A-Grounded-Grid-2-Kilowatt-PEP-Amplifier-LAMPOWO.pdf

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Two rugged Eimac 3-5002 high-mu triodes
are featured in Henry Radio's new 2K-3
linear amplifier. Henry designed the ampli-
fier around versatile Eimac power tubes
because these popular triodes are ideal
for grounded-grid operation at the
2
kW
PEP SSB input level, and at the
1 kW
rs of this new Henry rig
Eimac's line of power tubes for
advanced transmitters, write Eimac
Amateur Services Department or
contact your nearest
01
v.lrban
a grounded-grid
two-kilowatt pep
amplifier
This "full-house" linear
uses a pair
of 3-500Z's
in a highly
efficient circuit-
W6SAI
designs
it
for you
and
explains its operation
6
a
february
1969
The popularity
of
the
grounded-grid ampli-
fier is well established. This interesting cir-
cuit combines high efficiency, good linearity,
and minimum expense-three attributes not
usually found together in one amplifier.
A
quick on-the-air check reveals that an amaz-
ingly large number of amateurs are using
grounded-grid amplifiers and, judging from
the robust signals, are having much success
with them.
the grounded grid
To the uninitiated the grounded-grid or
cathode-driven circuit
i s
puzzling. A sim-
plified sketch of this amplifier is shown in
fig.
1.
The driving signal is applied to the
filament, or cathode, and the grid is at
ground potential, thus acting as a shield be-
tween input and output. When the cathode
is driven negative, the grid
i s
positive with
respect to the cathode, and plate current
flows. The driving voltage is thus added to
the plate voltage, and the resultant increase
in power input to the stage appears as feed-
through power in the plate circuit. The plate
circuit is conventional, being either parallel-
tuned or the more popular p i or pi-L
network.
At first glance i t might appear that the
grid receives no exciting signal. The driving
voltage is developed
between the grid and
cathode
of the tube, and i t matters little
which of these two elements is at ground
potential. That is to say, the grounded-grid
amplifier may be thought of as a grid-driven
amplifier "standing on its head." This is not
strictly correct, as other factors such as feed-
through power and grid-lead inductance en-
ter the picture. However, if you understand
the conventional grid-driven amplifier, i o u
can understand the grounded-grid amplifier.
The fact that the grid
i s
at ground potential
should cause you no distress.
If a well-shielded triode having a reason-
ably high amplification factor is used i n the
grounded-grid amplifier, neutralizing circuits
and bulky bias supplies can be eliminated,
reducing the amplifier stage to the basic es-
sentials needed to make it function properly.
The new Eimac 3-5002, and its older com-
panion the 3-4002, are ideally suited to this
class of service. They combine the charac-
teristics mentioned above with modest cost,
providing more watts output per dollar.
Of
interest to the user is that the drive
requirements of a grounded-grid, 2-kilowatt
PEP
amplifier using a pair of these tubes is
of the order of the output power level of
typical amateur exciters and transceivers;
that is, in the 100-watt
PEP
region. Bulky and
expensive power absorbers between amplifi-
er and exciter aren't required. Best of all,
a portion of the drive power appears as out-
put in the grounded-grid amplifier. This al-
most repeals the ancient law that says "You
can't get something for nothing." In this
case, it is nearly true!
A
pair of 3-500Z or 3-4002 power triodes
will run comfortably at
2
kilowatts input
5
3a
8
.-
$
E
[-ioTyT$
call
L.
.....---
3
.=
8+
T W O
CATHODE
ClRWT
W E D
PLATE
CIRCUIT
a
=
J
E
65
mm
4
v;
fig.
1.
The basic grounded-grid amplifier. The driv-
ing voltage, ec, is impressed between cathode and
grid. The grounded-grid circuit isolates input from
output and affords good shielding up to
100
MHz
or
so
in
tubes of the 3-5002 elass.
E
,-
2
>
8
3
6
.q
-
fu
--
u
m
E
E
0
3
m
PEP
(1000 watts average power) under con-
tinuous ratings in
CW,
ssb or RTTY service
without distress. "Fireman-quick" tune-up
style, wherein the equipment must be
tuned up in
30
seconds or less else the
whole works goes up in a puff of smoke,
is not necessary. This fire drill may have an
element of excitement about it, but it can
result in tube casualties when done in a
february 1969
Q
7
hurry, or under pressure, as i n the closing
moments of a
DX
contest,
Adequate
plate
dissipation, on the other hand, is insurance
against catastrophe from operator panic Or
error and provides a comfortable margin of
safety that leads to equipment reliability and
lower operator blood pressure.
fig. 2 (right). Schematic of the grounded-grid linaar
amplifier. RFC1 should be mounted reasonably in
the clear so that the series-resonant frequency doas
not
drop below 23
M H ~ .Capacitors
marked
c
are
disc ceramic, Cantralab DD-103. Power-line bypass
capacitors are Centra1.b DD16-103.
blower
13 cubic feet per minute at 0.2-inch back
pressure; use number-) impaller at 3100
rpm (Riplay 8472, Dayton 1C180 or Red-
mond AK-2H-OIAX)
Eimac SK-510 socket and SK-416 chim-
nay for 3-4002; Eimac SK-510 socket and
SK0406 chimney for 3-5002
two-kw pep amplifier
The Eimac 3-4002 and 3-5002 tubes are
air-cooled power triodes designed for zero-
bias, class-B rf amplifier service. They are
particularly well-suited for ssb linear ampli-
fier service. lntermodulation distortion prod-
ucts are 30 dB or better below one tone of a
two-tone test signal for either tube type.
