PHILIPS SOZ 139

GENERAL DESCRIPTION


DESCRIPTION OF A 40 kW BROADCAST TRANSMITTER

 [by: A. G. Robeer and B. Swets] - [source: Philips Communication News Vol. XII, No. 1, October 1951]

photo Philips SOZ 139 Editorial note

Calculation of the exact requirements concerning the transfer and increase of energy from one stage to another for a high-power transmitter, which is to be operated within the frequency range of 0.5 to 20 Mc/s, has become common practice. The various data of the valves to be used and the data which are necessary for correct circuit design to comply with modern requirements are well known, while in every up-to-date transmitter factory it is known how to combine a high-standard product with low manufacturing costs. Still, from time to time, some results of important developments concerning valve design and circuit design are published. The majority of these developments are related to safety precautions, frequency changing, cooling, of valves and circuits etc.
An entirely new idea in the design of transmitters is the application of instantuners (click-knobs) and other ingenious constructions which permit the frequency of a transmitter to be changed within the very short time of 30 to 60 seconds, thus avoiding the loss of time hitherto involved in the changing over from one frequency to another. This is a very important feature when considering that only one transmitter provided with instantuners is necessary instead of two or more transmitters of conventional design.
The authors of the present article describe a modern Philips 40 kW broadcast transmitter for 6 preset frequencies within the frequency range of 6 to 24 Mc/s, incorporating instantuners for all tuning elements, large coils of the latest construction with a continuously variable self-inductance, and newly designed triple contacts for heavy currents.

1. Introduction

Near the site of the medium-wave broadcast-transmitters of the Netherlands' Post Office at Lopik, a 40 kW highpower modulated shortwave transmitter has been erected. The frequency range of this transmitter is 6 to 24 Mc/s. Within this range six preset frequencies can be selected. The installation has been designed and built by Philips Telecommunication Industries, Hilversum (Netherlands), and is used for broadcasting to Indonesia, Surinam and the Dutch Antilles.
The installation has been designed according to the latest developments and is, for instance, provided with "instantuners" (click-knobs) by means of which an adjacent pre-set channel within the above-mentioned frequency range can be switched in within thirty seconds; a non-adjacent pre-set channel within one minute.
The high voltages exceeding 1000 V are supplied by valve rectifiers, while the voltages under 1000 V are provided by selenium rectifiers.
The transmitter consists of six frames which are mounted behind one separately mounted, large front panel, the control mechanisms and instruments being mounted on a control desk placed in front of this panel. The r.f. stages are mounted in cabinets while the remaining parts of the installation are mounted on frames or directly on the floor of the transmitter building. Fig. 1 shows a possible arrangement of the various parts of the installation in a suitable building.
To safeguard the attending personnel the energizing coil of the high-voltage circuit breaker is interlocked with door contacts of the high-voltage compartments, and the high-voltage leads are earthed when these doors are opened.
The transmitter is, of course, crystal controlled and any type of crystal or crystal oven can be used.
Essential components and contacts used in the r.f. stages are heavily silver-plated.
The water pipes which are necessary for the cooling system of the valves of the final amplifier stage, the interconnection cables and the supply cables for the installation can be fitted in a cable duct so that a cellar need not be provided.
The installation is constructed in such a way as to be dependable under the most severe climatic conditions.
The constructional layout, especially that of the front of the transmitter, allows easy adaption to almost any situation.

