photo Marconi B 6128S 1. INTRODUCTION

Marconi design embodies the important concept of evolutionary development. Before developing any new equipment, our design engineers make a careful study of current market requirements and available technology and integrate these factors with existing Marconi products in the most cost effective manner.

The Marconi transmitter, type B 6128S, is a product of this evolutionary design philosophy. It combines major features from its immediate predecessors, the B 6128, B 6127, B 6126, and B 6124 and integrates these with existing technology to provide a proven technically competent equipment capable of meeting all the needs of a modern broadcaster. The evolution has followed the pattern of Table 1 below:

Table 1: The Marconi B 6128S is the product of evolutionary development
Date of
Features No.
B 6124 1975 300 kW * New RF stage proved
* Frequency preset proved
* Class B modulator
* Solid state control proved
B 6127 1981 500 kW * PULSAM PWM modulator
* TH558 RF tube proved
B 6126 / 6131 1984 300/250 kW * ADVANCED PULSAM modulator proved
* Frequency follow system proved
* Simplified water cooling system
* Revised modular control circuits proved
B 6128 1987 500 kW * Improved B 6126 to allow SSB, (linear and EER) DAM etc 23
B 6128S 1993 500 kW * Derivative of the B 6128 to incorporate all solid state modulator .

Marconi also is a manufacturer of high power, high quality, single sideband communication equipment. The B 6128S transmitter makes full use of the designs, techniques and experience employed in its communication equipment.

No major elements of the transmitter are new or unproven. The main blocks of the B 6128S are drawn from existing equipment, as shown in Table 2.

Table 2: Proven technology is used in the B 6128S
Current Element Description
RF stages Same concept and design as used in Marconi types B 6124, B 6126, B 6127 and B 6128
Modulator Manufactured by Continental Electronics Corp and already fully proven on 500 kW shortwave and 600 kW mediumwave transmitters.
Frequency follow system Circuit design is identical to that used in B 6128
Frequency preset system As used in B 6128
Tube and component cooling As used in B 6128
Control circuits As used in B 6128

The current demands of short wave broadcasting have resulted in a requirement for a transmitter that must satisfy the following:

- High power

- Good overall efficiency

- Fast automatic frequency change

- Minimum number of valves (tubes)

- Fully automated control system

- Remotely controllable

- High reliability

- Easy to understand circuits

- Simplified fault finding with built in diagnostics

- Good access

- Easy maintenance

- Multiple sourcing of all components

- "User friendly" operation

- Capable of SSB operation

The Marconi B 6128S meets all of these requirements. It is the result of years of engineering experience backed by a thorough understanding of modern broadcasting requirements and techniques. It is a transmitter that has been designed to take on the challenges of the overcrowded HF spectrum and win.

2. THE B 6128S - High in Benefits, Low on Costs

The Marconi B 6128S 500 kW short wave broadcast transmitter is a highly efficient, automatically tuned design having a fast frequency change, requiring the minimum of maintenance and being equipped for remote control.

The transmitter is designed to help the operator.

The high reliability, the low capital cost, the low running cost, the low maintenance cost and low cost of alternative replacement tubes, results in a transmitter with the best value for money on the market today.

The B 6128S incorporates the latest technological advances but uses well tried and proven components throughout the design.

It produces a powerful 500 kW of carrier plus 100% modulation at all frequencies in the broadcast bands between 3.90 and 26.1 MHz.

The B 6128S automatically reduces power in two stages if the VSWR on the output feeder exceeds 2:1.

The three pi circuits in series in the RF output stage reduce spurious emissions and harmonics to more than 70 dB below carrier level and better than 80 dB above 40 MHz.

Frequency follow wave changing is achieved in an unequalled time of 12 seconds. When this time has elapsed the transmitter is radiating program, at reduced power, quickly followed by full power output.

Frequency preset facilities are provided with the ability automatically to store up to 4000 pretuned frequency settings. The transmitter can be in full power service on any one of them in less than 12 seconds.

The use of sliding radio frequency contacts has been deliberately avoided in high power stages in order to minimise maintenance and obtain the highest reliability.

High level anode and screen modulation is provided from an all solid state modulator. This is an extremely efficient form of PSM by virtue of the small forward loss and the small capacity and switching losses.

Solid state CMOS logic is used extensively in control circuits and is filter protected where necessary against radio frequency interference. This type of logic is ideal for remote computer control.

The solid state control circuitry is easy to understand.The equipment has a comprehensive set of diagnostics which immediately localise a fault, thus reducing maintenance costs.

Although the transmitter has a compact layout, all components are easily accessible. The B 6128S has an extremely well engineered safety system. Personnel are fully protected from induced voltages and currents in the outgoing feeder by an isolating and grounding switch.

The transmitter is not vulnerable to mains surges and is fully protected against damage. This is of vital importance in locations where the quality of the mains supply cannot always be guaranteed.

The B 6128S operates in ambient temperatures up to 50°C. Only the heat exchanger requires rating for the conditions on site. This is done to avoid an unduly large and expensive heat exchanger being used at temperate sites. On sites where the external temperature is likely to drop below 2°C (35.6°F) the external heat exchanger is in a secondary glycol/water circuit. This circuit is coupled to the primary water cooling circuit via a plate type heat exchanger.

The B 6128S operates with mean internal temperatures of less than five degrees centigrade above the transmitter hall ambient. The rise is substantially unaffected by the ambient temperature. Therefore, it will be possible to begin internal maintenance immediately on shutdown of the transmitter.

