifier bridge is disconnected by vacuum switches, leaving rectifier diodes in conducting condition so that only uni-directional current can flow in the fault or fault diverter circuit. When thyristors are used their operation leaves an open circuited rectifier bridge. Stored energy in the capacitors and reactors causes a damped oscillation around the filter and modulation components unless special precautions are taken to avoid this situation.

If required, a 12 pulse rectifier system may be used to limit the harmonic currents introduced into the mains supply system.

Power Supplies

Two mains supplies are required for the transmitter: one three phase, four wire low voltage supply at 380 or 415 V for auxiliaries, bias and low lower HT supplies; the other a high voltage three phase, three wire supply for the main HT rectifier.

The transmitter dc power supplies are mounted in the main cubicle with the exception of the main HT which is in the rear enclosure.

Reduction of rf output to approximately half and quarter power is provided by low level control of the HT. This can be carried out manually by front panel control or automatically.

All rectifiers employ silicon diodes tested to withstand direct short circuits.

Control and Monitoring

All control and monitoring functions required for normal operation of the transmitter are grouped together on the front panel. From here the full start up and close down sequence can be controlled by single button control during normal operation. The full tuning sequence when selecting new frequencies is also initiated at the control panel area, and a full set of status and fault diagnosis indicators are provided. An oscilloscope enables the rf and audio waveforms to be observed and can be used in setting modulation levels.

Operational controls, together with status and fault diagnosis indicators, can be repeated remotely.

The centre passageway through the transmitter provides access to controls, interlocks and pressure gauges for the water cooling system and the pneumatic system for the rf switches. It also provides access to the HT enclosure when positioned to the rear of the transmitter.

Although a failure of the incoming a.c supply will cause the transmitter to close down, if the period of failure does not exceed fifteen seconds the transmitter will re-start immediately upon the reapplication of power, without going through the normal starting delays.


Maintenance requirements have received a high priority in the design of this transmitter. The twin cabinet arrangement provides exceptionally good accessibility. Moreover, in the rf cabinet there is a rigid division down the centre between the rf circuits and their ancillaries. Thus all motor drives for capacitor tuning, switch actuators, and the water cooling system are positioned in very accessible positions to facilitate routine checks and maintenance.

Tube changing is rapid because of the low temperature of the components and the quick release self sealing hose connections.

All the low level solid state equipment is grouped in the modulator cabinet with easy access.

Major diagnostic aids are provided on the front panel. The control unit includes an array of l.e.d's providing a full set of status and overload indications.

The low level units are mounted on telescopic runners for easy access.

Modular type construction is used in the logic and control circuitry, permitting easy replacement or exchange of logic boards. In many instances logic boards are common, thus economising in spares holding.


It is common experience to those who operate high power high frequency transmitters that many of the problems encountered can be traced back to overheating, especially those concerned with high power rf contacts. This problem becomes even more severe in automatic frequency changing systems employing moving contacts, and even more so at high power levels and when circuit currents become large. This particularly applies to continuously variable inductors.

Such high power inductors are, therefore, avoided in the B 6128 where range changing is accomplished by well proven pneumatically operated rf switches with water cooled contacts, and fine tuning by water cooled variable vacuum capacitors. The two high power vacuum tubes operate on the 'hypervapotron' principle and the remaining tube is conventionally water cooled. The cooling system employed contributes directly to the reduction of 'down-time' for maintenance or tube changing, since the transmitter parts are cool to the touch and such measures as the use of special gloves for handling hot components are not required.

Distilled or demineralised water is used as a coolant and is pumped in a closed water circuit which includes a reservoir tank and an external heat exchanger. Normally the heat exchanger is air blast cooled. Alternatively, a water cooled heat exchanger may be more suitable. Both the type of heat exchanger and its position in relation to the transmitter, can be chosen to suit the site conditions.

The air cooling system delivers high pressure air to cool the seals of the modulator, penultimate rf and final rf tubes. In addition, both the main transmitter cubicles and the HT enclosure are cooled by low pressure air. An external air filter is supplied; this may be a roll type filter (if this is demanded by the environmental conditions) or alternative filters of a suitable type. The air cooling systems are protected by pressure switches and thermal overloads and a 'hold on' circuit ensures adequate cooling after shut down for a few minutes.