Philips SOZ 139 fig. 1 2. The driver unit

This unit consists of five stages which are mounted in one frame. Although only 2 kW is needed for the excitation of the power stage the output of the driver unit may be raised to 3 kW for a frequency range of 5,6 to 27 Mc/s. Fig. 2 gives the circuit diagram of this driver unit showing the valves and the frequency doubler stages. Stage I is crystal controlled; six crystals are mounted in a thermostat while special care is taken to obtain high stability. Stage II serves as a buffer stage. The stages I, II and III are mounted together in a drawer sliding on telescopic rails and are entirely screened. This drawer has been constructed so that inspection during operation is possible.
Philips SOZ 139 fig. 2 Philips SOZ 139 fig. 3 Philips SOZ 139 fig. 4 The r.f. stages IV and V which are mounted above the drawer are provided with forced air cooling so that, even at high ambient temperatures, the valves and circuits are adequately cooled. R.f. stage V is a class C balanced amplifier, the neutralization of which has been carried out in such a way that readjustment is not necessary for any change of operating frequency within the limits of the frequency range of the transmitter. In fig. 3 the circuit diagram of r.f. stage V is given, while fig. 4 shows the anode circuit of this stage. This anode circuit is tuned by short-circuiting parts of the coil and by means of a variable condenser. The coil consists of two sections between which a coupling coil can be moved. As a result of this construction the central spindle consists of two halves which are electrically connected with each other by means of contact discs and contact fingers. On the two central spindles are mounted a number of contact fingers which can thus be rotated in both directions resulting in short-circuiting a corresponding part of the coil. The anode circuit is inductively coupled to the final stage by one of three coupling coils which are mounted on a spindle. The required coupling coil can be rotated between the two halves of the anode circuit coil as already mentioned. The coupling coils are designed for connection to a symmetrical feeder with a characteristic impendance of 450 to 600 ohms.
Philips SOZ 139 fig. 5 Philips SOZ 139 fig. 6 As has already been stated, the condenser, the contactfingers for short-circuiting the coil of the anode circuit, and the coupling coils are adjusted by means of instantuners.
Fig. 5 shows the driver unit and in particular the r.f. stages IV and V, the drawer with the r.f. stages I, II and III (pulled out) also being visible. The panel which is visible at the extreme right contains the power supply. In this figure the various instantuners are also clearly visible. In fig. 6 a rear view of the driver unit is given from which various components of the power supply and also the coupling coils may be seen.

3. The final stage

The final stage delivers 40 kW carrier energy and is anode-modulated. It contains two water-cooled valves, type TA 12/35/03, in a neutralized push-pull circuit. These valves are provided with three-phase heater arrangements, by which, in combination with a suitable circuit, a hum level of the transmitted modulated signal of approximately -60 dB has been attained.
Philips SOZ 139 fig. 7 Fig. 7 shows the simplified circuit diagram of the final stage. The components which, after first being adjusted and then rotated to the desired position by means of instantuners, are each indicated by a character. The various main parts of the final stage are the grid circuit, the anode circuit and the valve assembly with the neutralizing condensers and the fixed tuning condensers.

a. Grid circuit (Figs. 7, 8 and 9)
For the frequency range of 5.8-13 Mc/s the grid circuit consists of the coil L1 and the variable condenser C1, while for the frequency range of 12—24 Mc/s a tunable Lecher system is applied.
By means of switch Sk1 the excitation voltage is fed either to the coil L1 or to a coupling link L3.
Philips SOZ 139 fig. 8 When the excitation voltage is fed to L1, one of three taps on the grid circuit can be selected with the aid of switch Sk1 in order to obtain correct impedance matching. For covering the entire frequency range with L1 and C1 the coil is provided with taps which can be selected by means of the switch Sk2. With the aid of condenser C1 the total frequency range is thus subdivided into smaller overlapping ranges.
When by means of switch Sk1 the excitation voltage is fed to the coupling link L3, the excitation energy is inductively transferred to the Lecher system which can be tuned by means of a variable short-circuiting bridge.
Philips SOZ 139 fig. 9 With the aid of switch Sk3 either L1 or the Lecher system is connected to the grids of the valves of the final stage. The coil L1 (which consists of two sections), the condenser C1 and the Lecher system form one unit. For practical reasons the Lecher wires are bent in a circular form. Since either the condenser C1 or the Lecher system is operated, the short-circuiting bridge of the Lecher system has been mounted, via two insulators, on the shaft of the condenser C1. This short-circuiting bridge can be rotated along the Lecher system over an angle of 360 degrees so that the insulators bearing the short-circuiting bridge can make one complete revolution. This has been accomplished by splitting the coil L1 as well as the condenser C1 into two identical parts between which the Lecher system is mounted. Short-circuiting of the coil and rotation of the contact fingers takes place in the same way as described for the driver unit. The Lecher system is made of a bent U-shaped metal profile one side of which serves as contact path along which the specially designed triple contact rotates. The construction of this contact will be dealt with further on.
Fig. 8 shows the grid circuit. At the left side the excitation voltage is fed to the circuit; the connection to the grids of the two amplifier valves can be seen at the right. In fig. 9 a more detailed picture is given of the right part of this circuit, whereas fig. 10 shows the grid circuit as it is mounted in the transmitter. In figs. 8 and 9 a clear view is also given of the front panel with the various drive spindles which are mechanically coupled to the corresponding instantuners.