The level of ionising radiation is reduced to a very low value by lining transmitter doors with lead sheet in strategic places. This is just one of the features that goes to make the B 6128S one of the safest high power transmitters available.


The transmitter radio frequency stages, with the exception of the synthesized exciter and wideband amplifier, are housed in the left hand cabinet. The penultimate and final RF stages occupy its front area with the output circuits occupying the rear. A grounded wall separates the output circuits from the variable capacitor control motors and the range switch actuators. These control mechanisms and the water cooling circuit distribution are housed in the "dead" area behind the grounded wall. Also located in this area is the compressed air distribution system for the range switch actuators.

The wideband radio frequency amplifier driving the penultimate RF stage is located above the centre gangway between the left and right hand rows of cabinets.

The synthesizer is housed in the transmitter control and monitoring area at the front of the right hand row of cabinets. This area contains all the metering and monitoring to tune the transmitter and establish that it is operating correctly. The front area is split into three vertical rows. The left hand row houses the transmitter control and overload modules located below the synthesiser. The servo control modules of the frequency follow tuning system are mounted below the control and overload modules.

The centre row accommodates the RF and modulator meters grouped together at the top. Immediately below is a servo override panel which allows the transmitter to be tuned by individual motors in an emergency. In this condition the motors are driven directly by a 50 or 60 Hz supply. The next panel down houses the remote control and monitoring interface (RCMI) option, if fitted.

The right hand row contains the monitoring oscilloscope at the top with the audio input unit below it. The modular control rack for the PSM modulator is housed below the audio input unit. The trip counters are located adjacent to this rack.

Approximately half way down the right hand row there is a series of modules which effects the coarse and fine tuning of the frequency follow system.

A comprehensive range of l.e.d.s is provided on the front panels of the modules for diagnostic purposes.

There is a fully integrated control and monitoring area at the front of the right hand row of cabinets.

The space behind the front control and monitoring area is taken up with "filtercons" and wiring connector blocks. These filtercons filter out radio frequency radiations from connections to sensitive and low level circuits.

Immediately behind the control rack is the cabinet containing the auxiliary power and distribution components. The multiway grounding switch is mounted in this cabinet.


4.1 Wideband Amplifier

Output from the synthesised exciter is fed to the wideband amplifier. This amplifier is mounted in the low pressure air duct above the centre gangway between the Radio Frequency and Modulator Cabinets. The wideband amplifier produces up to 100 W of carrier over the entire HF range of frequencies. The power required at any particular frequency is determined by a servo controlled level adjustment on the wideband amplifier input.

Protection is provided against loss of cooling air on the heat sinks and/or severe mismatch on the amplifier 50 ohm output by rapid switch off. The wideband amplifier is coupled to the penultimate RF stage by an impedance step up matching network, consisting of fixed input and output capacitors and a servo controlled variable inductor.

4.2 Penultimate RF Amplifier

The penultimate stage uses a water cooled tetrode type 4CW25000A, operating in Class B, which is located immediately below the final RF stage so that short connection paths are achieved. High tension supply to the stage is from a dedicated power pack but the supply for the screen grid and bias for the control grid are derived from those for the final RF stage.

Coupling to the final stage grid is by an impedance step down matching network. At frequencies of 17 MHz and above, the output tuning capacitor of the pi circuit is switched in series with the inductor, leaving the final stage input capacitor as the output matching element. The output power from the penultimate stage is approximately 5 kW. Half of this power is dissipated in damping resistors connected between the grid of the final RF stage and ground. This makes the final RF stage very stable and free from parasitic oscillations.

4.3 RF Final Stage

The final RF stage utilises a tetrode type TH558 or equivalent in grounded cathode mode. The tube is cooled by the hypervapotron principle and the anode (plate) has a dissipation rating of 500 kW. This dissipation rating provides a good safety margin against mistuning. The anode circuit consists of a tuned circuit in shunt with a pi tuned matching circuit. The combined "Q" of the two circuits is high thereby contributing to excellent harmonic frequency rejection and protection against cross-modulation from other transmitters.

The anode pi matching circuit is coupled to a double pi matching circuit via a section of 110 ohm coaxial line. The double pi tuning is arranged that it always presents a load of 110 ohm to the coaxial line irrespective of frequency or external load variations. Tuning of the final stage output circuits is achieved by monitoring the backward power on the 110 ohm line and controlling the tuning capacitors to give a near zero minimum value.

As well as enabling the final stage to deliver its rated power into varying loads at the transmitter output, the double pi tuning in conjunction with the anode tuned circuits, provides spurious and harmonic output rejection of at least 70 dB.

4.4 Low Pass Filter

The double pi matching circuit is connected to a low pass filter. This filter has input and output impedances of 50 ohm or 75 ohm.

The low pass filter is mounted on the roof of the RF cabinet, it is water cooled and comprises an eight section ladder network. It provides additional high attenuation in the FM and TV bands above 40 MHz and in the aircraft bands between them. It is a true fixed tuned low pass filter, derived from work on FM and TV harmonic attenuators.

The harmonic and spurious signal rejection of the final RF stage output tuning pi circuits combined with the low pass filter provide a spurious and harmonic level of approximately 90 dB or more below carrier depending upon the frequency and harmonic rejected. At frequencies above 100 MHz the rejection rises to at least 105 dB. Marconi engineers designed the RF output stage with three pi circuits in series in anticipation of a tightening of international regulations on spurious and harmonic emissions.

