The Tetrode

Part of Vacuum Tubes

The tetrode was the first four-electrode tube, adding a screen grid to the triode to eliminate the plate-to-grid capacitance that prevented stable RF amplification.

Why This Matters

The triode’s plate-to-grid capacitance created a fundamental problem for radio frequency amplification. Even a few picofarads of capacitance between the plate (output) and the grid (input) provides enough coupling at radio frequencies to create positive feedback, causing the amplifier to oscillate instead of amplify. Early radio receivers using triode amplifiers were unstable and prone to spontaneous oscillation that created interference for other receivers nearby. Controlling this instability required careful neutralization — adding an external capacitor to cancel the plate-to-grid feedback — that was tedious and had to be readjusted when tubes were replaced.

The tetrode solved the problem structurally. A second grid between the control grid and the plate, held at a fixed positive potential through a bypass capacitor, electrostatically screens the control grid from the plate. The plate-to-grid capacitance dropped by a factor of 1000 or more, eliminating the feedback instability. Receivers using tetrodes were stable and could achieve much higher gain without neutralization.

Understanding the tetrode’s operation — and its characteristic flaw (secondary emission distortion) — explains why the pentode superseded it and why beam power tubes were developed as an alternative solution.

Physical Construction

The tetrode has four electrodes: cathode, control grid (G1), screen grid (G2), and plate. The screen grid is wound similarly to the control grid — a helix of fine wire on support rods — but with coarser pitch (wider spacing between turns). It is positioned between G1 and the plate, approximately halfway between them.

The screen grid’s coarser pitch serves a dual purpose: it allows most electrons to pass through to the plate (while collecting 10-20% of them), and it provides electrostatic shielding of G1 from the plate. The shielding works because the screen is connected to a fixed DC voltage (through a bypass capacitor), creating a region of stable potential that electrons must traverse. The plate voltage variations cannot “reach through” this stable region to influence the electrons near G1.

The bypass capacitor from screen to cathode (or ground) is essential. It must provide a low impedance path to AC ground at all frequencies of interest. Without it, the screen voltage varies with the signal, partially un-doing the screening effect. For audio frequencies, a 0.1µF capacitor is adequate. For RF frequencies, use multiple capacitors in parallel (0.01µF ceramic in parallel with a larger value) to ensure low impedance across the entire frequency range.

Characteristic Curves and the Kink

The tetrode’s characteristic curves (plate current vs. plate voltage for different grid voltages) reveal a distinctive problem: the “kink” or “negative resistance region.” At low plate voltages (below the screen voltage), the curves dip down before rising — a region where increasing plate voltage actually decreases plate current. This kink makes the tetrode unsuitable for audio amplification and limits output voltage swing in RF circuits.

The kink arises from secondary emission. When electrons hit the plate at high velocity, they knock out secondary electrons — electrons ejected from the plate by impact. In a triode, these secondary electrons are immediately recaptured by the same plate, since there is no other positive electrode nearby. But in a tetrode, when the plate voltage drops below the screen voltage, secondary electrons see a positive screen grid and are attracted to it instead of returning to the plate.

This secondary emission current flows from plate to screen (electrons leaving the plate, arriving at the screen). The net plate current = primary current − secondary emission current. When secondary emission is large relative to primary current (low plate voltages, high plate voltage swing), the net current decreases as plate voltage decreases — the negative resistance characteristic.

The kink limits the usable output voltage swing. The plate cannot be swung below the screen voltage without entering the negative resistance region. For a circuit with 100V screen voltage, the plate must stay above 100V throughout its swing. This limits the negative peak of the output to roughly 100V below the B+ supply, reducing the maximum output power compared to a pentode (where the plate can swing much lower).

Practical Uses and Limitations

Despite the secondary emission problem, tetrodes found widespread use in receiver circuits where small signal swings kept the operating point above the kink region. In superheterodyne IF amplifiers operating at signal levels of millivolts, the plate never swings near the kink region and the tetrode performs excellently — providing high gain with excellent stability.

Tetrodes were also used in RF power amplifiers where the tuned output circuit provides a stable, high-impedance load that maintains the plate well above the screen voltage throughout its swing. The tank circuit’s reactance filters the distortion from the kink region, and the efficient Class C operation makes the secondary emission issue relatively minor.

The screen grid dissipation must be managed carefully. Secondary electrons arriving at the screen (during the kink condition) add to the normal primary electron collection, increasing screen current beyond the designed level. Screen dissipation ratings must not be exceeded: typically 1-3W for small tetrodes, with higher ratings for transmitting types. Operating a tetrode with the plate voltage significantly below the screen voltage for extended periods risks overheating the screen grid.

Modern “super tetrodes” and beam tetrodes (the beam power tube family) address the secondary emission problem through geometry rather than adding a third grid. The beam-forming plates direct electrons into tight beams that create a virtual cathode in the plate-screen space, repelling secondary electrons back to the plate even at plate voltages well below the screen voltage.

Neutralization Circuits

For circuits that use triodes or tetrodes in RF amplifiers where the residual plate-to-grid capacitance still causes instability, neutralization adds an external feedback path that exactly cancels the internal capacitance.

A small capacitor (the neutralizing capacitor) connects from the plate to the grid through an inverting network (typically a tap on the tank circuit tuned to provide 180° phase shift). When adjusted correctly, the signal fed back through the neutralizing capacitor exactly cancels the signal fed back through the plate-to-grid capacitance. The result is zero net feedback from plate to grid.

Neutralization was the standard technique for triode RF amplifiers before the tetrode made it unnecessary. With tetrodes, neutralization is only needed for the highest-gain RF stages operating above 30 MHz, where even the tetrode’s reduced plate-to-grid capacitance becomes significant.

Adjusting neutralization: with the B+ disconnected (no plate voltage), apply RF signal to the input and adjust the neutralizing capacitor for minimum signal in the output tank circuit. At perfect neutralization, the output tank receives zero signal from the input because the internal and external feedback paths cancel exactly. Reconnect B+ and verify the stage amplifies without oscillation. Re-check neutralization if tubes are replaced or the tank circuit is retuned.