b. Anode circuit
The anode circuit (see fig. 7) consists of a combination of fixed capacities and the variable coils L4 and L5.
For the frequency range of 12-24 Mc/s the tuning capacity is formed by the sum of the inter-electrode capacity of the valves, the stray capacity of the coils and the stray capacity of the wiring, whereas for the frequency range of 6-12 Mc/s the fixed condensers C3, C4 and C5 are added to this capacity.
Philips SOZ 139 fig. 10 The total inductance of the circuit depends on the position of the switches Sk6 and Sk7, which are connected with the coils L4 and L5 respectively. The inductance of coil L4 can be varied continuously by means of switch Sk6, the inductance of L5 being variable in steps by means of switch Sk7.
Philips SOZ 139 fig. 11 The switch Sk5 is used for short-circuiting in steps various parts of the coil L4 in order to prevent unwanted reflections.
A variable coupling with the feeder is obtained by means of switch Sk8. The contact of this switch can be moved along coil L5 continuously.
As shown in fig. 11, coil L4 consists of two parts which are mounted on horizontal ceramic rods, these rods being in turn supported by means of metal end plates. The end plates are mounted on vertical ceramic rods which carry two other end plates on which switch Sk6 is mounted. By means of this switch the tapping of coil L4 can be continuously varied.
Philips SOZ 139 fig. 12 For this purpose a special triple contact has been applied which can be moved inside the coil. As expressed by the name this contact always rests on the contact surface at three different points and is designed in such a way that sufficient flexibility is obtained. The triple contact, as already mentioned above in the description of the grid circuit, is shown in fig. 12a and b. On top is mounted a bush (1) in which are riveted three silver contacts (2). Since bush (1) is supported by a spherical surface (3) and is centered in a triangular hole (4) a "hinge action" of the contact with respect to its support is obtained. The pin with the triangular hole is pressed upwards through the action of a spring (5) which also determines the contact pressure. The stroke of the pin is limited by means of a screw (6) and a slot in the pin. The maximum stroke is sufficiently large to cover all practical tolerances of the contact surface. In order to ensure a reliable current path from the "hinged" bush to the fixed part of the contact (7), these parts are connected to each other by means of latten copper strips (8) which does not impair the flexibility of the contact.
Philips SOZ 139 fig. 13 Philips SOZ 139 fig. 14 Fig. 13 shows the driving mechanism of the triple contact. A threaded bar (10), the pitch of which equals that of coil L4, is mounted rigidly to the above-mentioned end plates (13) of the coil. This threaded bar carries a bush (11) which is mounted on ball bearings. Rotation of the bush (11), which is provided with a slot for driving the threaded body (9), results in a combined axial and radial displacement of the triple contact which is mounted on the threaded body. The bush (11) is actuated via a gearing. Fig. 12a gives a clearer picture of the threaded body (9), the fixed threaded bar (10) and the slotted rotatable bush (11). Fig. 12a also shows the springs with the silver-alloy contacts (12) which are mounted on the movable bush for ensuring a dependable current path from the threaded body (9) and the triple contact, to the fixed part of the circuit.
Philips SOZ 139 fig. 15 At the lower part of the supporting structure, bearings are provided for switch Sk5. This switch, which has four positions, consists of a metal shaft on which are mounted various triple contacts, 90 degrees apart. The mating fixed contacts of the switch which have the shape of blocks are welded to the coil.
A similar construction as used for L4 has been applied for L5 (fig. 14), except that in this case the shaft which carries the rotating triple contacts consists of two parts which are mechanically coupled to each other by means of an insulating coupling. This insulation is necessary since the rotating triple contacts of this pair of coils are connected with the feeders.
The switch Sk7 is mounted under L5 in the same way as Sk5 is mounted under the coil L4.
The cooling water of the transmitting valves flows through the profiled tube of which the coil is made so that the coil and the engaging triple contacts are adequately cooled.

Philips SOZ 139 fig. 16 c. The valve assembly with the fixed condensers of the anode circuit
In figs. 15, 16 and 17 the assembly is shown of the watercooled valves and the switch Sk4 for selecting the required fixed tuning condensers C3, C4 and C5 of the anode circuit, the connections between the anode coils and the valves also being visible (fig. 17). The valve assembly and the switch are combined into one single unit. Sk4 (figs. 7, 16 and 17) has four positions, three of which are used for connecting the vacuum condensers to the tuning circuit, whereas in the fourth position no capacity at all is added to the circuit. Switching of the condensers is obtained by vertically moving a balanced lever along two bar guides. In the above-mentioned three positions the triple contacts of the condenser (which can be clearly seen in fig. 16) engage the watercooled contact path of the valveholders.
Philips SOZ 139 fig. 17 The variable neutralizing condensers (figs. 15, 17 and 18) are each formed by the outer surface of a valveholder which has the same potential as the anode, and by a corresponding metal cylinder surrounding the valveholder. and consisting of a fixed part and a variable part. The fixed part of this cylindrical jacket is connected with the grid of the other valve by means of strips (fig. 18), while the variable part can be rotated on a vertically mounted hinge thus forming the adjustable section of the neutralizing condenser.
As a result of the proper electrical and mechanical lay-out of the neutralizing condensers these have only to be adjusted once, which adjustment holds for the entire frequency range.
As already mentioned on page 2, the valve assembly and all coils and contacts are heavily silver-plated.
The cooling system is closed. The main parts of this cooling system are the pump and a radiator, the latter being cooled by a blower.

