300 kW H.F Broadcasting Transmitter Type B 6126

photo Marconi B 6126 ADVANTAGES



The Marconi B 6126 H.F Transmitter combines the merits of high efficiency operation, and a fast frequency follow system, with a low initial cost and a low maintenance and running cost.

The transmitter is eminently suitable for the addition of the following facilities:
  1. Dynamic Power Output Control
  2. High Efficiency SSB Operation
  3. Dynamic Amplitude Modulation
The transmitter is designed for high reliability with a long average time between failures (MTBF).

Transmitter Overall Efficiency

The B 6126 operates at a very high overall efficiency. Depending upon the site ambient temperature and the frequency in use, the overall efficiency will vary between 64 and 70%.

The site ambient temperature dictates the size of the water to air heat exchanger and the electrical power to operate it. The final R.F stage efficiency varies across the frequency range of the transmitter.

Hence the reason why we give the overall efficiency between limits rather than quote the best that can be achieved.

Radio Frequency Stages

The Radio Frequency stages of the transmitter are based on a concept which has been born as a result of 46 years experience of high power short wave transmitter design.

This concept is based on high power R.F 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 has been proved in the first of our modern range of high power short wave transmitters, the 300 kW B 6124, where 17 are in service world wide.

The B 6126 has been evolved from the B 6124 using the same basic Radio Frequency design but having a pulse width modulator to replace the Class B modulator.

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 6126.

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.


The installation layout of the transmitter is extremely flexible, as apart from the basic cabinets housing the R.F, 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.

No underfloor ducts are necessary.

The transmitter can be supplied with a 50 ohm unbalanced R.F 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.


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 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 r.f 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 short connection paths to the final stage to be achieved. The coupling circuit utilises a water cooled inductor and pneumatically actuated off load r.f 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 TH537 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 of 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 R.F 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 250 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 r.f 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 r.f 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 cam 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 normal operation the transmitter is in the 'automatic' mode. In this condition 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

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

The H.F spectrum 3.95 to 26.1 MHz 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 100 watt 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 100 W 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.

HT and full screen voltage is applied plus programme if required. 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 samples are taken of output load match, and corresponding corrective action taken to maintain mimimum 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 will automatically reduce power until the v.s.w.r improves, permitting full power to be safely restored.

Manual Tuning

Provision is made to manually tune the transmitter from the front panel controls conveniently situated for reading the required meter indications.

In this case the tuning motors are operated from voltages controlled by reversing switches and utilises minimum circuitry for maximum reliability.

The Advanced Pulsam Modulator

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 6126. It is connected in series with a switching frequency suppression filter and introduced between the HT supply and the RF final stage to form a series PWM modulator.

By applying constant current drive to the control and screen grids of the final modulator the full capability of the tube can be exploited without overdissipating the grids. This being so it is possible to optimise the size of the tube. This optimisation allows a smaller tube to be used in the B 6126 modulator 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 a pulse. The TH555 pyrolytic graphite gridded tube chosen for the final modulator has a very low forward resistance in this state.

Audio is processed to form pulses at the switching frequency by a solid state encoder located at the transmitter front panel. As the grid deck potential varies between zero and H.T voltage, when modulation is applied to the final R.F stage, the audio processed signal is connected to the grid deck through fibre optical circuits.

For the same reason filament, grid bias and screen supplies for the TH555 are fed to the grid deck through transformers with a large air gap between primary and secondary with the secondaries having low capacity to ground.

The filter connected to the output from the grid deck converts the pulses back to the original audio wave shape and a string of diodes connected between tube cathode and ground discharges the filter between positive pulses.


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 H.T rectifier in preference to thyristors because of their simple circuit requirements, proven reliability and cost effectiveness.

The transmitter tetrodes are rated to accept full H.T voltage applied at the correct point in the starting sequence. Under fault conditions a.c 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 H.T supplies; the other a high voltage three phase, three wire supply for the main H.T rectifier.

The transmitter d.c power supplies are mounted in the main cubicle with the exception of the main H.T which is in the rear enclosure.

