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ALL ABOUT VALVES: TETRODE, PENTODE     

     WOOD  BAKELITE  CHASSIS  ETHICS  DIODE, TRIODE  TETRODE, PENTODE  STANDARDS PHOTOGUIDE  VIDEOS

PILOT  BUSH  EKCO  COSSOR  PHILCO  BAKELITE HISTORY VALVES DIODE, TRIODE  TETRODE, PENTODE  STANDARDS     

 

The TETRODE

Internal 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 frequency range.

NOTES ON PENTODES

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 is 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 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.

NOTES ON BEAM TETRODES

The 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.

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.

 

 

 

 

 

 

 

 

Number of Electrodes

Classification

2

diode

3

triode

4

tetrode

5

pentode

6

hexode

7

heptode

8

octode

 

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