The AA8V Twinplex Regenerative Receiver

Posted on Feb 5, 2014

The antenna coupling capacitor is a critical component in a regenerative receiver without an RF stage, like the Twinplex. In such a receiver, the antenna is an integral part of the regenerative detector, and for proper operation the coupling to the antenna must be easily adjustable. Increasing the capacitance increases the coupling and vise versa.

The AA8V Twinplex Regenerative Receiver
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After passing through the coupling capacitor, the signal is applied to a parallel resonant circuit consisting of L1 and the bandset and bandspread capacitors. The resonant circuit shorts to ground all signals except that to which the circuit is tuned (resonant frequency. ) The resonant frequency is determined by the inductance of L1, which can be changed by switching coils, and the settings of the bandset and bandspread capacitors. The bandset capacitor is used to set the portion of the band you wish to tune, and the bandspread is then used for the fine tuning. Because the antenna is part of the detector circuit, changing the antenna or the antenna coupling also affects the frequency to which the receiver is tuned. This is one of the shortcomings of this type of circuit. For more information on this, see my page on How to Operate a Regenerative Receiver. The output of the resonant circuit is coupled to the grid of the first triode through a 100 pf capacitor. The grid and cathode of the triode function as a diode and rectify the signal, much as the crystal diode does in a crystal radio. The rectifying action puts a negative charge on the grid, and this charge would build up and eventually make the grid so negative that the tube would be cut off. To prevent this, the 2. 2 Megohm resistor bleeds some of the charge off, leaving a small negative voltage (grid bias) on the grid. The grid bias helps to prevent the tube from...

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