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Also see
VHF instability
Neutralization
HF instability, which is instability at or near the operating frequency of the amplifier,
is the most damaging type of problem. This is because the primary energy storage
system, the tank circuit, is resonant with high Q at or near the operating
frequency.
If we look at network analyzer
swept measurements of a tank circuit, we find the maximum voltage
at any point in the tank circuit occurs at or near the operating frequency.
Voltages at VHF are very low, because impedances are, by definition of it being
a low-pass network, require high series impedances and low shunt impedances.
Some amplifiers have designed-in HF stability problems. The Yaesu
FL2100 is one example of an amplifier with poor RF design. Un-neutralized
amplifiers using tubes with high feedthrough capacitance are another source of
problems. Any amplifier with high feedthrough (or feedback) capacitance is a
candidate for HF instability if the load is removed while the amplifier tube or
tubes are drawing quiescent current. Dentron amplifiers with four 811A and four
572B tubes, as well as the Collins 30L1 with four 811A’s, are examples of
relatively unstable amplifiers (along with the FL2100).
These are all
amplifiers with low
control-grid parallel-resonant
frequencies. For
example here is plot of tube feed-though:
The lower the
line the better the
isolation and the
better grounded the
grid is.
Marker 2 at 116
MHz is the VHF peak
in anode-cathode
isolation. This is
where the tube is
most stable and
least likely to
oscillate. This
socket has an
exceptionally low
grid inductance.
Grid pins connect
directly to a foil
groundplane.
Marker 4 at 202
MHz is a minimum in
isolation. Isolation
is only 10 dB! This
is the most common
frequency range for
a 3-500Z to
oscillate at.
Note: The peak
at 410 MHz is an
artifact of the
coupling system to
the anode and
cathode. 350 MHz is
the highest usable
frequency of the
test fixture.
Look at the
exceptional HF
isolation of a
3-500Z with this
socket. Feedthrough
is in the order of
-80db to
-90dB. This is why a
3-500Z does not need
neutralized.
In contrast, here
is the 811A tube:
The 811A, even
with a directly
grounded grid, has
only -22
dB isolation from
input to output at
84 MHz. A
single tube is open
for oscillation at
any frequency above
50 MHz.
Isolation below
30 MHz is only -40dB
for a single tube.
As tubes are
parallel isolation
decreases. This is
why four parallel
tubes need
neutralized at HF.
The loss going
less than zero dB is
caused by impedance
transformation
(transmission line
effect) in the test
fixture and tube.
The peak at 4 is
from the same effect
noticed in the
3-500Z trace. The
peak at marker 4 is
a test fixture
artifact.
572B’s have very
similar isolation.
The following is
a 3-500Z socket and
tube in a Heathkit
SB-220 amplifier.
Note the change in
HF isolation caused
by floating the
grids. Nothing gets
better in the 3-500Z
when the grids are
grounded through
capacitors. As a
matter of fact
everything gets much
worse!
The cure for HF
instability is to remove intentional undesired feedback.
Ground the grids
directly. If you
have a particularly
poor tube design,
you will have to
neutralize the PA!
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