04 Tube Operation.pdf - Kambing UI

3.13 STABILIZING THE AMPLIFIER
3.1 3. 1 Testing for parasitic oscillations
In the case of an r-f amplifier, it will be necessary to investigate not
only the possibility of self-oscillation, but also lack of feedback on
the fundamental frequency. The basic steps of checking for self
oscillation are threefold:
(a) The amplifier should be operated without r-f excitation
and without fixed grid bias, with light loading and with low
voltages applied to the anode and to the screen of a tetrode or
pentode.
The voltage should be high enough to develop full anode
dissipation. For this test, grid leak bias should be used. If the
screen and anode voltage supplied cannot be adjusted directly
to low voltages, suitable series resistance should be used,
either in series with the rectifier output or transformer primary
so that the voltages developed at the tube will be low.
Light bulbs of the correct size will serve as resistors in series
with the primary of the rectifier transformers. The r-f circuits
should be tuned off-resonance to see if self-oscillation of the
amplifier can be started. The indication of any current by the
grid milliammeter means that self-oscillation is present.
(b) The frequency of the self-oscillation must be determined.
A simple neon bulb will indicate whether the frequency of
oscillation is high or low. The lower the frequency the more
orange will be the glow. A purple color indicates a VHF or UHF
oscillation. With a little experience, it will become possible to
guess the approximate frequency very closely. A wavemeter or
an oscilloscope will then determine the exact frequency.
(c) After the frequency of oscillation is measured, it remains to
identify the circuit supporting the oscillation. The circuit must
then be altered without disturbing the performance of the
amplifier on the normal frequency of the amplifier.
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