BBC BROWN BOVERI SK 51

GENERAL DESCRIPTION

100 kW Short-Wave Broadcast Transmitter
Type SK 51 F3
Type SK 51 C3

photo BBC Brown Boveri SK 51

Introduction

The BBC SK 51 F3 (C3) transmitter is the smallest unit in the range of equipment fitted with only four tetrode tubes. Solid-state devices are used for the low-power audio and radio frequency circuits, control circuits and rectifiers.

The transmitter employs the standard simple proven Class B modulation system using a modulation transformer only. Very careful design of all stages together with application of a unique pre-emphasis circuit results in exceptionally high efficiency for a short-wave transmitter of this rating.

Use of a condensed vapour cooling system for the power tubes greatly enhances reliability.

Main features

Transistorized RF and AF preliminary stages
Only four tubes
Solid-state rectifiers throughout
Solid-state control system
Choice of fully automatic (C3) or manual motorized (F 3) tuning
Veritron system for smooth high-voltage control
Choice of 50 ohm unbalanced or 300 ohm balanced output
Simple operational procedure
Condensed vapour cooling
Rapid and easy installation
Small floor area
Low running costs: very high efficiency

Circuits RF stages

The RF drive unit may be a synthesizer (C3 and F3) or a simple crystal oscillator (F3 only). The only requirement is that it deliver an output of 1 V r.m.s. on 50 ohms and has a stability and accuracy to accord with a customer's specification. The drive unit maybe installed in the transmitter or can be located externally if required.

The signal from the drive unit is fed to series of solid-state broadband amplifiers. They deliver approximately 10 watts to the grid of the air-cooled penultimate RF amplifier tube type BBC CQL 2.

The final tube requires approximately one kilowatt of driving power to deliver its rated 100 kW over the full frequency range of 3.9-26.1 MHz. Modulation is applied to both anode and screen-grid. The tube operates with grounded cathode and feeds into a Pi-circuit. A harmonic filter is provided which ensures the transmitter conforms to CCIR Radio regulations. A TV filter is available as an optional extra when more rigorous filtering is requested.

The standard output impedance is 50 ohms unsymmetrical (75 ohms as alternative). A balun which fits inside the cabinet transforms to 300 ohms balanced when this impedance is required.

AF stages and modulator

A standard 600 ohms input is provided for the AF chain (1 mW or 4 mW). If requested, a peak clipper may be connected at the input for trapezoidal modulation. A series of solid-state amplifiers is followed by two tetrodes (type BBC CQK 25) in a zero grid-current push-pull Class B system. Overall negative feedback ensures that harmonic distortion and hum remain within acceptable limits. The negative feedback has a stability margin of at least 4 dB.

Power supply

The standard power supply is 380 V / 3 phase 50 Hz. Other voltages and frequencies can be accommodated on request. Twin HV transformers feed two series connected silicon diode rectifiers in a Graetz circuit. The phases of the two transformers are shifted by 30°.

This arrangement not only reduces interference feedback into the mains but requires a minimum of filtering capacity which in turn reduces the protection requirements for the diodes and eliminates the necessity for a crowbar protection of the final tubes. The diodes are capable of withstanding a full short-circuit. Their reliability is, however, further enhanced by using a thyristor-control led convertor type BBC Veritron to feed the primary windings of the HV transformers. The Veritron allows a smooth variation of the HV rectifier output voltage from zero to the required 11 kV. In this way the star-delta and short-circuit switching requirements usual with diode rectifiers are completely eliminated.

Cooling system

The final RF tube and the two modulation tubes are condensed vapour cooled. Distilled or demineralized water is used as coolant. It is pumped in a closed circuit through the tubes and the main tank coils. All necessary reservoir tanks, valves and supervision devices are included. Either an air or water-cooled heat exchanger is provided for the secondary cooling.

A single blower is used for general cooling and ventilation of the transmitter cabinets. The RF driver tube BBC CQL 2-1 and the large capacitors are additionally cooled by a small high-pressure blower within the cabinet.

Safety system

The personnel safety system has been designed to fulfill IEC Publication 215. It comprises

a. Electric door contacts

b. Door key interlock panel

Before high voltages can be applied, all doors giving access to dangerous voltages must be locked and the keys inserted in a lock panel. Only when all keys are inserted can a switch for earthing all high voltages (including charges on capacitors), be opened.

c. Earthing hooks in all compartments

In addition a blinking light system gives warning whenever, for tests or other reason, an interlock contact is bridged.

Protection

The transmitter is fitted with comprehensive protection circuits, the operation of which is initiated by sensors which monitor overvoltage, overcurrent, excessive VSWR or cooling system failure. The high-speed action of the Veritron in the event of an overload on the HV rectifier renders vacuum switches unnecessary while the very low filter capacity in the transmitter obviates the need for crowbar protection of the large tubes against flash-arcs.

A three-shot re-cycling overload system forms part of the protection measures.

