capacitance limits the range of frequencies that the triode can amplify. Simply
put, the higher the frequency, the less the gain of the valve. This is due to
the internal capacitance of the electrode structure. The tetrode has an
additional grid called the screen. This reduces internal capacitance and
improves frequency response compared to the triode. The grid usually has a
bypass capacitor (not shown on the diagram) which further reduces
inter-electrode capacitance and allows still greater amplification and a wider
The pentode has a further electrode, called the
suppressor grid, placed between the screen grid and the anode. This grid is
connected to the cathode and is therefore at cathode potential. In all valves
but especially the tetrode, due to its higher gain, secondary emission takes
place when electrons bombarding the anode dislodge other electrons. These
‘wandering’ electrons cause no trouble in diodes and triodes as there is no
positively charged screen nearby to attract them and they return to the anode.
In the tetrode the collection of these electrons lowers the current and
therefore the gain of the valve, limiting the valve performance. The suppressor
grid repels these relatively low-speed electrons back to the anode without
obstructing the normal flow of electrons. The pentode, therefore, can provide
considerably higher gain than the tetrode.
Many pentode valves are in multiple format (duplexed
is the American term) i.e. the
functions of more than one valve are enclosed within the same envelope. One typical combination valve
double-diode pentode. The diodes are commonly used for signal detection and AGC
(automatic gain control) and the pentode for power output or AF amplification.
Superhet designs using this type of power output triode are sometimes called
'short' superhets, as effectively, one separate stage of valve amplification is saved.
This made the sets less expensive to make - and perhaps to purchase (but not
always, as manufacturers sometimes put more money into the overall quality).
Sets using the duplex diode/pentode valve as an AF amplifying stage feeding the output valve
stage are classed as 'long' superhets and often (but again, not always) formed
the designs at the mid-to-higher-priced end of the market.
ON BEAM TETRODES
action of the beam tetrode audio output valve is similar to the pentode but instead of
a suppressor grid, beam-forming plates concentrate the electron flow and
increase the total power output.
main purposes for which valves are used in domestic broadcast receivers may be
classified roughly as: voltage amplifiers (RF, IF and AF); frequency changers;
demodulators; AF power amplifiers; power rectifiers; and tuning indicators.
Within these groups further subdivision is possible between those with
variable-mu or straight-line characteristics, directly and indirectly heated
practice, however, valve assemblies are additionally classified according to the
number of electrodes they contain, the heater of an indirectly heated valve
being omitted from that number.
single, double or multiple valve assembly may be contained within one envelope,
for example a double diode triode, a triode pentode, a double triode etc.