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COOLING TECHNIQUES IN TRANSMITTER
Introduction
In modern A.M. transmitters power valves are used in the PA and modulator stages, which are condensed vapour cooled ceramic tetrodes. In the old generation transmittes, triodes are used in the PA, modulator and exciter stages. Both the tetrodes and triodes tubes are capable of being operated at high voltages (11 kV DC) and large anode current of the order of 50 Amps. They also draw large filament current of about 620 Anps at 24 volt CQK-350. Hence the tubes dissipate large amount of power which require effective cooling.
The CQK series of transmitting tubes are tetrode specially designed for transmitters and power amplifiers used in broadcasting.
The tube is installed vertically with the heating connections at the bottom. Handle and transport the tube with utmost care : vibrations and external impacts can cause invisible damage. Avoid sudden movement. Slowly insert the tube in the connection head so that sudden impact is avoided. If the dead weight of the tube is not sufficiently to overcome contact resistance in the connection head, apply gentle pressure. The ceramic parts must be always kept clean. If necessary, they should be cleaned with alcohol or acetone but no circumstances should they be rubbed with emery paper.
The contact surfaces are coated with a heat resistant lubricant film, which does not attack silver. Electrical connections and connection head are provided with contact rings for all electrodes including the anode. The connection head is stationary. It supports and locate the tube, which can be inserted into the connection head only in a certain position. This position is determined by the guide groove on the anode.
Cathodes
All the ceramic tetrodes used in AIR transmitters are directly heated thoriated tungsten cathode. The filament voltage should not vary beyond +5% of the rated filament voltage. The filament voltage must always be measured at the concentric contact rings using sub-standard volt meter. The cathode cum filament has only a very small resistance when cold. Hence the filament voltage is applied and increased smoothly as per the design of the transmitter. In some transmitters, the filament voltage is applied in steps. In some transmitter, the design of the filament transformer is such, it will restrict the surge current to 3 to 4 times the normal steady current.
Screen Voltage
The screen grid current can become dangerously high, even at normal screen grid voltage, when the anode voltage is lower than that of the screen grid. Hence the screen grid supply will be switched ON only when the anode voltage has become about 40% or so of the anode voltage.
Temperature
A separate air cooling has been arranged to control the temperature of the ceramic cylinder and all metal ceramic seals in addition to the condensed vapour cooling.
There must not be any high frequency on the supply leads. To ensure this filament RF by pass condensers are provided.
Cooling System Used in Transmitter
In high power A.M. transmitter, lot of power is dissipated in the valve as the input power is not fully converted into output R.F. power due to the efficiency of the amplifier which never reaches 100%. Hence the valves have to be cooled. In addition filaments are drawing large current of the order of 210 Amps at 10 volt for CQK valve. Hence they also have to be cooled. The dissipated heat in the valves also circulates in the concerned cubicle and heat develops there. Hence some kind of cooling has to be provided to the transmitting equipment. Different types of cooling are used in AIR transmitter at present.
a) Air cooling
b) Vapour cooling
c) Condensed vapour cooling
a) Air Cooling
At present forced air cooling is used in AIR transmitters. A blower sucks the air through an Air filter and a guided duct system and the forced air is passed on to the required transmitting tubes. There has to be minimum air flow to cool the valves. Hence there will be an air operated Air Flow Switch (Relay) AFR : the AFR will close only when sufficient amount of air has been built up with the blower. Otherwise, AFR will not close and filament cannot be switched on. Sometimes, if the filter is not cleaned, sufficient air may not go out of the blower. Hence the blower needs periodical cleaning.
Caution :
Do not by pass the AFR on any account. In case the AFR has failed but the blower is throwing the required amount of air, the AFR may be shorted to bring the transmitter on air. But AFR should be replaced at the earliest. At times if the 3 phase supply is phase reversed the blower may be running. But air may not go to the valves and AFR will not close. On such rare occasions, through Anemometer or by a paper flop the direction and the amount of air can be checked and suitable steps can be taken to rectify the phase reversed. At present, transmitting tubes like BEL 3000, BEL 6000, BEL 25000 etc. are forced air cooled types of valves used in AIR transmitters.
b) Vapour Cooling System
This system is used in 100 kW BEL Transmitters. For very high power valves and efficient cooling, air cooling is not sufficient. Hence some of the valves like BEL 15000, BEL 75000 etc. are cooled by vapour cooling. (Hence called Vaptron). Here the principle of heat required to convert water into steam at its boiling point is used (Latent heat of steam). The valves are kept in a in-tight water container filtered and de-ionized water. This water has high resistivity and comes in contact with anode. The water containers called "Boilers" are provided with inlet and outlet pipes.