Vacuum Capacitors in Parallel

When two or more capacitors are connected in parallel, the inductance of the connecting conductors acting with the capacitors form a tuned circuit.

In high current circuits, it is normal to parallel two or more vacuum capacitors. Care must be taken, due to the low loss of the vacuum capacitors and the heavy copper straps paralleling the capacitors, to ensure that the frequency of this series high-Q resonant circuit is above the operating frequency. If the frequency of the series resonant circuit were allowed to become equal to the operating frequency, high currents could be generated which would result in damage to the capacitor.

All vacuum capacitors have inductance within the connections (Figure 3), the incidental tuned circuit of two 1000 pF capacitors in parallel can be kept well above 30 MHz. At low capacities of 50 to 100 pF, the resonant frequency can be kept well above 100 MHz.

Figure 4 is a graph of resonant frequency vs. capacity of two CVEP-2000 ceramic capacitors with both standard and low inductance connections. The resonant frequency of the resulting 20 MHz to 135 MHz from a maximum to minimum capacity.

This pair of vacuum capacitors would operate in the plate tank of a high powered 2-30 MHz transmitter with no difficulty. At the low frequency, maximum capacity would be used and the incidental resonance within the tank capacitors would be several times the operating frequency. At the high frequency, minimum capacity would be used and the high frequency, minimum capacity would be used and the incidental resonance would have increased until it was still several times the operating frequency.

The physical size of the components in a high power 30 MHz final amplifier may cause problems because of added stray inductance.

This stray inductance of a plate blocking capacitor and its straps for shunt feed is often as great as the desired tank inductance. The output capacitance of the tube (frequently over 100 pF), the plate tank tuning capacitor, and the stray inductance associated with the plate blocking capacitor make an incidental tuned circuit that could be marginal to the overall circuit (Figure 5).

In circuit Figure 5A the blocking capacitor at high frequencies and high power may be required to carry very heavy current. By using the circuit of Figure 5D, the plate tank capacitor can be connected adjacent to the plate (less stray inductance) and become more effective. The plate blocking capacitor is moved closer to the antenna circuit. While more blocking capacity is required in Figure 5D than in the circuit of Figure 5A, it only has to carry the output load current. However, the plate tank tuning capacitors will have the DC plate voltage applied to them in addition to the RF voltage. A capacitor with DC voltage capability should be specified when ordering units for these applications. The peak RF working voltage should be equal to the sum of the applied DC and the peak RF voltage.

The plate blocking capacitor in a shunt fed RF amplifier sees the DC plate voltage plus the peak modulating voltage plus a small amount of RF voltage. At higher frequencies and powers it will usually see high RF currents as well.

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