PHILIPS K.V.F.H. 10/12

GENERAL DESCRIPTION

INTRODUCTION

The special features to short-wave communication have not failed to encourage the use of short-wave transmitters for broadcast services. Especially in tropical countries short-wave transmission has outstanding merits, as relatively little interference is experienced from atmospheric disturbances.

A short-wave broadcast service is particularly useful when very large ranges have to be covered. In many countries densely populated centres as well as vast districts with a relatively small population are likely to be found. To provide for an adequate broadcast service under such conditions, short-wave transmitters having a very large range even in daylight, can be used to advantage.

From a technical point of view the use of a short-wave transmitter offers additional facilities because of the fact that very effective aerial systems can be utilised: if necessary the radiated energy can easily be "beamed" in a certain direction.

With the "world wide" broadcasting stations PCJ and PHI Philips' Radio have gained a vast experience in the design and construction of short-wave broadcast transmitters. This experience, combined with the results of intensive research work carried out for a number of years by Philips' Laboratories, has resulted in the development of a range of short-wave transmitters, specimens of which are now in use in various parts of the world.

The type K.V.F.H. 10/12 short-wave broadcast transmitter of 10 kW rating is the latest addition to this range of standard equipments. An abridged description is given overleaf.

TECHNICAL DESCRIPTION

The aerial power of type K.V.F.H. 10/12 short-wave broadcast transmitter is 10 kW for the unmodulated carrier wave. To allow for a high degree of modulation - the transmitter can be modulated up to 100% without impairing the quality of reproduction - the principle of "high-power modulation" is applied, i.e. modulation is effected in the final H.F. stage.

FREQUENCY STABILITY

For a short-wave broadcast transmitter it is of extreme importance to keep the carrier frequency constant within close limits. The use of quartz crystal control - the crystal being kept at a constant temperature by means of a thermostat - makes the carrier frequency constant within 0.005% for temperature variations of not more than + or - 10° C, and voltage variations not exceeding + or - 10%. Apart from the very close regulation of the carrier frequency to be arrived at by crystal control, there are other possibilities that tend to impair the frequency-stability of a transmitter. Reaction from subsequent H.F. stages, for instance, may seriously impair the frequency-constancy of the drive-circuit. In modern transmitting circuits this type of reaction is prevented by the use of a separator stage comprising a screen-grid valve.

Reaction from the modulated H.F. power stage upon the drive circuit, which may cause frequency-modulation resulting in bad quality of the transmision, must be prevented as well. This is done by incorporating a separator stage which, in conjunction with the frequency doubling-stage, safeguards the transmitter from frequency-modulation.

MODULATION SYSTEM

The modulation of the transmitter is effected in the anode circuit of the H.F. power-amplifier stage, laboratory research having conclusively shown the superiority of this method of modulation. As the method of "high-power modulation" would be rather expensive when using a modulator in "class A" setting, the modulation sytem is equipped with modulator valves working as "class B" low-frequency amplifiers. The "class B" modulation system is of advantage in two respects: 1. the efficiency of the modulator valves is high; 2. the anode input of the valves is a function of the modulation-depth and decreases as the latter is decreased. A few figures to illustrate these points may be quoted: at 100% modulation-depth the anode input of the modulator valves is about 12 kW, the anode efficiency is 40%.

Extreme care has been taken to attain high fidelity of the modulation. At a modulation-depth of 80% the frequency characteristic is practically linear between 35 ond 8000 c/s; at 10,000 c/s the deviation is 2.4 db. The harmonic distortion at a modulation-depth of 80% does not exceed 4% for the frequency range from 100 to 8000 c/s.

The modulation-depth is checked by means of two volume-indicators bridged across the line leading to the sub-submodulator, the scales of the measuring instruments being calibrated in percentage modulation. One of the indicators shows the mean value of the modulation-depth; the other instrument only deflects when the degree of modulation is over a certain value, for instance, when it exceeds 80%. The latter instrument consequently serves to warn the operator that the mean value of the modulation-depth is set too high for a particular type of transmission. The information the operator receives from these indicators is not entirely complete, for it will not tell him of discrepancies in the quality of the programme. The arrangement of monitoring facilities is such that the operator has an aural check at important points of the modulation system; this check is effected by means of a three-valve amplifier and a loudspeaker.