A reflectometer section is located at the end of the low pass filter. The strip line couplers in it provides information on the carrier forward and backward power. From these parameters the VSWR of the following 50 or 75 ohm transmission line is determined and used to control both the tuning and carrier power level of the transmitter.

4.5 Balun

Where it is desired to operate the transmitter into a balanced antenna feeder system a balun can be provided. In this case the transmitter will be built for a 75 ohm output impedance and the balun, giving the standard 4:1 step up ratio, added so that the transmitting system has a nominal output impedance of 300 ohm. It is rated for 550 kW carrier power plus 100% modulation and can tolerate a VSWR of up to 2.0:1 at its output under full load conditions.

The balun is a wideband device covering the frequency range 3.9 to 26.1 MHz. It is tuned by two range switches for coarse setting, and fine tuning is implemented with a single variable capacitor. Switching and tuning is under the control of the main transmitter control system so that the balun's tuned frequency will track that of the transmitter to which it is connected. All tuning is effected during the first stage tune of the transmitter, that is, within 5 seconds of a frequency change command being given and before the re-application of the HT supply.

4.6 Output Power and Load VSWR

The transmitter will deliver a power of 500kW into a nominal 50 or 75 ohm load, having a VSWR not more than 2:1, over the broadcast bands between 3.9 and 26.1 MHz. Where a balun is fitted this VSWR is reduced to 1.8:1 at the 300 ohm balun output. If the VSWR of the load increases to more than 2:1 but less than 3:1 the transmitter automatically reduces to half power (250 kW).

If the VSWR increases to more than 3:1 the transmitter automatically reduces to quarter output power (125 kW).

If the VSWR exceeds 4:1 the transmitter will begin its trip sequence and will lockout.

If, before lockout occurs, the VSWR improves to better than 3:1 the transmitter will automatically increase to half power then to full power as it improves to less than 2:1. No operator intervention is necessary and this feature has been of considerable benefit at locations where severe weather conditions cause temporary degradations of VSWR.

Differentiation between the reducing and increasing VSWR power change levels avoids hunting.

4.7 Automatic Load Matching

In normal operation gradual changes of VSWR are compensated by the automatic load matching facility built into the frequency follow tuning system. This system maintains the correct output circuit tuning to enable the transmitter to deliver the transmitter rated power into VSWRs up to 2:1. Normal service can therefore be maintained during conditions of varying VSWR without the need for operator intervention.

4.8 RF Penultimate and Final Stage Tuning

Inductors are shown diagrammatically as single turn coils with shorting bars across them. In practice the inductance required is obtained by putting RF switches across a pair of parallel pipes which, in the case of the final RF stage output circuits, run down the side of the RF cabinets are folded back and finally short circuited at the end - a single turn coil in fact. Then, as the RF switches are closed, the active length of the single turn coil is reduced.

This unique concept of producing a variable inductance is the result of nearly 40 years experience in designing high power short wave circuits.

This experience has proved that if a high power radio frequency contact moves on power it will also burn. The burn may be very hard to see at first but over weeks and months the burning gets progressively worse and unless the coil is regularly maintained holes will eventually appear in it. Many different forms of sliding contact have been tried, some extremely sophisticated and well engineered, but the result is still the same.

Therefore Marconi engineers decided that high power RF contacts in a design must:

- not move when carrying current;

- be well cooled - preferably with water;

- have adequate pressure between contact surfaces.

These three requirements were applied in 1973 at the start of the current short wave transmitter series. Since adopting this technique we have virtually eliminated the problems associated with RF contacts.

Water is passed through the pipes (inductances) and through the shorting bars (RF switches). In this way both the fixed and movable RF contacts are kept cool.

The RF switches are closed by compressed air actuators which apply pressure to the contacts continuously when the contacts are made.

Pneumatic operation allows a controlled rate of contact closure with a self cleaning action of the contact resulting in considerably reduced maintenance requirements.

When the correct inductance has been selected fine tuning is accomplished by vacuum capacitors. Vacuum capacitors in the output circuits are water cooled. With this technique the inductance required for a particular band of frequencies is selected when the transmitter is not powered.

Fine tuning is accomplished by variable capacitors. Sliding or rolling radio frequency contacts carrying heavy current are avoided.

4.9 Isolating and Grounding Switch

On a short wave site with multiple transmitters and antennas it may not be sufficiently safe to simply ground the outgoing feeder. It is possible that the ground is put on a current antinode and there may be a voltage present in the transmitter caused by induced voltages, even though the feeder at the transmitter output has been grounded.

Marconi therefore installs a feeder isolating and grounding switch on the transmitter output feeder for absolute protection to operating and maintenance staff.


5.1 General

A good quality modulator is of vital importance in providing quality sound transmission. Audio bandwidth and distortion are critical parameters that affect the overall intelligibility and perceived quality of a service. These facts have not been compromised in the design of the B 6128S.

The modulator is of solid state technology and employs the pulse step modulation principle to provide high level anode modulation of the associated RF amplifier and, as such, supplies the DC plate voltage to develop the RF carrier power and the audio voltage to develop the modulation sidebands. Functionally the high efficiency solid state modulator serves as both the RF amplifier anode power supply and the high level audio source.

