Lighthouse Electric Tutorials

Vacuum Tube Theory

Part 3.
Development of the Tetrode and Pentode Tubes.

In a triode, the three elements, anode, grid and cathode are made of metal and separated by a dielectric,
the vacuum. A capacitance between electrodes, called Interelectrode Capacitance
is established. The largest capacitance value usually occurs between the plate and cathode
because of the large area and distance involved. Smaller capacitances are formed between cathode-grid
and grid-anode. The plate-cathode capacitance has the greatest effect on the tube operation. At higher
frequencies this capacitance will couple part of the AC signal from the plate back to the grid.
Such reaction or Feedback, affects the gain of the stage and under certain load conditions causes
the amplifier to become unstable. To countermeasure this occurence a forth element was installed.
A second grid, the Screen Grid was placed between the control grid and the plate
and the Tetrode tube was invented.

The screen grid is splitting the big cathode-anode capacitance into two small series
capacitances. Connecting capacitors in series reduces the total capacitance to a value
smaller than either of the two. The interelectrode capacitance was greatly reduced.
To keep conduction of the tube, the screen grid must be operated at a positive potential.
However, if operated at plate potential it would draw current from the plate current stream.
Therefore the screen is operated at a lower potential than the plate. For adequate shielding
of the control grid, the screen potential should have no AC or signal component. When
current in the tube increases, the screen grid will draw current too and go toward negative
potential. A negative-going electrode between the control grid and the plate will decrease the
current flow (gain). To overcome this undesired effect, a bypass capacitor is connected between
the screen grid and ground, maintaining a steady DC potential. The positively charged
grid will accelerate electrons across the tube even during negative going plate potential.
The screen grid has the ability to isolate the effect of plate AC variations on tube conduction - anode
current is nearly independent of anode voltage.
Since a relatively large change of plate potential causes very little change in plate current,
the internal PLATE RESISTANCE is much larger than that of a triode. For the same reason
the Amplification Factor is greater for a tetrode than for a triode. This is a big advatage of screen grid tubes.

Tetrodes found little acceptance however, because of a very undesirable effect,
theSecondary Emission. Electrons traveling from the cathode to the plate are accelerated
to such an extend , that they dislodge electrons from the plate's surface. Dislodged electrons
form a cloud between the screen grid and the plate. When the signal at the control grid is going positive, the plate
potential will drop and the secondary electrons will be attracted to the screen grid. Tube conduction (Gain)
will decrease due to lost current drawn by the screen grid. This effect is shown below
Secondary Emission

Fig.3-1 Plate voltage-plate current characteristic of a tetrode for fixed control and screen grid voltages.

Below A the electrons velocity is low and will not dislodge secondary electrons. Secondary emision happens
between A and C (dip in plate current). Above D, plate potential is high enough to atract all
electrons and this is the only useful  operating range for amplifiers. Because of this, tetrodes
have very limited applications.

The Pentode The negative effect of the secondary emission has been reduced by insertion of a third grid, the Suppressor, between plate and screen grid. The suppressor grid is usually internally connected to the cathode and is made with wider mesh spacing then the other two. Because of it's location between two positive electrodes (screen grid and plate), it has little effect on plate current. Only secondary electrons are affected by the suppressor grid and repelled back toward the plate. Tubes of this type are known as pentodes.

Power Pentodes and Beam Power Tubes For power amplification, a special type of tubes was developed, the power pentode and beam power tube. The power pentode, besides having a large cathode and plate area, has a special in-line grid alignment which poses little obstacles for the electron stream. The tube has the advantage of high power output and high power sensivity. Beam Power Tube is a special type of pentode with beam-forming plates in place of the suppressor grid. The beam forming plates and grid support structures are connected to the cathode. Electrons that are emited from the sides of the of the cathode are repelled away from the grid supports and forced into the main electron stream by the negative charge on the beam-forming plates. Any electrons emitted by secondary emission are also repelled back to the plate. This increases the total current flowing to the plate. For this reason, power pentodes and beam-power tubes are most suitable for power amplifiers.

Next: Tube Characteristics and Operation

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