GENERAL ARRANGEMENT

The transmitter K.V.F.H. 10/12 comprises five high-frequency stages; the modulation system Consists of three low-frequency-stages.

For a short-wave broadcast transmitter which has to give service during the greater part of the day, it will be necessary to work on different wavelengths for different parts of the day. To provide for a quick change-over from one wavelength to the other, two sets of crystals, two thermostats and two oscillator valves have been fitted, the switching-on of either of the circuits being effected by a single manipulation. Screen-grid valves type E 452T are used as oscillator valves in the drive circuit.

Each of the thermostats contains three crystals: a crystal for the nominal carrier frequency and two crystals respectively for frequencies of a few kc/s higher and lower than the nominal frequency. In case of interference from another transmission on the nominal frequency it is therefore easy to change over to another carrier frequency of a slightly different value and thus evade interference.

In the separator stage two screen-grid valves type Q.B. 2/75 are used. These valves are set to work without running into grid current. Apart from preventing feedback from subsequent H.F. stages on to the drive-circuit, the use of screen-grid valves tends to simplify both construction and adjustment of the transmitter, as no neutralising devices are needed. Furthermore a screen-grid valve only requires a small amount of excitation power, so that a small drive-valve will suffice.

The third H.F. stage, an H.F. amplifier, contains two screen-grid valves type Q.B. 2/75. As already mentioned, the use of screen-grid valves offers several advantages and materially assists in bringing about stability of the transmitter. Moreover, this type of valve gives a high degree of amplification.

The fourth H.F. stage, the frequency-doubling stage, incorporates two triode valves type T.A. 10/5000 K. The grids of these valves are excited in counterphase, the anodes being connected in parallel. This type of circuit provides for a very effective system of frequency-doubling.

The fifth stage, the H.F. power-amplifier, comprises two triodes type T.A. 12/20000 K arranged in a symmetrical manner and consequently allowing for a perfect neutralising scheme.

The frequency-doubling and H.F. power-amplifier stages as well as the modulator stage comprise water-cooled valves. Spare valves are already mounted in the housings of the stages mentioned, thus enabling the spares to be put into operation with a minimum delay.

All H.F. stages, with the exception of the power-amplifier, are designed to cover a waverange from 80 to 160 metres; the final stage can be tuned from 40 to 80 metres.

The modulation system consists of three stages made up of the bulb-submodulator with a valve M.C. 1/60 for transformer-coupling to the sub-modulator, incorporating two type M.A. 4/500 valves.

The sub-modulator is transformer-coupled to the modulator comprising two valves M.A. 12/15000, which is also transformer-coupled to the anode-lead of the fifth H.F. stage, thus providing anode modulation in the H.F. power-amplifier stage.

POWER SUPPLY

The filament energies for the different stages are obtained in the following ways:

The filament current of the crystal oscillator is derived from a metal rectifier with smoothing unit;
The filament currents of the other stages, includog the modulation system, are derived from a motor generator giving a power of 400 amps at 30 volts; normally the generator supplies about 395 amps at 25 volts.

The anode powers are derived from a set of rectifiers:
a rectifier containing a 506 valve and smoothing unit for the anode voltage of the crystal oscillator;
a rectifier containing 3 valves D.C.G. 5/2500 and 3 valves D.C.G. 4/400, supplying 500 milliamps at 2000 volts and 250 milliamps at 4000 volts respectively for the screen-grid and anode voltages of the separator and H.F. amplifier stages and of the sub-sub and sub-modulator stages;
a rectifier containing 4 valves D.C.G. 10/15, supplying 5 amps at 8000 volts for the anode voltages of the frequency-doubling and final H.F. amplifier stages and the modulator;

The grid biases are derived from another group of rectifiers;
a rectifier comprising 2 valves D.C.G. 4/400 and delivering 100 milliamps at 500 volts for the grid biases of the separator and H F. amplifier stages and for the sub-sub and sub-modulator;
a rectifier comprising 2 valves D.C.G. 5/2500, delivering 1 amp at 1000 volts for the grid biases of the frequency-doubling and H.F. power-amplifier stages;
a rectifier containing 1 valve 1768 and delivering 1.3 amps at 150 volts for the grid biases of the moduIator valves.