5.2 Audio Input Unit

The audio input unit routes the audio signal through a variable attenuator to the audio processing unit. The unit will select one of two incoming lines to modulate the transmitter.

The demodulated radio frequency audio also is routed through the audio unit. A variable attenuator on the front panel controls the demodulated audio level to the remote monitoring point. A "U" link allows the circuit to be monitored locally for test purposes.

5.3 Pulse Step Modulator

The modulator consists of four major elements as follows:

  1. The two power transformers, located in the modulator vault with 11 kV, 3 phase primaries, supplying the individual PSM switching modules via multiple mutually isolated secondaries. Transformers with alternative primary voltages can be supplied although, in order to maintain the primary current levels below reasonable limits, it is not recommended that the primary voltage be below 3.3 kV.

    Primary tappings on these transformers can allow the secondary loading to be represented to the line input as equivalent to a 12 pulse rectifier system, thus reducing the amplitude of harmonic injection on the power lines.

  2. The modulator switching module bank, also located in the modulator vault, contains 48 modules of identical design, whose outputs are connected in series.

    Each module is fed by a stepped down 3 phase line voltage from its own individual secondary winding. This voltage is rectified and smoothed and is then available as a switchable pulse step increment at the output. The output voltage from the modulator is primarily governed by the number of switched on PSM modules. When a required level of HV is such that it cannot be acurately provided by an integer number of modules, then the level is achieved by application of PWM/PDM to a module to provide for the intermediate value between two integer levels.

    Transmitter operating power selections are implemented by controlling the mean voltage output from the modulator. The selected voltage is stabilised against power line variations with feed-forward correction.

    When a module is switched off, its output terminal characteristic reduces to a low impedance with a very low voltage drop due to forward conduction of its filter current continuity diode.

    The PSM switching bank is arranged as four vertical columns of twelve modules each. Forced air cooling up the insides of the columns is provided by two centrifugal fans, split ducted, to supply two columns each.

  3. The modulator filter consists of two parts, first the small inductors which are the series connection between each module and reduce the transient charging currents when an individual module is switched on and the maln low pass filter which is located on top of the PSM switch banks and removes the PSM switching components and reconstitutes, with appropriate bandwidth, the required analogue HV to be applied to the RF output stage.

  4. The PSM control unit samples and converts low level audio and carrier signals into time related independent ON/OFF control signals for the individual PSM modules. By this means, the number of switched-on modules and hence the instantaneous HV pulse step amplitude is defined.

    The modulator control unit is housed in the main transmitter control rack and consists of series of circuit cards each with particular functions.

    The noise reduction circuit reduces the level of hum on the output signal. In order to reduce hum two methods can be used, first by the use of large and complex power supplies and, second, by the use of negative feedback. Large power supplies are a size and expense load when 48 are required and negative feedback can produce undesirable phase shifts. The modulator is linear and negative feedback is not required for distortion control. The noise reduction board samples the transmitter output and the audio input and applies feedback only when there is no audio present. Since the audio masks the noise no feedback need be applied during modulation conditions.

    The incoming audio is processed in the audio path board. The audio is AC coupled and peak limited. A high pass filter removes all low frequency disturbances but permits the resolution of a 50 Hz square wave. From this point in the programme chain the modulator is DC coupled. A DC voltage is added to the audio to represent the carrier level; this voltage may be fixed or adjusted to provide different carrier levels or dynamically altered to give controlled carrier modulation.

    The modulator converter is the analogue to digital conversion unit. The audio input is compared by 48 different voltage comparators each with a unique reference voltage. The main reference voltage comes from the audio path board and determines how many of the 48 modules are switched on at any time. A clock pulse from the audio path board latches the outputs which are summed and passed to the ring modulator.

    The ring modulator establishes a sequence for the rotation of the switching modules. The purpose of this rotation is to ensure that all switch modules are equally used in the series voltage chain and to aviod any module that is non functioning. The 48 outputs from the ring modulator are routed to the optical transmitter board which contains the optical transmitters for controlling the power switch modules. Also on the optical transmitter board is the circuitry for the fast shut down of the modulator in case of transmitter faults thus avoiding the need for a thermionic crowbar device and placing a more gentle load on the power supplies.

5.4 Modulator Safety

As described above, the bulk of the PSM consisting of the mains transformers, the switching bank and the audio filter is contained within the modulator vault. A screened high voltage cable feeds forward reconstituted HV to the RF output tube via the transmitter earth switch.

For safe access to the modulator vault a mechanical cable link between the transmitter earth switch and a multi-contact earthing mechanism on the PSM switch bank allows all switching modules to be grounded when the transmitter earth switch is operated. A key interlock system prevents access to the vault until the earth switch is properly in the 'earthed' position.


The carrier level of the final RF Amplifier can be changed simply by altering the DC voltage applied to the modulator's flash converter.


7.1 Dynamic Amplitude Modulation (DAM)

Dynamic Amplitude Modulation is a method of controlled carrier modulation which provides a means of conserving energy by decreasing the transmitter carrier power at low levels of modulation. CHRIS

Theoretically the carrier has to be of only sufficient amplitude to support the modulation superimposed upon it. Therefore at low levels of modulation only a small carrier is required. However, with a low carrier the a.g.c. of the distant receiver operates and increases the received noise and interference.