4. The modulator

The modulation energy necessary for a modulation depth of 100 percent is approximately 32 kW. The modulator applied can deliver 40 kW and consists of four stages. A block diagram of this modulator is shown in fig. 19 in which the valves are also indicated. Except for the separately mounted modulation choke, the modulation transformer and the condenser belonging to it, the modulator is built in one frame (See fig. 20). For a detailed description of the principles on which the modulator has been designed reference is made to an article which has been published earlier in this periodic.
Philips SOZ 139 fig. 18 The input signal, arriving via a line with a characteristic impedance of 600 ohms is adjusted to the required value for 100 percent modulation (1,55 Volt r.m.s.) by means of an H-attenuator on the control desk.
M II is a balanced amplifier in the anode leads of which resistances are applied. For coupling M III with M IV a cathode-follower circuit is used which has the well known advantage that, notwithstanding considerable grid currents will flow in the valves of M IV, large damping of the preceeding amplifier stage M III is not necessary for obtaining distortionless and full excitation of this amplifier. In addition, no audio transformers are necessary in the cathode-follower circuit, thus avoiding phase shifts which would otherwise impede the action of the feed-back circuit M IV - M I, since the magnitude of these phase shifts is dependent on the frequency; for a certain frequency range the feed-back action may be reversed, thus causing instability.
The balanced amplifier of the final stage in which two valves TA 12/35 are used is operated in class B. The filaments of these triodes are also supplied from a threephase system. The anode voltage of the TA 12/35 valves is 12000 V which is supplied by a valve rectifier which will be discussed further on. Except for a d.c. voltage of 1000 V which is obtained by means of a selenium rectifier, the other d.c. voltages for the modulator are also supplied by valve rectifiers.

Philips SOZ 139 fig. 19 Philips SOZ 139 fig. 20 5. The 12 kV rectifier

For a modulation depth of 100 percent and a modulation frequency of 1000 c/s the total input power of the final stage of the transmitter and the final stage of the modulator amounts to 120 kW, the applied anode voltage being 12 kV. This power is supplied by a rectifier which is designed for a continuous output power of 144 kW at 12 kV. This rectifier operates with directly heated grid-controlled mercury-vapour rectifier valves, type DCG 5/30, in a threephase Graetz arrangement.
Voltage control is obtained by varying the phase shift between the grid voltage and the anode voltage with the aid of a phase-shifting device. This phase-shifting device is controlled from the control desk.
By means of an adequate smoothing filter consisting of a choke coil with condensers, a ripple voltage of 0.05 per cent is obtained for an output power of 120 kW.
A special filter has been inserted for protecting the high-voltage supply-transformer against eventual peak-voltages arising from commutation.
For safeguarding the rectifier valves against overload and against a possible back-voltage a rapidly acting electro-mechanical protective device has been provided. This protective device operates in such a way that immediately after the occurrence of a failure the high tension is suppressed during a fraction of a second after which it is rapidly restored again. In case the failure is persistent the high voltage is automatically switched off. In addition, a fault-locating system facilitates supervision and repair.
Each of the six rectifier valves is mounted in a separate support together with its own filament transformer and with the auxiliary apparatus for firing the valve. These supports are suspended to the main structure and in case of a failure may therefore be rapidly replaced by a spare support.
At the other side of the supporting frame are mounted the low-voltage apparatus and the components of the interlocking circuits. The rectifier is placed directly behind the front panel of the transmitter with the valve supports in the front, so that the correct operation of the valves can be checked via a window which is provided in the front-panel.
The phase-shifting device is mounted in the space behind the front panel; the high-voltage supply-transformer, the choke coil, the smoothing condensers, the protective devices and the earthing switch being placed in separated high-voltage cells.

