Talk about the Miller effect of the tube

The voltage amplification tube that drives the strong discharge tube has a lower output impedance than the output swing to 74V or more. This is because the Miller Effect of the strong discharge tube is relatively high! That is the reason for the interelectrode capacitance!

We know that the larger the interelectrode capacitance of the vacuum tube, the worse the high frequency response, and the greater the distance between the poles of the strong discharge tube. Therefore, the interelectrode capacitance is much larger than that of the general small voltage amplification tube. The output impedance is low and the effect is relatively small, while the Miller effect of the three-pole pump is larger than that of the quadrupole or the pentode, so the output impedance of the driver stage is lower.

The high-frequency response of the strong-release tube is not only related to the capacitance between the stage and the stage of the strong-release tube, but also related to the output impedance of the front driver stage. The lower the output impedance of the driver stage, the more the Miller effect of the power tube can be ignored. Therefore, in addition to the large output swing, the driver stage also has a low output impedance.

In fact, there is such a relationship between any two-stage amplification, not just the power stage and the driver stage, but the inter-electrode capacitance of the power tube is large, so the output impedance of the driver stage becomes more important.

We can use the load resistance of the previous stage and the inter-electrode capacitance of the latter stage to calculate the approximate -3db high-frequency cut-off point. The formula is:

Fc=1/2πRL{Cin+Cgp(1+A)}

=159/RL{Cin+Cgp(1+A)}

among them:

Fc = -3db high frequency cutoff frequency of the next stage vacuum tube, the unit is KHz.

RL = screen load resistance of the previous stage vacuum tube, the unit is MΩ.

Cin = the input capacitance of the vacuum tube of the latter stage, that is, the capacitance between the gate and the filament, in units of pf.

Cgp = capacitance between the gate and the screen of the vacuum tube of the latter stage, the unit is pf

A = gain of the vacuum tube of the latter stage.

It can be seen from the above formula that the lower the screen load resistance of the previous stage, the smaller the Cgp of the latter stage, the lower the gain, and the higher the high frequency cutoff point. In other words, the higher the high frequency response.

 How do you choose the vacuum tube of the voltage amplification stage (driver stage)? Of course, it is necessary to select a vacuum tube with a low output impedance to obtain a better frequency response.

What kind of vacuum tube output impedance is low, and under what circumstances is the output impedance low?

The lower the screen resistance (screen resistance) of the vacuum tube, the lower the output impedance.

The lower the screen load resistance, the lower the output impedance.

The higher the current, the lower the output impedance.

By bypassing a capacitor with a capacitor, the output impedance is greatly reduced.

The following methods can also reduce the output impedance, but it does not conform to the "eight-point" principle, so it is for reference only:

Parallel vacuum tubes reduce output impedance.

Use the Cathode Follow circuit to reduce the output impedance.

With the SRPP circuit, the output impedance can be reduced.