Two Tube AM Transmitter

Posted on Feb 6, 2014

I finally completed a working AM transmitter. The transmitter`s design is based on several ideas, but mainly based on Phil`s Li`l 7 Transmitter design. It is not always easy to achieve oscillation in LC-based transmitters without the right combination of tubes and coils. The tubes used in this transmitter is a 6J5 and 6SA7 th

Two Tube AM Transmitter
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at operates at around 1400kHz on the AM band. Below is the schematic diagram of the transmitter. The antenna coil was salvaged from the oscillator circuit in a typical old radio with a 455kHz I. F. frequency. The coil generally has two windings; one with a larger DC resistance than the other and is indicated as the larger winding in the schematic. If there is no oscillation present on the antenna of the 6SA7 then reversing the connections to one of the coil windings usually does the trick. The plate current by the 6SA7 causes the larger winding to generate a magnetic field that induces current in the other coil. The coil has to be the correct polarity to push the grid negative to reduce plate current. When the plate current reduces then the grid current reduces and allows more plate current to flow. The oscillations are controlled by the LC circuit consisting of the grid coil and the 150pF capacitor. The trimmer capacitor in parallel with the 150pF allows small range adjustment of the carrier frequency. If oscillations do not start, flipping around windings might help. Modulation is achieved by feeding audio in the second grid of the 6SA7 tube so the plate current is controlled by the audio signal. As a result, the oscillator signal and the audio signal are superimposed upon each other resulting in an amplitude modulated signal, hence it is an AM transmitter! The 6J5 is a triode designed for general purposes, but in this...

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