50W AM Valve RF Transmitter circuit

Posted on May 3, 2017

This transmitter consists of 5 in total bulbs. 6L6 works as an oscillator. Subsequently the oscillation is guided to the guide grid of 807 which acts as the ultimate amplifier and is the transmitter output lamp. The amplifier consists of the ECC83 double-stage as pre-amplifier and 6L6. The power supply feeds both the transmitter and the modulator. 5R4 is used as a rectifier lamp. The coil L1 has been requested by the market as a coil of 6SA7 or 12SA7. The middle reception of the coil is associated with the 6L6 cathode. One end of the coil (closest to the middle reception) is grounded and the other one is connected to one arm of the variable capacitor C1. The variable capacitor C1 is 1000pF (parallel to the two coaxial) so that the oscillator (600-1800 KC) is tuned in the entire mid wave range.

50W AM Valve RF Transmitter circuit
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The radio frequency chuck at the 6L6 rise is 2.5mH ​​for 60mA. Variable capacitor C2 is also 1000pF with somewhat thiner sheets than C1 so that no sparks are generated between the capacitor sheets during the transmitter operation.

As shown in the drawing, both C2 reinforcements have a high voltage so it must be insulated from the chassis. The coil L2 consists of 50 coils. The diameter of the coil is 4cm and the wire thickness is 1mm. One coil is next to each other. The low frequency amplifier modulates the carrier wave from the 807 protective grid. Thus, a 90-100% configuration is achieved.

The low frequency amplifier of the transmitter receives a crystalline microphone. The potentiometer R1 is used to adjust the volume of the sound. The potentiometer R3 together with the capacitor, which is in series with it, is the tone tuning system. The transformer is 5Ω (primary impedance) for 70mA. The secondary transformer stays dead while the transmitter is operating or we use it to control the amplifier sound with the loudspeaker.

Transmitter power adapter has primary 220V and secondary 2 x 750V for 250mA, 6,3V and 5V. The choke filter choke is 15H for 250mA. At the ends of the filter is a voltage divider consisting of two resistors of 15KΩ for 20W each. The switches Δ1 and Δ2 are single-pole single-acting. D2 is not located along with the R1 potentiometer. Switch D1 serves to turn off the high voltage by letting the yarn work, so we can listen to other transmitters.

The function of the transmitter is as follows: With switches Δ1 and Δ2 closed, we change C1 until we hear the characteristic blowing of the oscillation at the desired frequency. Next, set C2 until the indicator light E2, which is in line with the antenna, flashes to the maximum. At this point we have the maximum radiating power of the transmitter. With a good antenna the range of this transmitter is great and the performance is very satisfactory. The indicator lights E1 and E2 are for 6.3V. Also all resistors are 1W unless otherwise noted in the drawing.

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