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To adequately understand the operation of triodes and tetrodes employed in the amplification of RF, it is first necessary to understand the operation of a diode. The reason for this first step is simple - power triodes and tetrodes are really nothing more than electronically regulated diodes.

A diode vacuum tube has two operational elements - a cathode and plate, hence the word diode. Simplistically a diode will allow current to flow from the cathode to the plate but will not permit electrons to flow in the opposite direction. Most diodes also have a third functional element called a heater. The purpose of the heater is to heat the cathode, thereby loosening the atomic bonds that hold electrons in place.
When a negative voltage is applied to the cathode and a sufficient positive voltage is applied to the plate, current flow will occur as electrons, loosened from their atomic bond, are drawn off the cathode, across the small space between the operational elements, and onto the plate to complete the circuit.

If we apply alternating current to the diode vacuum tube, current will flow, as described, during the portion of the cycle that the plate voltage becomes positive and the cathode respectively negative. During the remainder of the cycle, current will not flow. The result will be pulsating direct current - current flowing in only one direction, albeit interrupted.
To understand the triode, at this point, consider that a third operational element is added - a control grid. The control grid is physically situated within the tube between the cathode and the plate, and often looks similar to a piece of hardware cloth.
If we connect the cathode to the negative terminal (B-) of a high voltage supply source (HVS) and the plate to the positive terminal (B+), at some point, as we increase the voltage, current will begin flowing. We can observe the current flow by placing an ammeter in series with the negative supply terminal and the cathode (we could place it in the B+ circuit as well, but to do so would present a potential shock hazard).

To bring the control grid into play, attach a separate, adjustable low voltage supply (LVS) between the grid and the cathode. To start with, attach the negative terminal of the LVS to the grid and the positive terminal to the cathode. Adjust the HVS to increase the voltage applied between the cathode and plate to the point that a small amount of current begins flowing through the tube (as indicated by the ammeter). Increase the voltage delivered by the LVS (attached between the grid and cathode) and note the current flowing between the cathode and plate will decrease accordingly. By continuing to increase the LVS voltage the current flowing between the cathode and plate may be halted altogether.

By bringing everything back to zero and reversing the polarity of the LVS, attaching the grid to the positive terminal and the cathode to the negative terminal, we can use the grid to control the current in much the same manner as before. The difference is that the HVS will be increased to the point that current just begins to flow, and then decreased to the point that it stops. Now, increasing the LVS will cause current to flow between the plate and cathode. Additionally, current will also flow between the cathode and grid. The sum of the current flowing from the cathode to the plate and grid will be indicated on the ammeter if the meter is situated between the cathode and the negative terminal of both supplies.
Next consider attaching a small adjustable super low frequency (1 cycle per second) AC supply (ACS) between the grid and cathode. With the ACS set at zero, adjust the HVS to permit a small amount of current to flow between the cathode and plate. Now, by slowly increase the ACS a corresponding increase in the current flowing through the plate circuit (cathode to plate) will be indicated on the ammeter. In addition, the ammeter will rapidly fluctuate in cadence with the frequency of the AC voltage applied to the grid circuit - in the span of each second in time, it will indicate a relatively large current increase and a proportional decrease. Although a voltage reversal is occurring at the grid, the plate circuit current indication will not, however, indicate a current reversal (remember that electrons can only flow in one direction between the cathode and plate).

The important lesson, here, is that small voltage variations applied to the grid to make a large change in the amount of current flowing between the cathode and plate. This differential - utilizing a small fluctuation to effect a large change is what amplification is all about. And, the only fundamental difference between the example cited here and the basic principal behind the amplification of RF, is the frequency of the amplified signal

The next step in the process is to consider what to do with the amplified signal. As we have seen, applying a small signal to the grid nets a large variation in current at the plate. And while the signal at the plate constitutes fluctuating DC - current that rises and falls in amplitude - it is not alternating in both directions and is of little use in its present form. Our task, at this point is to extract as much of the AC or fluctuating component as possible. To accomplish this, we must first place something in the Plate/B+ circuit that will resist the flow of fluctuating or pulsating direct current. The component of preference is a choke - in this instance, an RF Choke. By placing an RF Choke in series with the Plate/B+ circuit, we effectively permit the DC component to pass, but restrict the AC component (remember that RF is simply high frequency AC).

Our final task is to extract the AC component. By placing a capacitor in line with the plate, as shown above, we can effectively permit the AC component to pass, while blocking the DC component. On the output side of the blocking capacitor, pure AC is developed between the AC Plate component and the fill provided by the B- terminal of the HVS.


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