Tuning system

1. F3 version
To change frequency a band switch is turned to the desired band. Coarse tuning follows automatically.

A selector is then switched to each of eight flip-flop switches consecutively. By moving each switch to left or right the correct position of the corresponding motorized element (coil or capacitor) is set by observing a number (000 to 999) on a LED display.
Once all eight elements are correctly positioned depression of a push-button brings the transmitter "on air" on the new frequency. The time required is approximately 3 minutes.

2. C3 version
a. Stored frequency
To change to an already stored frequency, one selects the corresponding store (two digits). The frequency appears in a display window. The command to change frequency is given by depressing a push-button and this command is executed in less than 60 seconds.

b. Unstored frequency
A store is chosen by touch-dialling two digits. If it is already full, the stored frequency is cancelled by a push-button. The desired frequency is touch-dialled (5 digits) and is put into the empty store by depressing another push-button. All this occurs without interrupting the operation of the transmitter. When the moment to change to the newly stored frequency arrives, the procedure is as described under a. above.

Construction and layout

The basic transmitter comprises two BBC standard MNS cabinets. They are transported completely assembled and are interconnected on site. The left cabinet contains the RF driver and final stages with the output circuit and harmonic filter.

The right cabinet includes two hinged racks in front. The left-hand rack contains the complete control and supervision system. That on the right carries the RF drive oscillator and preliminary stages and the AF preliminary stage unit. At the rear one finds the modulation stage, the 380 V auxiliary distribution board and the auxiliary rectifiers.

The two cabinets require a floor area of 3.6 m x 1.6 m.

On site, the remaining components are normally placed behind the two main cabinets. A row of four standard cabinets contains the mains switchgear and distribution system, the HV diode rectifier and the HV filter capacitors. Suitably placed near these components one finds the two HV rectifier transformers, the modulation transformer and the voltage regulator (when supplied). The whole installation requires a total floor area of approx. 30 m2.

Remote control

Both the F3 and C3 models are inherently suitable for remote control.

With the C3 version the frequency can also be changed from a remote point.

TECHNICAL SPECIFICATIONS
Carrier power output	100 kW over full frequency range
Frequency range	5.9-22 MHz (extension to 3.9-26.1 MHz if required)
Output impedance	50 ohms unbalanced or 300 ohms balanced
Permissible standing-wave ratio (SWR)	1.7 at 300 ohms balanced output 2.0 at 50 ohms unbalanced output
Harmonic radiation	The harmonic radiation meets the requirements of the CCIR Radio Regulations (1976 Edition, Appendix 4)
Modulation capability	a. Continuous sine-wave modulation between 100 and 5000 Hz - ten minutes per hour: 100% - fifty minutes per hour: 76% b. Short-period modulation between 50 and 10 000 Hz: 100%
AF response at nominal power output referred to m = 70% and f = 1000 Hz	a. With f between 60 and 7 500 Hz the response remains within +0.5 to -1 dB b. With f between 50 and 10 000 Hz the response remains with +1 to -2 dB
AF distortion	at nominal output power, m = 90% and f between 100 and 5000 Hz the distortion factor does not exceed 3%
Carrier noise level below m = 100%	a. unweighted -55 dB b. weighted (CCITT Rec. P53) -65 dB
Carrier shift	with sudden application or removal of 100% modulation the variation in the carrier voltage will not exceed 5% (mains voltage drop excluded)
AF input level	for m = 100% modulation, f = 1000 Hz across 600 ohms input impedance: 4 mW (+6 dBm) or 1 mW (0 dBm)
Mains supply	380 V, 3 phase a. Permissible voltage fluctuation: with TQ regulator ±10%, without regulator ±3% b. Mains frequency and tolerance (other voltages and frequencies if required): 50 Hz ±2 Hz
Mains power factor	not less than 0.9 at 100% modulation
Overall efficiency a. at 0% modulation not less than b. at 30% sine modulation not less than c. at 100% sine modulation not less than	3.9-15 MHz 66% 63% 60%	15-22 MHz 65% 62% 58%
Required time for frequency change	SK 51 F3 about 3 minutes SK 51 C3 max. 1 minute
Specifications may change without notice

TUBE COMPLEMENT
RF stages		AF stages and modulator
Number	Type	Number	Type
1	CQK 50-2	2	CQK 25-2
1	CQL 2-1

THIS TYPE OF TRANSMITTER IS INSTALLED IN THE FOLLOWING COUNTRIES
	ITU	Country	ITU	Country
	BEL	BELGIUM	COD	CONGO DEMOCRATIC REPUBLIC
	CUB	CUBA	DNK	DENMARK
	EGY	EGYPT	ETH	ETHIOPIA
	FIN	FINLAND	HNG	HUNGARY
	IND	INDIA	IRN	IRAN
	I	ITALY	KOR	KOREA SOUTH
	MLA	MALAYSIA	MTN	MAURITANIA
	MOZ	MOZAMBIQUE	HOL	NETHERLANDS
	POR	PORTUGAL	RRW	RWANDA
	SEN	SENEGAL	SRB	SERBIA
	SNG	SINGAPORE	AFS	SOUTH AFRICA
	SUI	SWITZERLAND	TZA	TANZANIA