The AA8V 6AG7 Amplifier

Posted on Feb 5, 2014

The input coupling capacitor allows the input signal to pass through to the grid of the tube while preventing the input source from potentially shorting the grid bias to ground. If the grid bias were shorted out, the 6AG7 would overheat and it would be destroyed. The value of this capacitor is not critical at all. As long as the reactance of the c

The AA8V 6AG7 Amplifier
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apacitor at the lowest operating frequency (in this case 4. 5 ohms at 3. 5 MHz) is small compared to the value of the grid resistor, the circuit will operate correctly. The grid resistor allows the grid bias developed by the cathode resistor to pass through to the grid, while providing a load for the input signal. The value of this resistor is not critical. As long as it is high enough to prevent too much loading of the source, while still allowing the grid bias through, the circuit will operate fine. The value of the cathode bias resistor is critical. Cathode current (sum of plate and screen currents) passing through this resistor causes a voltage drop across the resistor. This voltage drop is the bias applied to the grid of the tube. The bias is critical. In this amplifier the bias is chosen so that the amplifier requires the least amount of drive possible. This means making the bias as small as possible, while preventing the tube from overheating. The various currents and voltages in the tube were measured while the value of this resistor was varied. 180 ohms was finally chosen as this kept the plate dissipation safely under the maximum value of 9 watts. While the cathode bias resistor generates the tube bias, it also offers opposition to the signal that ultimately flows to the plate (the output signal). This causes negative feedback, which, in this case, is undesirable. The solution is to connect a capacitor from the...

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