Reduction of R.F output to approximately half and quarter power is provided by low level control of the H.T. 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 R.F 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 R.F switches. It also provides access to the H.T 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.

Pneumatic System

Maintenance requirements have received a high priority in the design of this transmitter. The twin cabinet arrangement provides exceptionally good accessibility. Moreover, in the R.F cabinet there is a rigid division down the centre between the R.F 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 R.F 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 6126 where range changing is accomplished by well proven pneumatically operated R.F 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 R.F and final R.F tubes. In addition, both the main transmitter cubicles and the H.T 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 6126. In the final R.F amplifier or modulator an incipient flash-arc triggers the 'crowbar' and also opens the six vacuum switches between the H.T transformer and rectifier bridge.

Arc detectors are strategically positioned to detect any arcing occurring in the R.F 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 a.c 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 R.F feeder and for high voltage supplies.

The equipment conforms generally to IEC215.

The outgoing R.F 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.

Power output 300 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
Power output variation Automatic reduction of power output to approximately 50% or 25% for load VSWR's up to 3:1 and 4:1 respectively
Operating frequencies Frequencies within the range 3.95 MHz to 26.1 MHz.
Full performance applies to the broadcast bands defined by WARC 1979 as follows :
3.95-4.00 MHz
4.75-5.06 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 10s for a change within the same band.
Not exceeding 35s for a change from one band to another.
Typical frequency change time 20s.
Tuning method By frequency following techniques to any frequency in the specified range as determined by the synthesiser. A manual tuning facility is also provided.
Type of transmission Amplitude modulation, d.s.b (CCIR Classification A3)
Modulation High level anode and screen modulation using the Advanced Pulsam PWM modulation system.
R.F output load impedance 50 ohm unbalanced, max V.S.W.R 2:1 or 300 ohm balanced, 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.
R.F load variation Automatic adjustment of output coupling and tuning is provided to maintain full power into a mismatch not exceeding 2:1 for a 50 ohm load, or 1.8:1 for a 300/328 ohm load.
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 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, 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. Standard frequency of 1 MHz at a level of 0.45 V to 2.5 V rms into an impedance of 50 ohm unbalanced.
Carrier shift Less than 5% amplitude between 0 and 100% modulation measured at 400 Hz with constant mains voltage.
Audio input impedance 600 ohm balanced (nominal).
Audio input level control 19.5 dB in 0.5 dB steps by front panel controls.
Nominal audio input level With the front panel control set at centre position, a preset attenuator allows 40% modulation to be obtained from a 400 Hz tone at any level from -5 dBm to +10 dBm
Audio frequency response ±1 dB, 50 Hz and 6000 Hz +1 -2 dB between 6000 and 7500 Hz relative to 400 Hz when transmitter modulated at 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 400 Hz.
Output Monitoring R.F at approximately 10 to 16 V peak carrier into 50 ohm unbalanced. In operation, this feeds the envelope monitor. A demodulated output provides 0 dBm ±3 dBm (adjustable) into a 600 ohm balanced load at 100% modulation.
Equipment operating conditions Ambient temperature : 2° to 50°C max.
Daily average 45°C
Yearly average 35°C
Maximum Altitude : 2300 m (approx. 7500 ft)
Maximum Humidity : 95%
Transmitter rating 100% sine wave modulation for 10 mins per hour, followed by 70% modulation for 50 mins. per hour OR 75% sine wave modulation continuous.
Incoming power supply Auxiliary circuits : 380 V or 415 V 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 or 11 kV 3 phase.
Variation of supply voltage Auxiliary circuits : ±10%
Main H.T 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 adjustments 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 the 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.
Overall power consumption and efficiency

Power Output (kW)
Power Input (kW)
Overall Effy %

Sine Wave
40% Mod
Sine Wave
100% Mod
A carrier shift of 2.5% amplitude at 100% is assumed
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 4CM300,000GA or TH537 1 TH555
1 4CW25,000A


ITU Country
ITU Country