In the Radio Engineers Handbook, 1st edition by Terman, on page 316,
sub-section 13 is listed; "Effect of Gas Upon
Tube Characteristics" which is quoted below.
"Very small traces of gas in vacuum tubes affect the characteristics adversely
in a number of ways as a result of the
positive ions produced in the tube by collision between the gas molocules and
the electrons flowing to the anode.
The positive ions travel in the opposite direction from the electrons, and
normally end their existence by falling into
the cathode or negative control grid. Electrons that bombard the cathode tend
to destroy the emission of thoriated-
tungsten and oxide coated cathodes. Positive ions collected by the negative
grid result in grid current, which limits
the resistance that may be inserted in series with a negative grid, and which
also introduces noise.
Positive-ion currents to the grid limit the d-c resistance that may safely be
placed in series with the control grid and the cathode, because the voltage
drop that such a grid current produces across the resistance has a polarity
that makes the
grid less negative than would otherwise be the case. Thus if the tube begins to
liberate gas, with resulting positive-ion
grid current, the grid becomes less negative, thereby increasing the space
current. This increases the number of positive
ions produced, and will cause additional grid current, and still greater
reduction in the negative grid potential. If the
resistance in the grid circuit is high enough, this process can become
cumulative, and in some types of tubes can easily
result in the destruction of the tube as a result of excessive plate current
caused from loss of grid bias. The maximum
resistance that is permissable to place in series with the grid electrode
depends upon the grid characteristics and the
method of obtaining bias. It is of the order of several megohms in small tubes
used for voltage amplification at audio and
radio frequencies. With small power tubes, such as the output tubes of radio
receivers and public address systems, the
allowable grid resistance is much less, particularly if a fixed bias is
employed in the grid-cathode circuit, since self-bias
provides an automatic protection against excessive increase in plate current".
End Quote.
Best,
Will
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