Practically, the carrier level is kept to 50 or 60% of full carrier for all levels of modulation up to 60%. Then, as the modulation increases, the carrier is increased linearly to full carrier at 100% modulation.

This system can be used when operating the 500 kW B 6128S. An audio processing unit is switched into circuit and applies DAM to the transmitter.

The DAM unit monitors the incoming audio amplitude and sends a control signal to the solid state modulator.

In operation the input audio level is set so that 100% modulation can be achieved on programme peaks with the transmitter power at full level. with the DAM system activated the carrier will fall to a pre-set level in the absence of modulation and rise to a level necessary to support the instantaneous level of the programme audio. The level to which the carrier will be reduced in the absence of programme is adjustable from 0 to 6 dB.

7.2 Amplitude Modulation Companding (AMC)

Dynamic Amplitude Modulation (DAM) reduced the transmitter carrier level for low levels of modulation. Consequently, the distant receiver fitted with automatic gain control (a.g.c.) increases its gain. This often results in a raised level of noise and interference signals which can, in some cases, be troublesome during periods of low modulation of the transmitter.

The British Broadcasting Corporation have developed another system of carrier control to reduce mains input power to the transmitter and to improve reception in the target area. This system is known as Amplitude Modulation Companding (AMC). With AMC the full carrier amplitude is used for low levels of modulation and is reduced progressively as the amplitude of the modulating signal increases.

The carrier level operates the receiver a.g.c. and the receiver gain is reduced when the modulation in the transmitter is low; consequently the noise and interfering signals are attenuated and quiet program passages in the transmission can often be heard with greater clarity. A small increase in carrier power can, under certain circumstances, cause a much greater reduction in the levels of interfering signal. When the audio input to the transmitter is high the receiver a.g.c. increases gain. Hence, the loud program passages are sufficient to overcome the interference.

When used in the "enhanced" AMC mode the carrier at zero modulation is raised to 625 kW and drops in accordance with an approximately linear law to 300 kW at 100% modulation. A typical carrier compression at 100% modulation is 2.5 dB.


The SSB modes of H3E and R3E are available as optional extras on the B 6128S. The SSB system used in known as Envelope Elimination and Restoration (EER). In this system the carrier frequency is phase modulated while the RF amplifier is amplitude modulated in the normal way. This retains the high efficiency of the class C RF amplifier and the automatic RF amplifier tuning is the same for AM and SSB.

The SSB signal is generated on two boards contained in the modulator control rack. The SSB generator board takes the audio signal and produces a SSB signal at an intermediate frequency which is the reference from the RF synthesised drive. This SSB signal is then split into its amplitude and phase components and sent to the Equaliser card. For the correct operation of the EER system it is essential that the phase and amplitude components arrive at the modulated final RF amplifier with the correct amplitude and time relationships. It is the function of the equalisation card to provide corrections for any time and amplitude differences. Once processed the amplitude component is sent to the audio path board and the phase component returns to the RF synthesiser where it is up-converted to the carrier frequency.


9.1 Water Cooling

Water cooling is used extensively in the B 6128S. It is common experience to those who operate high power transmitters that many of the problems encountered can be traced back to overheating, especially those concerned with high power RF contacts. The aim of the Marconi design engineers was to keep all parts of the transmitter cool except the anode (plate) of the final RF tube which is deliberately allowed to run at a high temperature in the region of 70°C to 90°C. This is done so that high grade waste heat is available and may be used for building heating in temperate or cold climates.

High power radio frequency inductors, vacuum variable capacitors and RF switches are all water cooled because water is a much more efficient heat transfer medium than air and, particularly in hot and dusty climates, is easier to handle. Unlike the final RF tube these components are cooled to run only a few degrees above ambient temperature. This reduces thermal stressing and greatly adds to the transmitter reliability. Where practical several components are connected in series to form a branch, these branches are then connected in parallel.

Water feeds to components at high DC potential are made using high grade polyester reinforced silicon rubber hose.

Leak detectors are fitted in various positions within the transmitter so that, in the unlikely event of water leakage, the transmitter will be shut down and the pump stopped.

The final RF stage tube requires low filament voltage applied during the time the transmitter is off power. This is known as the "black heat" condition. In this way thermal stresses on the tube heater (filament) are reduced when the transmitter is brought up to full operating condition. Hence the guaranteed life and actual achieved life of the tube are improved.

A small convection circuit is connected in parallel with the main cooling pipes to the final RF Amplifier tube. The convection circuit dissipates the heat produced by the tube filament (heater) operating in the "black heat" condition when the pump is inoperative.

9.2 Air Cooling

Low pressure air is used to scavenge the transmitter cabinets and the modulator vault. High pressure air taken from within the cabinets or vault, is directed to tube seals and the PSM switch bank. These switches are arranged in four columns of 12 modules each. Air is forced up the insides of the columns by two centrifugal fans, split ducted, to supply two columns each.


10.1 Transmitter Run Up Modules

The transmitter run up sequence is accomplished by a number of modules. The modules are:

- Cooling

- Filaments Initiate

- Aux 1

- Aux 2

- Aux 3

- Interlocks Complete

- HT

- Transmitter ON

Each module is connected to the parent rack by a ribbon cable and connector. It is therefore possible to withdraw a module on power without the use of an extender board.

Each module has green and amber l.e.d.s on the front panel. The amber l.e.d. at the top of each module indicates that the module function has been completed. It is easy to see where the run up sequence has been interrupted, or where the transmitter has failed, by looking along the top amber l.e.d.s from left to right.

