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Another point to note is that the diagrams tend to be simplified - where 'filament' is used, it may also mean 'cathode'. It is important to remember that the earliest valves only worked from DC sources and it was the development of the electrically insulated heater instead of a plain filament that allowed AC capable valves.

The DIODE The diode in schematic form

This is the simplest type of valve, having just two electrodes – anode and cathode (filament in the case of battery valves, as shown in the diagram). Schematic showing diode rectifier valve (tube) action

The electrodes are enclosed within an evacuated envelope – bulb – usually of glass, the connections to the electrodes passing through this envelope via airtight seals.

The hot filament or cathode generates an invisible cloud of electrons in the space around it. A positive potential on the anode attracts these and a current flows from cathode to anode. A hard vacuum is created within the envelope in order to allow free movement of the electrons as they pass from cathode (filament) to anode and also to prevent destruction by oxidation of the heating elements.

The DIODE as a RECTIFIER

Under no conditions can current flow from ANODE to CATHODE in any diode. The device is a ‘one-way VALVE’.

Increasing the positive potential will increase the flow of electrons from cathode to anode but if the anode is made negative, all current flow will cease. You can see from this that the positive-going section of the AC sine-wave will cause current flow, but the negative-going half (shown as a broken line) will stop all current flow. As current only flows in the one direction, the result is a pulsing but direct current output. The addition of a reservoir capacitor across the output helps ‘fill in’ the gaps between the pulses by charging on the pulses and discharging in the gaps between them. This is improved further by either a choke or a resistor in series with an additional capacitor, called the ‘smoothing’ capacitor. The choke/resistor-capacitor circuit forms a ’time-constant’ that filters even more of the residual AC ripple. Choke is best, having a low resistance at DC, unlike the resistor which tends to waste power, but the resistor is often used because it is cheaper.

The TRIODE

Triode valves (tubes) shown in schematic form Diagram showing how vacuum tube auto-bias works The triode is a two electrode valve with a third electrode, called the grid, placed between the anode and cathode. The grid is usually a mesh or spiral of fine wire extending the full length of the cathode. The spaces between the wire spirals are quite large in order not to impair the passage of electrons from cathode to anode. The grid is used to control the flow of current through the valve. This action controls the anode current. By maintaining the grid at a negative potential, it will tend to repel electrons (like forces repel). The less negative, the less repulsion and the greater the flow of electrons. The more negative, the more repulsion and the smaller the flow of electrons. It is important to note that the grid is assumed to be always negative WITH RESPECT TO THE CATHODE. It cannot therefore collect electrons and small changes in potential at the grid can cause large changes in current flow through the valve.

This diagram shows the typical construction of a triode electrode assembly. Note that the filament in this illustration is directly supplying electrons, being heated by the ‘A’ battery, the grid has a negative potential supplied by the ‘C’ battery and the anode has an HT potential supplied by the ‘B’ battery.

These battery terms were commonly used in the USA. In GB the ‘A’ battery is called the LT (low tension) battery, the ‘C’ battery the grid-bias battery and the ‘B’ battery the HT (high tension) battery. Showing the construction of a triode valve (tube)

The variable LT resistor ‘R’ sets the operating voltage, therefore the temperature, of the filament. Although in early days such regulating rheostats were used, they were soon outmoded as advancements were made in valve manufacturing technology.

Interelectrode capacity effects limited the gain of the triode at higher frequencies.

Automatic biasing

Grid-bias batteries were inconvenient even in battery sets, and unsuitable for use in mains powered ones. Automatic biasing was developed to get around this problem. In the triode, the cathode will be slightly positive due to the current flow through the resistor R2. The capacitor C1 keeps the potential steady. The grid is therefore negative with respect to the cathode. The value of the resistor R2 needs to be chosen with care if the valve is to be correctly biased. Resistor R1 removes any charge caused by the supply signal positive-going swings causing the grid to act as a second anode, creating grid current. Resistor R3 is the load resistor across which the signal is developed that mirrors the input signal as an inverted signal with much greater voltage swings.