The transmitter embodies comprehensive protection circuits whose operation is initiated by sensors which monitor overcurrent, overvoltage, high v.s.w.r or malfunctioning of the cooling system. A three shot recycling overload system is incorporated.

While great improvements have been made in reducing the hazard of internal flash-arcs in high power vacuum tubes, the manufacturers nevertheless specify a protection requirement should a flash-arc occur. These requirements are more than adequately met by the B 6128. In the final rf amplifier or modulator an incipient flash-arc triggers the 'crowbar' and also opens the six vacuum switches between the HT transformer and rectifier bridge.

Arc detectors are strategically positioned to detect any arcing occurring in the rf power circuits.


To ensure the safety of all personnel, a system of mechanical and electrical interlocks is used. With the main isolator open, access to all parts of the transmitter is available with complete safety. When closed, filaments, fans and control circuits can be powered with certain doors open.

However, where voltage over 50V ac are applied, covers and warning notices are fitted. Before high voltages can be applied, all doors have to be locked and the keys returned to an interlock panel. When all keys are located the earth switch and power switch can be operated, allowing power to be switched on. While power is on it is impossible for a key to be released.

Earthing is provided for the rf feeder and for high voltage supplies.

The equipment conforms generally to IEC215.

The outgoing rf feeder is isolated by a feeder earthing and isolator switch.


For ease of installation and transport the transmitter is built up from two cubicles, each of which dismantle into two sections which can be conveniently handled.
Each of the four sections is built on a robust steel base.


The installation layout of the transmitter is extremely flexible, since, apart from the basic cabinets housing the rf modulator and low power supplies, the high voltage and cooling equipment can be positioned to suit a single or double storey layout or can be adapted to suit an existing building.

The transmitter can be supplied with a 50 or 75 ohm unbalanced rf output or a 300 ohm balanced output. In the latter case a balun is supplied and this can be conveniently mounted immediately above the transmitter and forms the first part of the outgoing feeder to the station matrix.

Power output 500 kW, +0.2 dB, -0.4 dB into a matched 50 ohm unbalanced load, or a 300 ohm balanced load with external balun, at nominal mains voltages.
RF output load impedance Full power is delivered into a 50 ohm or 75 ohm unbalanced load max. v.s.w.r 2:1, or 300 ohm balanced load, max. v.s.w.r 1.8:1 using the external balun.
The balun can be designed for an impedance of 328 ohm for special cases.
Power output variation Full power up to 2:1 v.s.w.r (or 1.8:1 at balun output). Automatic reduction power output to approximately 50% or 25% for load v.s.w.r's up to 3:1 and 4:1 respectively.
In A3E mode, operation at half and quarter power is possible, selected by remote control, or locally.
Operating frequencies Any frequency, in 100 Hz increments, within the range 3.90 MHz to 26.1 MHz.
Full performance applies to the broadcast bands defined by WARC 1979 as follows :
3.90-4.00 MHz
5.95-6.20 MHz
7.10-7.30 MHz
9.50-12.05 MHz
13.60-13.80 MHz
15.10-15.60 MHz
17.55-17.90 MHz
21.45-21.85 MHz
25.60-26.10 MHz