The first amber l.e.d. from the left which is extinguished shows the module at fault. The green l.e.d.s on the module indicate the locality of the fault. For example if the Aux 3 top amber l.e.d. is not lit and the "range switch air" and "coarse tune complete" l.e.d.s fail to indicate, the fault lies in the auto tune system and will be found in the air supply to the range switches or in the Aux 3 module itself. The latter is unlikely but can quickly be checked by substituting a spare module.

The run is completed automatically or can be stopped at any step by using the sequence switch on the sequence module.

If the switch is in the "remote" position the full run up is automatic. In any other position the run up has to be accomplished manually by pressing the "aux" start pad then, when the aux 1 top amber l.e.d. has indicated that filament delays have been completed by pressing the HT start pad.

Transmitter output power level is selected manually on the power control module in the modulator control rack and the power obtained indicated on the power level module.

In the "remote" position of the sequence switch control is passed either to the ECMP or to the RCMI if these are fitted. However all indicators on the modules are unaffected by the control point.

In addition there are two subsidiary modules to aid the run up sequence. They are:

- AC/DC driver module

- Rapid restore module

The AC/DC driver module is the interface between solid state control circuits and power relays.

The rapid restore module holds completed filament delay circuits with a small floating battery and ensures that the transmitter will be immediately repowered provided that a mains failure does not last longer than 30 seconds.

The control modules are annotated "A", "B", "C" etc. and the rack positions where they are placed are annotated similarly to aid the operator to identify locations.

The transmitter overload modules are located in a row below the control modules. Transmitter failures and HT trips are indicated by amber l.e.d.s on the module front panels.

Transmitter lockout indication is located on the centre double panel. This panel also carries a test circuit and indicator so that all overload operating levels can be checked.

Switches are provided for lockout reset and overload indication reset.

The overload modules do not have potentiometers to adjust overload settings. These potentiometers are grouped together on the "set levels" module.

Overload modules can be replaced instantly without adjustment or can be interchanged in an emergency. In the latter case panel indicators will be incorrect but the transmitter will function and be fully protected.

The modules controlling the modulator are mounted together in a frame which holds up to 18 control cards arranged in two rows of 9 each. This frame also holds the SSB generator and equaliser boards when fitted.

A further row of modules houses the:

- Remote-In module for accepting remote commands

- Remote-Out module for giving remote indications

- Alarm Module for indicating low water level and dirty air filter

- Two modules for Remote Ready.

These modules carry a number of green l.e.d.s which, when lit, indicate that strategic controls and switches are positioned correctly for transmitter remote operation. All green l.e.d.s have to indicate before the overall top amber l.e.d. on module "Remote Ready 2" is lit.

10.2 Remote Control and Monitoring Interface (RCMI)

An optional remote control and monitoring interface (RCMI) can be built into the transmitter local control and monitoring panel (LCMP). The RCMI allows the transmitter to be controlled from the local operations control centre (LOC).


The 500 kW B 6128S transmitter is capable of automatically tuning itself to any frequency in the broadcast bands selected on the synthesized exciter. It also will tune to discrete frequencies outside the broadcast bands if these are programmed into the system during manufacture and sub-unit test.

Marconi call this system "frequency follow".

When the transmitter has tuned to a frequency all the tuning settings are stored in a memory for instant recall thereby allowing a fast frequency change back to the stored frequency. A total of 4000 frequency settings can be stored. In the automatic mode the system tunes the transmitter for full output and maximum overall efficiency.

The tuning system continually adjusts for changes in the impedance and phase angle of the load thereby ensuring that the maximum power is delivered to the antenna.

The frequency follow tuning time is 30 seconds or less to full power output and 40 seconds to completion when the overall transmitter efficiency is optimised, for frequencies not previously used.

The frequency change time to a previously stored frequency is 12 seconds or less.

11.1 Automatic Tuning System

The frequency follow automatic tuning system utilises the following modules which are mounted in the local control and monitoring panel (LCMP):

- Drive Level

- Auto Tune

- Frequency Counter

- Coarse Tune Memory

- Range Switch

- Servo Control

- Fine Tune

Having set the synthesized exciter to a specific frequency the ACT(ion) pad on the exciter is pressed and a "Tune initiate" pulse is sent to the auto tune module.

The auto tune module switches off transmitter high tension (if it was on) via the modulator switch modules and supplies a "drive mute" signal to the 100 watt broadband amplifier via the drive level module. Bias and screen voltages also are removed.

The frequency counter then measures the incoming frequency from the synthesizer and addresses the coarse tune memories.

The coarse tune memories contain component setting data applicable to bands of frequencies, each approximately 25 kHz wide, across the broadcast bands between 3.9 and 26.1 MHz. There is a memory for each tuning control and inductance shorting bar. The memories identify the 25 kHz band in which the synthesizer frequency lies and sends component setting signals to the servo control modules and the range switch modules.

Simultaneously each tuning capacitor and the inductance shorting bars automatically take up the setting applicable to the incoming frequency.

The transmitter is then tuned to within 25 kHz of the optimum tune setting.

The "coarse tuning complete" signal is sent to the auto tune module and the bias and screen supplies to the transmitter are restored. The broadband amplifier muting is removed and the fine tune system activated.

The penultimate RF amplifier grid and plate are fine tuned by detecting the change of slope of the tuning curve.

