Passive Grid
An example of a passive grid used at VHF/UHF frequencies include the 4CX250B; an example of a twin tetrode would be the QQV06/40A. The tetrode has a screen grid which is between the anode and the first grid, the purpose of the screen grid is to increase the stability of the circuit by reducing the capacitance between the first grid and the anode. The combination of the effects of the screen grid and the damping resistor often allow the use of this design without neutralization.
The signals come into the circuit through a capacitor, they are then applied to the valve's first grid directly. The value of the grid resistor determines the gain of the amplifier stage. The higher the resistor the greater the gain, the lower the damping effect and the greater the risk of instability. With this type of stage good layout is less vital.
Passive grid design is ideal for audio equipment, because audio equipment must be more broadband than RF equipment. A RF device might be required to operate over the range 144 to 146 MHz (1.4% of an octave) while an audio amp might be required to operate over the range 20 Hz to 20 kHz, a range of three orders of magnitude.
Read more about this topic: Valve Transmitters
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