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). 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
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 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.
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.
The diagram centre right shows the elements of
a triode. Note that the 'cathode' in this illustration is actually
the filament, directly
supplying electrons, being heated by a cell or a battery.The grid has a
negative potential supplied by the grid bias battery and the anode has an
HT potential supplied by the HT battery.
Battery terms commonly used in the USA differ
from those in the UK. In America the LT battery is called the ‘A’
grid-bias battery is the
‘C’ battery and the
HT (high tension) battery is the
‘B’ battery .
The variable bias resistor sets the operating
point of the valve.
at bottom right shows a basic triode amplifier stage. Signal is
applied across A-B, causing small signal variations at the grid
which control the electron flow through the valve from cathode to
anode. Variations in current at the anode are developed into voltage
changes across Ra. The amplified signal appears at X.
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.