Half Final Stage RF HT is applied and the final tank circuit, final load, feeder tune and feeder load are fine tuned.

At this stage, provided that the forward output power from the transmitter exceeds the preset threshold the modulation is applied and if the load VSWR is less than 2:1 the transmitter may be switched to full HT.

The time from the start of the tuning sequence is now 12 seconds.

The fine tuning system continues to operate until an optimum tuning result is obtained. The full 500 kW forward power is achieved within 30 seconds from start of the tuning sequence and optimum efficiency from the final RF stage obtained in another 10 seconds.

Optimum tuning settings can be retained in a memory and can be recalled for frequencies previously used.

In effect the coarse tuning procedure then is applied although the tuning signals to each component are optimised. Hence the change to a previously used frequency can be accomplished to full power in 12 seconds.

11.2 Manual Tuning

If for any reason the automatic frequency change system fails, three methods of manually tuning the transmitter are available

When switched to manual operation the transmitter can be tuned by operating the increase/decrease switches on the servo control units. The transmitter electrode feeds, forward and reverse output power and VSWR meters located immediately above the servo controls can be observed during the tuning process.

If a servo control unit fails and no spare is available immediately, the problem can be overcome by using the servo override panel. The tuning control motor' connections are brought to this panel when the servo override key is turned and a particular motor can be selected using a switch. Having selected the motor it can be powered from 7.5 VAC by operating the increase/decrease switches.

The transmitter can be set to a frequency previously used if no power exists on the equipment. Each tuning motor carries a mechanical counter to indicate the position of travel. Provided that these counter readings have been recorded for the frequency required, the motor gearboxes can be cranked to the setting using the handle supplied.


Silicon diodes are used in all rectifiers. Applied peak voltages are 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.

The main HT transformers have delta connected primaries and twenty four secondary windings. Extended delta taps enable the two primary windings to be phase offset by +15° and -15° respectively. This presents the effect of 12 pulse rectification to the power line and much reduces the level of rectification harmonics conducted back to the power source.

Under fault conditions power to the high voltage stages of the transmitter is disconnected by very rapid switch off of all the modulator switch modules. This rapid switching is necessary in order to reduce the energy in any flash arc below a level acceptable to the valve manufacturer.


Two mains supplies are required for the transmitter: one three phase four wire low voltage supply at 380/415 V for ancillaries, bias and low power HT supplies; the other a high voltage three phase three wire supply for the main HT transformers. A typical high voltage supply will be 11 kV. The incoming mains frequency may be 50 or 60 Hz.

All DC power supplies are mounted in the main auxiliary cubicle except for those associated with the modulator where the power supply is on board each switching module.

This allows a very simple layout in the power vault with two transformers and one switching assembly, thus cutting down installation time.


The transmitter embodies comprehensive protection circuits, utilising optical fibers where applicable. The operation of these circuits is initiated by sensors which monitor overcurrent, overvoltage, high reverse power, or malfunctioning of the cooling system. A three shot recycling overload system is incorporated in HT protection circuits.

Great improvements have been made in reducing the hazard of internal flash arcs in high power vacuum tubes but tube manufacturers specify a protection requirement to cover this eventuality. The specified requirements are more than adequately met in the transmitter. In the final RF amplifier an incipient flash arc triggers the rapid shut down circuit in the modulator optical transmitter board and by switching off the 48 power modules the HT voltage is rapidly brought to zero.

Arc detectors are strategically positioned to detect any arcing occurring in the RF power circuits. Their function is to interrupt HT as described above in order to remove the arc. If the arc is persistent the transmitter locks out.


The strict rule applied to the design of all Marconi transmitters is that anyone should be able to operate and maintain the transmitter without the possibility of coming into accidental contact with harmful voltages or mechanical parts. This rule has been applied for many years so that when the International Recommendations for Safety, IEC-215, were published, Marconi equipment already conformed to the requirements.

The basis of the Marconi safety system was introduced in 1938 using an electro mechanical arrangement. The system has been improved in detail over the years but the original principles still apply.

To ensure the safety of all personnel, a system of mechanical and electrical interlocks is provided. When the mains supply isolator is open access to all parts of the transmitter is possible with complete safety. When closed, access to the auxiliaries cabinet containing the DC power supplies is prevented but filaments, fans and control circuits can be powered with the other doors open. However where potentials in excess of 72 V peak are present covers and warning notices are fitted. These potentials do not exceed 250 V r.m.s.

To Gain Access To HT Enclosure :

  1. Operate HT STOP

  2. Release Kaba key associated with 11 kV circuit breaker or the isolator and earthing unit and insert into position on the earth switch panel.

  3. Turn AUXILIARY ISOLATOR to OFF. Check that red lamp is off.

  4. Operate EARTH switch SE, with delay plunger and remove EXTERNAL INTERLOCK key.

  5. Insert this key in HT Enclosure door and turn.

  6. Open HT Enclosure and place P0 key in pocket.

To Gain Access To The AC Distribution Cabinet :

Proceed with 1 - 4 above, then:

  1. Switch Transmitter to AUX. STOP.

  2. Set transmitter isolator to OFF, release both SA Castell keys in double lock. Place one in pocket and use the other, with one Kaba key, to open the cabinet door.

For Entry To The RF Cabinet :

Proceed as 1 - 4 above, then use one or more Kaba keys to open door(s) and place one key in pocket.

