Valve (Tube-Based) Amplifi ers 347evolution from the cathode, or any other electrodes that may become hot in use. The
solution to this problem is the inclusion of a small container, known as a boat, mounted
somewhere within the envelope, but facing away from the valve electrodes, which
contains a small quantity of reactive material, such as metallic calcium and magnesium.
The boat is positioned so that after the pumping out of the envelope has been completed,
and the valve had been sealed off, the getter could be caused to evaporate on to the inner
face of the envelope by heating the boat with an induction heating coil. Care is taken to
ensure that as little as possible of the getter material fi nds its way on to the inner faces of
the valve electrodes, where it may cause secondary emission, or on to the mica spacers,
where it may cause leakage currents between the electrodes.
While this technique is reasonably effective in cleaning up the gas traces that arise during
use of the valve, the vacuum is never absolute, and evidence of the residual gas can
sometimes be seen as a faint, deep blue glow in the space within the anode envelope of
a power output valve. If, however, there is a crack in the glass envelope, or some other
cause of signifi cant air leakage into the valve interior, this will become apparent because
of a whitening of the edges of the normally dark, mirror-like surface of the getter deposit
on the inside of the valve envelope. A further sign of the ingress of air into the valve
envelope is the presence of a pinkish-violet glow that extends beyond the confi nes of the
anode box. By this time the valve must be removed and discarded to prevent damage to
other circuit components through an increasing and uncontrolled current fl ow.
11.1.8 Cathode and Heater Ratings
For optimum performance, the cathode temperature should be maintained, when in use,
at its optimum value, which requires that the heater or fi lament voltages should be set
at the correct levels. Since the voltage of the domestic AC power supply is not constant,
the design ratings for the heater or fi lament supply must take account of this. However,
this is not as diffi cult to do as it might appear. For example, Brimar, a well-known valve
manufacturer, makes the following recommendations in their Valve and Teletube Manual :
“ the heater supply voltages should be within 5% of the rated value when the heater
transformer is fed with its nominal input voltage, provided that the mains power supply is
within 10% of its declared value. ”
An additional requirement is that, because of inevitable cathode-heater leakage currents,
the voltages between these electrodes should be kept as low as possible and should not