6. The central switchboard

The mains supply leads (3 x 380 V, 50 c/s) are connected to the installation via a circuit breaker with thermal and magnetic overload protection. After this circuit breaker the single rail system is split into two rail systems, of which the first is connected to those parts of the installation that are not sensitive to voltage fluctuations, whereas the voltage-sensitive parts of the transmitter have been connected to the second rail system. When the voltage fluctuations do not exceed ± 5 percent, both rail systems can be easily connected to each other by means of connecting strips. In case larger voltage variations occur, a voltage stabilizing device must be incorporated.
On the switchboard are further mounted the magnetic switches for the grid supply and for the anode supply units, and also the necessary protective apparatus associated with these switches. The switches are operated from the control desk or, if required, from the corresponding panel of the transmitter.
The switchboard is constructed as a single unit.

7. The frontpanel of the transmitter

The driver-unit, final stage, modulator, rectifier and switchboard described above are mounted in standardized frames which are placed behind a separate frontpanel (figs. 21a and b) which fits to the inner panelling of the transmitter building.
Except for the rectifier panel, each panel is provided with a draw-shutter. When the draw-shutter has been opened, the stage under consideration is accessible from the front and can be tuned and inspected. The comparatively dark space behind the panel is then lighted by a lamp which is automatically switched on at the moment the draw-shutter is opened. Where required, the draw-shutters are provided with glass panels, thus allowing inspection when the shutters are closed; the tuning means, however, are not visible. For safety reasons the rectifiers are provided with fixed glass panels.
The constructional lay-out of the various sections of the frontpanel is such that it can be matched to any situation while maintaining the overall aesthetical aspect.

Philips SOZ 139 fig. 21 8. The control desk

All operating mechanisms for switching the transmitter, which previously has been tuned to the desired frequencies, and all control instruments, are located on the control desk which is placed in front of the transmitter (figs. 21a and 22).
The operating mechanisms consist of a number of push buttons and signalling lamps which permit the following consecutive switching operations, viz.: switching of the pumps for the cooling water, of the blower for lowering the temperature of the circulating water, and of the various required voltages. The voltages which are switched on the control desk can be read on the corresponding voltmeters mounted on the control desk. The correct switching sequence is secured by means of an interlocking circuit. Since the position of each contact of the interlocking circuit is indicated, a fault can be located immediately.
Further are mounted on the control desk a level-indicating instrument on which the level of the incoming signal can be read, an oscilloscope for monitoring the modulated carrier, and a meter on which the amount of energy which is transferred to the feeders can be read.
The temperature of the cooling water before entering the cooler jacket of the valves, the temperature of the water at the output nozzle of the cooler jacket, and the waterpressure at the exhaust nozzles of the pumps can be read on meters.
The operating controls and the various meters are mounted on a horizontally hinged section of the control desk (fig. 22) which in turn is mounted on a support serving as writing desk. The rear side of the desk contains the necessary terminal strips and interconnections etc.

Philips SOZ 139 fig. 22

Philips SOZ 139 fig. 23 Philips SOZ 139 Table I and II
Philips SOZ 139 fig. 24

TECHNICAL SPECIFICATIONS
Frequency stability The frequency stability easily meets the Atlantic City specifications.
Hum Depending on the load of the various phases of the mains, the total hum of the transmitter with the modulator lies between -62 and -57 DB (with a modulation depth of 100 percent as reference level).
Amplitude distortion Fig. 23 gives the modulation depth as a function of the modulator input voltage, for a modulating signal of 1000 c/s, an output of 40 kW and an operating frequency of 20 Mc/s.
Attenuation distortion The attenuation distortion as a function of the modulation frequency at 20 Mc/s, with 1000 c/s modulation frequency as reference frequency, is given in fig. 24.
Non-linear distortion The non-linear distortion for an output of 40 kW and for an operating frequency of 20 Mc/s is given in table I.
Performance of the modulator Table II gives the frequency distortion and the non-linear distortion of the modulator when loaded with 1800 and 2400 ohms respectively. These values hold for a modulator output power of 35 kW.
Harmonics Harmonics radiation is well within the limits of the Atlantic City specifications.

TUBE COMPLEMENT
RF stages AF stages and modulator Rectifiers
Number Type Number Type Number Type
2 TA12/35 2 TA12/35 6 DCG5/30
2 TB3/2000 4 PB3/800

2 PB2/200 2 PB2/200

3 PE04/10 2 PE04/10

1 ECH21





THIS TYPE OF TRANSMITTER IS INSTALLED IN THE FOLLOWING COUNTRIES

ITU Country
ITU Country
flag HOL HOL NETHERLANDS