Before potentials in excess of 250 V r.m.s. can be applied all doors have to be locked and the keys returned to an interlock panel. The keys in the interlock panel are trapped when the grounding switch is operated to remove the ground from HT lines. Conversely the grounding switch cannot be operated until all keys are present, and turned, in the interlock panel.

When the grounding switch has been moved to the ungrounded position it is possible to close the power isolator. The auxiliary supplies of the transmitter then can be powered followed by the main high voltage DC supply.

The outgoing RF feeder is grounded and isolated by a switch, positioned immediately behind the low pass filter in the case of 50 ohm systems or at the antenna end of the balun in the case of 300 ohm systems. It is coupled into the safety interlock system. It is not sufficient to ground the outgoing feeder unless it is isolated as well, because induced voltages in the feeder system could appear at the grounding point as a node and it still will be possible to suffer an RF burn when working on the RF stage and its output circuits.


Maintenance requirements have received a high priority in the design of the B 6128S. The twin cabinet arrangement provides exceptionally good accessibility. Moreover in the RF cabinets there is a rigid division down the centre acting as a ground plane between the RF circuits and their ancillaries. Thus all motor drives for capacitor tuning, switch actuators and the water cooling system are positioned in readily accessible places to facilitate routine checks and maintenance.

Tube changing is rapid because each tube plugs into a socket and has quick release self sealing couplings on the plate water cooling inlet and outlets.

The built in crane facility allows the tube to be changed in less than 10 minutes. This time may be reduced to 5 minutes if two operators are used.

All the low level solid state equipment is grouped together in the control and monitoring area with comprehensive l.e.d. indications on the module front panels and the internal printed boards to aid fault location.

It is a quick and easy operation to replace these line replaceable units (LRU).

Power Output 500kW +0.2 dB, -0.4 dB carrier power into a matched 5O ohm unbalanced load or, with external balun, a 300/328 ohm balanced load, at normal supply voltage.
Automatic adjustment of output coupling and tuning is provided to maintain full power into a VSWR not exceeding 2:1 for a 50 ohm load, or 1.8:1 for a 300/328 ohm load.
Automatic reduction of power output to approximately 50% and 25% occurs for load VSWRs of 2:1 and 3:1respectively.
Types Of Transmission Amplitude modulation, double sideband (ITU Classification A3).
Amplitude modulation, double sideband, with programme controlled carrier (ITU X3E)
Single sideband, with -6 dB or -12 dB carrier reduction (ITU H3E or R3E) (optional)
Modulation High level anode and screen modulation using an all solid state modulator.
SSB modulation is by the Envelope Elimination and Restoration (EER) method.
Carrier Shift Less than 5% amplitude between 0 and 100% modulation measured at 1 kHz with constant mains voltage.
R.F. Harmonics And Spurious Radiations The mean power of any spurious r.f. 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 modulator 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.
Operating Frequencies Frequencies 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
9.50-9.90 MHz
11.65-12.05 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 Not exceeding 30s (typically 12s) for a change between any two frequencies in the range.
This is the time taken for 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.
Drive Synthesiser type, integrated with frequency change system, mounted in the transmitter.
The drive may be demounted and installed in a central area, or capable of frequency selection from 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 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.
Audio Input Impedance 600 ohm balanced (nominal).
Return loss not less than 20 dB from 50 to 7500 Hz.
Normal Audio Input level 100% modulation is obtained from a 1 kHz tone at any level from 0 dBm to 15 dBm, by adjustment of a preset control.
Audio Frequency Response ±1 dB between 50 Hz and 7500 Hz (A3E)
±1 dB between 100 Hz and 4500 Hz (H3E & R3E) relative to 1 kHz when transmitter modulated to 75%.
Audio Frequency Harmonic Distortion (A3E) 2.5% at 50% modulation between audio frequencies of 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 R.F. at approximately 2 V peak carrier (at 500 kW) into 50 ohm unbalanced.
A demodulated output provides 0 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 minutes per hour, followed by 70% modulation for 50 minutes per hour.
75% sine wave modulation, 60 Hz to 7500 Hz, continuously.
Incoming Power Supply Auxiliary circuits:
380 volts or 415 volts (to be specified with order), 3 phase, 4 wire, 50 Hz. Equipment for 60 Hz can be supplied to special order.
Main H.T. Rectifier:
Arranged to suit customer's supply.
Normally 3.3 kV, 4.16 kV or 11 kV, 3 phase.
Variation of Supply Voltage Auxiliary circuits: ±10%
Main H.T. Supply:
The transformers are 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. Winding are provided so that the phases of the two primaries are offset by ±15°.
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 additional 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 71% averaged over the range of frequencies in the broadcast band at 500 kW, when used with cooling equipment optimised for operation at up to 300 m above sea level and at a maximum ambient temperature of 35°C.
Equipment Operating Conditions Ambient temperature
Maximum daily average
Maximum yearly average
Maximum relative humidity
Maximum altitude (ASL)
1°C to 50°C
2000 m (approx. 6600 ft)
Equipment Storage Conditions Ambient temperature (with all cooling fluids drained)
Maximum relative humidity
Maximum altitude (ASL)
-40°C to 60°C

10000 m (approx. 32500 ft)
Maximum Dimensions (Excluding heat exchanger and air handling system)
HT Enclosure (typical)
(includes cooling space)




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 4CW25,000A


ITU Country
ITU Country