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Vacuum Tubes 807 and 1625 are commonly used by hams as power amplifiers. Both tubes are audio power amplifier beam tetrodes in the output stage of high power amplifiers but they are suitable for high frequencies also. Both tubes are equal in their characteristics except for base connection and filament voltage.
807 valve requires about 6.3 volts for filament at 0.9 amperes while 1625 requires 12 volts at 0.4 amperes. 807 requires 5 pin socket and 1625 needs 7 pin socket. Other details are exactly same for both the vacuum tubes.
Typical voltages are given below Anode - 600 to 700 volts Screen - 250 volts Anode Current - 100 ma Screen Current - 6 ma Grid Voltage - 45 for class C - 35 for use as linear amplifier
For CW operation grid block keying is used. In the key up position a negative voltage about 100 volts is applied to grid of PA, the anode current is cut off. When you apply -45 volts to grid the anode conducts and signals goes out. The key can also be connected to the cathode but since there is a high voltage on cathode risk of getting shock is present and also the heavy current may damage the key contacts.
For phone operation the grid voltage is made -35 volts so that it can be used to drive SSB signals also. For getting a clean signal in the output the screen grid should be given a stabilised voltage of 250 volts. The screen grid voltage can be stabilised by either using voltage regulator tubes or zener diodes.
The plate current of the tube depends on the screen grid voltage. When the voltage is 250 volts the anode current is 100 ma. When the screen voltage is reduced to 125 volts the anode current is also reduced to 50 ma. In screen modulation, the screen voltage is set to 125 volts and anode current to 50 ma. For modulation transformer we use an audio transformer having 4 ohms primary and 5000 ohms secondary. Such a transformer is used in old valve receivers using EL84 vacuum tube as power amplifier. The output from the audio amplifier using TBA810 is connected to the 4 ohms winding of the modulation transformer and the 5000 ohms winding is connected in series with the screen.
The gain of the audio amplifier is adjusted so that the peak audio voltage across the 5000 ohms winding is 125 volts. During the positive swing of the audio voltage, the voltage at the screen will be 125 + 125 = 250 volts and the anode current will be 100 ma. During the negative of the audio voltage, the screen voltage will be zero. Hence the anode current will vary from zero to 100 ma. The average anode current will be 50 ma and the transmitter output will be halved. This is the one disadvantage of screen modulation but the main advantage of screen modulation is that audio power requirement is very less compared to plate modulation.
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