The Tetrode
The tetrode was
developed in response to the need for valves that could operate at higher
frequencies than the early triodes. In the US Bureau of standards in 1919 a
mathematical analysis of the triode revealed the detailed nature of the
inter-electrode capacitance problem that caused a falloff in performance as
frequency of operation was raised.
Armed with this
knowledge various solutions were tried. Wide separation of the connecting
leads gave rise to the Marconi-Osram valves Q
and V24 that were effective at up to 2
MHz.
Some bi-grid vales had
been produced. Either a space charge grid of close wound fine wire was placed
between the filament and the main grid to reduce the space charge and enable
lower anode voltages to be used or two open concentric open helix grids were
used one to amplify RF and the other to amplify AF so trying to make best use
of the single electrode stream. See Early
Bi-Grid Valves K4 and Q
The eventual solution
was a development of the work carried out in Germany in 1919 by Walter
Schottky working for the Siemens and Halske Company and trying to develop a
higher gain type of valve. His solution was to put a protective net between
the grid and the anode and to bias it with a positive voltage less than the
anode voltage. His valve did not itself solve the problem as the net was not
a complete screen and the valve did not improve high frequency performance.
In 1926 in a research
lab in the General Electric Company in the USA produced two types of
experimental valves that tried to screen the anode and control grids and
achieved a very low inter-electrode capacitance. The first successful
screened grid valve produced in the United Kingdom was the Marconi-Osram S625 in 1927. The design, by Round, of the
S625 was radically different to the German work. A metal cylinder enclosed
the anode with metal gauze over the end of the cylinder pointing to the grid.
The double-ended S625 was both technically successful and commercially
expensive to employ because of its two base caps. The first single ended UK
screened tetrode was the S215 the fifth
connection was made by taking the anode to the top cap.
When this screen is maintained at a steady positive voltage, it is found that the amplification factor
of the valve, as compared with the triode, is very much higher, the impedance
is also greatly increased.
The reason for this increased amplification lies in the fact that the anode
current in the tetrode valve is far less dependent on the anode voltage than it
is in the triode. In any amplifier circuit, of course, the voltage on the anode
must be expected to vary since the varying anode current produces a varying
voltage-drop across the load in the anode circuit. A triode amplifier suffers
from the disadvantage that when, for instance, the anode current begins to
rise due to a positive half-cycle of grid voltage swing, the anode voltage falls
(by an amount equal to the voltage developed across the load) and the effect
of the reduction in anode voltage is to diminish the amount by which the
anode current would otherwise increase. Conversely, when the grid voltage
swings negatively the anode current falls and the anode voltage rises.
Because of this increased anode voltage the anode current is not so low as it
would have been if it were independent of anode voltage. This means that the
full amplification of the triode cannot be achieved. The introduction of the
screen grid, however, almost entirely eliminates the effect of the anode
voltage on the anode current, and the amplification obtainable is thus much
greater.
A screen is found to function best when its voltage is below the mean value of
the anode voltage. Most of the electrons from the cathode are thereby
accelerated towards the anode, but some of them are unavoidably caught by
the screen. The resulting screen current serves no useful purpose, and if it
becomes excessive it may cause overheating of the screen. The total cathode
current is equal to the sum of the screen and anode currents.
Another important effect occurs when a screen grid is introduced into a triode:
provided that the screen is kept at a constant voltage (not varying with the
signal) it reduces the capacitive coupling between the control grid and the
anode and therefore helps to eliminate unwanted feedback in amplifier circuits
especially at radio frequency. To take full advantage of this feature the screen
grid is made with a finer pitch or smaller mesh size than would be necessary
merely to obtain greater amplification, and auxiliary electrostatic shields are
built into the structure in an attempt to reduce the grid-to-anode capacitance
to the lowest practicable value. If the size of the apertures in the screen is
made too small the electron flow to the anode will be seriously impeded, but
with a reasonable compromise the residual capacitance between control grid
and anode can be made 1,000 times smaller than in a triode. The improved
stability in a radio frequency amplifier depends on the constancy of the screen
voltage, and it is for this reason that thorough capacitive by-passing of the
screen to earth (ie decoupling) is so important.
For radio frequency
amplification with a resonant anode load the screen tetrode was effective,
but for audio use the tetrode suffers from the disadvantage that, when the anode voltage swing
is so great that on downward peaks it falls below the screen voltage, there is a
flow of secondary electrons from the anode to the screen, the effect of this
secondary emission is to cause a drop in anode current and a rise in screen
current, which in ordinary amplifier circuits results in serious distortion and a
reduction in useful power output. The search began for ways of keeping the benefits of the tetrode
but to eliminate the kink.
There is another type of tetrode, known as a space-charge grid tetrode, in
which a positively charged grid is interposed between the control grid and the
cathode. The purpose of this positive grid is to overcome the limiting effect of
the negative space charge and thus enable the valve to operate efficiently
with very low anode voltage (for example, a 12 volt supply as used for mobile
equipment).
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