Time for frequency change For autotune : Not exceeding 30s (typically 20s) for a change from any two frequencies inthe specified range, this time is taken to the restoration of programme modulation. (At full power, approx 45s).
Tuning method Pre-selection of up to 4000 frequencies to be stored in a memory, or by frequency following techniques to any frequency in the specified range as determined by the synthesiser. Settings are then automatically stored in the pre-selection memory. A manual tuning facility is also provided.
Type of transmission Amplitude modulation, double sideband (ITU Classification A3E)
Amplitude modulation, double sideband, with programme controlled carrier (ITU X3E)
Single sideband, with -6dB or -12 dB carrier reduction (ITU H3E or R3E).
Modulation High level anode and screen modulation using the advanced Pulsam pulse width modulation system for A3E. SSB modulation is by the Envelope Elimination and Restoration (EER) method.
RF harmonics and spurious radiations The mean power ot any spurious rf emission at frequencies up to 40 MHz when working into a matched test load will not exceed a value of -70 dB relative to the unmodulated carrier.
For harmonics related to the switching frequency the level will be -80 dB relative to the unmodulated carrier.
For harmonics and spurii above 40 MHz the level will be -80 dB relative to the unmodulated carrier.
Drive Synthesiser type, integrated with the frequency change system, mounted in the transmitter. The drive may be demounted and installed in a central area, and is capable ot frequency selection trom a remote point. Internal frequency reference normally supplied but external 1 MHz standard providing a level of 0.45 to 2.5 V r.m.s into 50 ohm. unbalanced can be used if required.
Frequency stability
  1. With internal standard 1 part in 108 per day. Less than 1 part in 107 over the temperature range -10°C to +55°C.
  2. With optional high stability internal standard 5 parts in 1010 per day. Less than 1 part in 108 over the temperature range -10°C to +55°C.
  3. With external standard. Will not degrade long term stability characteristics of applied external standard. See 'Drive' above.
Carrier shift Less than 5% amplitude between 0 and 100% modulation measured at 1 kHz with constant mains voltage.
Audio input impedance 600 ohm balanced (nominal). Return loss >20dB from 50 to 7500 Hz.
Nominal audio input level 100% modulation is obtained from a 1 kHz tone at any level from O dBm to 15 dBm, by adjustment of a preset control.
Audio frequency response ±1 dB between 50-Hz and 6000 Hz +1, -2 dB between 6000 Hz and 7500Hz relative to 1 kHz when transmitter modulated to 75%.
Audio frequency Harmonic Distortion 2.5% at 50% modulation and 4% at 90% modulation between 50 Hz and 7500 Hz.
Noise and residual modulation At least 56 dB below the level corresponding to 100% modulation by a sine wave signal at 1 kHz.
Output monitoring RF at approximately 2 V peak carrier (at 500 kW) into 50 ohm unbalanced. A demodulated output provides O dBm +3 dBm (adjustable) into a 600 ohm balanced load at 100% modulation.
Transmitter rating 100% sine-wave modulation, 60 Hz to 7500 Hz, for 10 min per hour, followed by 70% modulation for 50 min per hour.
75% sine-wave modulation, 60 Hz to 7500 Hz, continuously.
Incoming power supply Auxiliary circuits : 380 V or 415 V (to be specified with order) three phase, 4 wire, 50 Hz (Equipment for 60 Hz can be supplied to special order).
Main HT rectifier : Arranged to suit customer's supply. Normally 3.3 kV or 11 kV three phase.
Variation of supply voltage Auxiliary circuits : ±10%
Main HT supply :
The transformer is provided with taps in order to compensate for the difference between the normal supply voltage to the transmitter where this differs from the nominal voltage.
The range of adjustment is ±6% in 3% steps. Having selected the correct tap setting the transmitter will remain operational throughout short-term supply voltage variations of ±6% with respect to normal value. In addition full performance (except power output) will be maintained with variations of ±2% of nominal value.
Variation of supply frequency ±4% reference nominal frequency.
Overall power factor of equipment Better than 0.9.
Auto-restoration The max period of supply failure for immediate automatic restoration is 15s.
Overall Efficiency 65% averaged over the range of frequencies in the broadcast bands at 500 kW, when used with cooling equipment optimised for operation at up to 300m above sea level and at a max ambient temp. of 35°C.
Environmental conditions Operational:
Ambient temperature: 1° to +50°C max.
Average temperature over any 24 hour period not to exceed 45°C and, over one year, not to exceed 30°C.
Maximum Altitude : 2300m (approx. 7500 ft) above sea level.
Maximum relative Humidity : 95%.
Storage and Transport:
Ambient temperature range, with all cooling fluids drained, -40°C to +60°C.
Maximum altitude: 10000 m (approx. 32500 ft.) above sea level.
Maximum relative Humidity: 95%.
Maximum dimensions (excluding heat exchanger and air handling system):

HT Enclosure
Width (mm)

Depth (mm)

Finish Cabinet
Control panels
Meter panels
Dark grey
Light grey
Morning Mist
Legend English.
Specifications may change without notice

RF stages AF stages and modulator
Number Type Number Type
1 4CM500,000G or TH558 1 4CM300,000GA or TH537
1 4CW25,000A


ITU Country
ITU Country