Grid resonance
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Grid resonance |
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The single
largest cause of VHF
instability in
amateur power tubes
is grid-to-anode
feedback. This even
occurs in grounded
grid amplifiers.
Control grid
construction, lead
lengths inside the
tube, sockets, and
socket wiring to chassis ground dramatically affect grid
impedance at higher
frequencies.
One way to
measure
effectiveness of
grid grounding is
measuring S21 with
the grid grounded.
By applying a
leveled source that
sweeps frequency at
the cathode and
placing a sensitive
detector at the
anode, an isolation
vs. frequency
isolation level can
be plotted. This
plot will show the
frequencies where
the grid is not
effectively
grounded.
Example 1,
3-500Z E F Johnson
Socket
Directly grounded
to chassis with
shortest possible
connection.
Notice low
impedance dip at
95MHz, a peak at 121
MHz, and a
reasonably broad
increase in grid
impedance all the
way up to 404 MHz.
Test have shown this
tube/socket
combination without
suitable parasitic
suppression can
oscillate anywhere
from 110 to 250MHz,
depending on anode
impedance and exact
layout.
3-500Z Ameritron
Socket
The Ameritron
socket uses a
ground plane to
connect all grid
pins. It has short
wide socket
connections. Here is
the same tube with
only a socket
change.
The grid is most
ineffective
(“parallel
resonant”) at 202
MHz. There is a
series-resonance
effect or low
impedance at 116
MHz. Tests
prove this socket
and tube
combination, without
any anode parasitic
suppressor,
oscillates at about
200 MHz in the AL-80A
or AL80B amplifier.
3-500Z Johnson
Socket with three
500pF grid
capacitors
This arrangement
is typical of
amplifiers like the
SB-220 and Kenwood
TL-922 in that the
grid is not directly
grounded. The grid is
grounded through
500pF capacitors
with one inch of
total lead length.
Notice the large
increase in VHF feed
through, indicating
the grid is no
longer effectively
grounded at
frequencies above
50MHz.
A suppressor with
many more turns is
required to
stabilizing an
amplifier with this
grid arrangement.
The grid is very
well grounded at 30
MHz.
3-500Z and
socket with long
grid leads
I attempted to
use a grid dip meter
to measure grid
resonance. This In
this only case where
my GDO meter would dip.
Again this is a
poor grid connection
arrangement, and the
only one where I saw
a reliable dip at
the socket grid
pins. In this case
the amplifier could
oscillate somewhere
between 100 and
250MHz, depending on
the anode system.
3-500Z with
three 33pF
capacitors
Reducing
capacitor size to
33pF looks like
this:
This creates a
dip at 85MHz but at
the cost of much
poorer performance
across the rest of
the spectrum. This
is not a good
arrangement.
Of all these
configurations the
best is a direct
connection from grid
to ground with the
shortest possible
leads. This will
make the least
troublesome
amplifier.
811A Tube with
direct grid ground
Here is the same
test procedure with
an 811A tube.
We can see the
811A has much higher
feedthrough than a
3-500Z. The grid is
poorly grounded
through a long
internal grid lead.
The poor
grid-to-ground
connection makes the
811A tube much more
difficult to
stabilize. An 811A
with long anode lead
length tends to
oscillate on 50-60
MHz. The 811A, with
four tubes in
parallel, is also
unstable at HF
without
neutralization.
811A with grid
through 500pF
capacitor
Here is an 811A
with the grid
grounded through a
500pF capacitor.
The 500pF
capacitor doesn’t
make a great deal of
difference because
the grid is already
poorly grounded.
Also see HF stability
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