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Oscillator instability in vhf transmitters

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Instability in crystal oscillators using tubes with high values of grid-cathode (input) capacitance, Cgk, is often caused by too much feedback. In many tubes, Cgk is on the order of 10 or 15 pF, a sufficient amount of capacitance to permit the crystal to oscillate. If additional capacitance is added to the circuit, often 10 or 15 pF, too much feedback will result and the oscillator will self-oscillate independently of the crystal. This effect is all too common in v.h.f. exciters using circuits similar to Fig. 1.

Fig 1
Fig. 1. Typical first stage in a v.h.f.transmitter. V1 may be the pentode section of a 6CX8, a 5763, 6CL6, or other popular types.

The cure is simply to remove the external capacitor, C1, from the circuit and employ only the internal capacitance (Cgk) of the tube.

Instability of the oscillator circuit can be detected by pulling out the crystal, applying operating voltage to the stage, and coupling an indicating-type wavemeter to the tank coil. If h.f. energy is present with the crystal removed, remove C1 from the circuit.