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  • Stu WØSTU

Vacuum Tubes (G6A10)

The 2023-2027 General License question pool asks about the inner workings of vacuum tubes used as amplifiers:

G6A10: Which element of a triode vacuum tube is used to regulate the flow of electrons between cathode and plate?

A. Control grid

B. Heater

C. Screen grid

D. Trigger electrode

a triode vacuum tube diagram showing electrons moving from cathode to plate through the intervening control grid
A triode vacuum tube control grid helps to vary the current that will flow from cathode to plate. The control grid voltage is determined by the transmitter signal voltage.

Vacuum tube amplifiers are widely used in amateur radio to boost RF signal transmission power. The basic operation of a vacuum tube is much like a field effect transistor (FET), with a transmitter signal voltage controlling a large current to boost the power of transmission to the antenna.

Triode: A triode vacuum tube has three primary elements: Cathode, Plate, and Control Grid. A heater, or filament, is energized near the negative potential cathode to cause electrons to be released. The positive potential plate, or anode, attracts the released electrons, so a current flows from cathode to plate. This current can become quite large for high power operations.

a positive voltage on the control grid increasing the current flowing through the control grid to the plate of the triode vacuum tube
A positive voltage on the control grid will increase the flow of current from cathode to plate.

A control grid is positioned between the cathode and plate. This grid is essentially a screen of fine filaments or wires connected to a controlling voltage from the transmitter. By varying the voltage on the intervening control grid the current flowing from cathode to plate may be commensurately varied. When the voltage of the control grid is positive it will help to accelerate electrons from the cathode. The vast majority of these will accelerate rapidly toward the control grid and pass right through its gaps and on to the positive potential plate. When the control grid voltage is negative it will repel the cathode’s electrons, reducing or terminating the current flow to the plate. As the voltage of the signal to the control grid varies across a range of values associated with the signal of transmission the current through the vacuum tube mimics this signal and provides amplification for a powerful transmission.

Tetrode: A related question pool item (G6A12) involves a four-element vacuum tube called the tetrode. The tetrode is nearly identical to the triode except for the addition of another grid called the screen grid. The screen grid is positioned between the control grid and plate, usually closer to the control grid than to the plate. Normally the screen grid is connected to a positive DC voltage slightly less than that of the plate.

tetrode vacuum tube configuration with screen grid between control grid and plate
A tetrode vacuum tube includes a screen grid between the control grid and plate to help reduce parasitic capacitance between the tube's elements.

The screen grid’s purpose is to reduce capacitance that arises between the control grid and the plate. Such parasitic capacitance can cause the tube’s circuit to become self-resonant at some frequencies, and it will reduce the tube’s achievable gain at higher frequencies. This problem is called the Miller Effect, and the screen grid helps to resolve it. A triode amplifier will typically require some type of “neutralization” circuit outside of the vacuum tube to avoid the detriments of the Miller Effect.

Configuration: Although these graphics depict the cathode, grids, and plate in a flat configuration within the vacuum tube enclosure, the more common configuration is a concentric circular or oval arrangement for these elements. The heater and cathode are the inner-most elements, surrounded by the grids, and the plate encircling the outer perimeter. Electrons flow from the inner cathode out to the surrounding plate through the grids.

End-on view of vacuum tube structure, showing concentric rings of cathode, control grid, screen grid, and plate.
Typical vacuum tubes have a concentric ring structure, as shown in this end-on view.

The answer to General License question G6A10, “Which element of a triode vacuum tube is used to regulate the flow of electrons between cathode and plate?” is “A. Control Grid.”

-- Stu WØSTU


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