valves

The TETRODE

The Pentode

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 duplexed, with other valve elements. The diagram above shows one typical combination valve, a 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 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 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 higher-priced end of the market.

NOTES ON BEAM TETRODES

 The action of the beam tetrode 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.

The diagram on the right shows the construction of a beam tetrode output valve and the action of the beam plates in focusing the electron beam onto the anode (plate).

Valve Types

 The 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 types, etc.

In 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.

A 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.