Tube Internal Construction
Tube Construction
Cathode – this is the ‘power house’ of the tube as it is coated with a material which, when heated, releases electrons. The entire operation of the tube relies on controlling their movement (hence the term ‘Thermionic’).
Heater – this is an electrically heated filament which provides thermal stimulus to the cathode.
Grid – depending on the type of tube, their may be one or more grids present, each with a specific function, however all audio tubes contain at least one control grid. The control grid is made negatively charged with respect to the cathode in order to restrict or prevent the flow of electrons in the valve.
Anode – this electrode is made positive with respect to the cathode (usually by a few hundred volts), and serves to collect the electrons which are allowed to pass through the grid.
Vacuum – audio grade tubes will only operate satisfactorily if there are virtually no air or gas molecules in the envelope. To achieve this, the glass envelope is evacuated to a very high degree during manufacture, which is known as a ‘hard’ vacuum. If small amounts of gas are present, their molecules will ‘ionise’ due to the high voltages present and can give rise to electrical noise or overheating in worse cases. Under these conditions, the valve is said to have ‘gone soft’. Each valve contains a small amount of a reactive metal such as barium (known as the ‘getter’), which is burnt inside the envelope after evacuation in order to ‘mop-up’ the last remaining gas molecules. If the envelope is cracked or broken, air will enter and cause the heater to oxidise and ‘burn out’; it will also disrupt the electron flow and the tube will cease to conduct current. It will be seen from the description of the control grid that if an alternating potential audio signal is applied to the grid, it will modulate the flow of electrons to the anode. Electrical current is defined as electron flow; therefore the signal applied to the grid will produce a modulated current in the anode circuit. Under ideal conditions, this current will be an exact replica of the input signal, but of much greater amplitude. Power valves are optimised to produce large current swings at the anode for relatively small control grid signals. This large output current is used to drive a loudspeaker via a matching transformer.