The rectifiers mentioned sub 1) and 3) are contained in the panel housing the H.F. stages of the installation. The rectifiers mentioned sub 4) and 5) are fitted in the frame containing the modulation system. The remaining grid-bias rectifiers are built as a separate unit.

THE CONTROL POSITION

The important supervisory indicating instruments and also the modulation pre-amplifier and the monitoring amplifier are mounted at a centrally located control position.

When the transmitter is in operation the engineer on duty is normally seated at the control desk, for which reason the controlling and checking instruments are concentrated on same to enable him to watch the performance of the whole installation in a most effective way.

The control position has been given the form of a table. The modulation pre-amplifier, the monitoring amplifier and the power-supply unit for these apparatus are mounted at the rear of the table; the vertical front panel of the amplifier also bears the indicating instruments of the modulation meters.

On the left are the instruments for checking the temperature of the cooling water and thermostats, on the right the regulation device for the transmitter high-tension supply and an emergency control, the centre of the table remaining free to be used as a writing-desk by the engineer in charge.

CONSTRUCTION

The high-frequency stages are constructed as a single-frame assembly. The framework is subdivided into three parts. The left part contains the drive-stage and thermostats, the separator and H.F. amplifier stages and the frequency-doubling stage. The middle part houses the H.F. power amplifier and the right part the feeder-line coupling and tuning-unit.

The main switchboard is made up of four separate panels. The first panel bears the controls of the regulating system used to reduce the variations of the supply mains voltage to as small a value as possible. The second panel contains the main automatic switch and the control for putting the regulating system for the mains voltage into action. On the third panel are the switches for the motor generator for the filament current and the machines of the cooling-plant. The fourth panel bears the main switch for the high tension. Checking instruments are systematically arranged at the top of the respective panels.

The frame assembly houses the modulation system and the high-tension rectifiers. The panel also bears the controls for regulation of the filament voltages, the checking instruments being aranged at the top of the panel. Furthermore, there is a frame containing three rectifiers for the grid biases of different stages.

Centrally arranged is the control position.

The high-tension and engine-room. The high-tension part of the power supply installation is built in a separate cage. From left to right: the induction regulator for the high-tension supply, the filament current converter, the booster for regulation of the mains voltage and the cooling-plant. There are condensers of the smoothing system, the modulation transformer, the coupling condenser and the modulation choke.

The cooling-water circulating system shows the water-flow control system with relays for switching off in case the rate of water supply should drop below a certain minimum value, and furthermore the circulating pump system and ventilator.

A SPECIMEN TRANSMITTER

of the type described has been supplied to the Neth. East Indian NIROM Broadcasting Company. The station is operating at Tandjong Priok near Batavia (Java). The equipment comprises two aerial systems, one for a wavelength of 50 metres and another for a wavelength of 80 metres. During daylight transmissions are performed at 50 metres; at night the 80-metre wavelength is used.

Broadcasts are being received over the greater part of the Neth. East Indies (distances up to 1500 km) with good strength and excellent quality. Extracts from reports, vouch for the general high performance of this type of Philips transmitter.

TUBE COMPLEMENT
RF stages		AF stages and modulator		Rectifiers
Number	Type	Number	Type	Number	Type
2	TA 12/20000 K	2	MA 12/15	1	506
2	TA 10/5000 K	2	MA 4/500	3	DCG 5/2500
4	QB 2/75	1	MC 1/60	5	DCG 4/400
1	QC 05/15			4	DCG 10/15

THIS TYPE OF TRANSMITTER IS INSTALLED IN THE FOLLOWING COUNTRIES
	ITU	Country	ITU	Country
	IND	INDIA	INS	INDONESIA
	BRM	MYANMAR