A
comparison of the 3-4002 and 3-5002
types is given in Eimac Amateur Service
Bulletin number 35*.
A circuit for either the 3-4002 or 3-5002
is shown in
fig.
2.
The tubes are cathode
driven with grids at both rf and dc ground
potential.
A
tuned-cathode network achieves
an optimum impedance match to the exciter
and reduces intermodulation distortion to a
minimum. The plate pi-network circuit
i s
suitable for operation on amateur bands be-
tween 80 and 10 meters. Separate meters
monitor grid and plate current.
A
VOX-
operated circuit reduces plate dissipation
during standby. Power output is better than
1200 watts PEP on all bands, and driving
power is about 75 watts PEP. Plate poten-
tials between 2500 and 3000 may be used,
with reduced output for plate voltages as
low as 2000.
sockets
table
Cathode
circuit
components.
Tl,lO meters 0.15 pH.
4
turns number-14 AWG
enamelled wire on
'12''
form,
'12''
long
(National XR-50 form); parallel ca-
pacitance: 200 pF, 1 kV silver mica;
resonate to 28.7 MHz
T2,lS meters 0.15 pH. Same as 11, resonate to 21.3
MHz with 470-pF, 1-kV silver-mica
capacitor
73, 20 meters 0.31 pH. 6 turns number-14 AWG
enamelled wire on National XR-50
form; parallel capacitance: 470 pF, 1
kV silver mica; resonate to 14.3 MHz
14, 40 meters 0.31 pH. Same a r 13, resonate to 7.2
MHz with 940 pF (two 470 pF, 1 kV
silver micas in parallel)
~5.80
meters 1.3
I r ~
13 turns
.
number-18
AWG
enamelled wire on National XR-50
form; parallel capacitance: 940 pF,
as in 14
table 2.
Plate circuit components.
combination 1 B&W 650A bandswitching inductor
the
plate circuit
The amplifier plate circuit may be of sev-
era1 forms. The easy and more expensive
assembly is to use a bandswitching pi-net-
work inductor. The less-expensive but time-
consuming procedure
i s
to make your own
bandswitchinn network
from
commerciallv
-
available
coil
stock,
copper
tubing and
a
heavy-duty
switch'
Either approach will
and
your choice will probably
be made on an economic basis. Data for
either type of circuit
is
given in the parts
list, table
2.
One idea that hasn't been popularized
is
designing the amplif~erfor single-band oper-
combination 2 Air-Dux 195-2 pi-inductor
combination 3 homemade tapped inductor; 60 me-
tars, 13.6 pH; 40 meters, 6.5 pH; 20
meters, 1.75 pH; 15 meters, 1.0
SH;
10 meters, 0.8
p~
Combinations 2 and 3 should be tapped and trimmad
to resonate as follows:
80
meters, 210 pF; 40 meters,
105 pF; 20 meters, 52 pF: 15 meters, 30 pF; 10 metars.
30 pF. Inductor should be parallel resonated and
grid dipped with these capacitance values before
.lacement in circuit. For combination
3,
the
lo-.
15-
and 20-meter coil should be wound with 3/16-inch
diameter copper tubing, and the 80- and 40-mater
coil wound with number-12 AWG wire.
'Available
from
the
author
upon request
8
Q
february 1969
1
! ?
!q
2%
&
SI
001
-
5
tv
Jt
ANTENNA
xC3
m
-
RFC
2
c7
17
pH
1
2~-
TI
-
- -
$+
8
+
C1
C2
C3
C4
C5, C6
C7, C8
L1, L2
250 pF, 4.5 kV (Johnson 154-16)
1500 pF (LaPointe Induslries, Rockville.
Connecticut, type J-1500-S-30)
500 pF, 2500-volt mica (surplus)
.001 pF, 5 kV (Centralab 8585-1000)
.01
PF,
500-V mice (two ElMenco CM-30
i n parallel)
.001 pF, 6 to 10 kV (Centralab DD60-102)
see table 2
RFC3
RFC2
mounting, series resonant at 25 MHz; grid
dip with terminals shorted
1.7
(J.
W. Miller RFC-144 or Ohmite
2-144) homemade: approximately 20 turns
number-28 AWG enamelled wire wound on
look, 2-wall resistor
for series filament operation: 12% bifilar
turns number-12 AWG enamelled wire
wound on ferrite core 3" long,
'12''
diam-
eter (Indiana General CF-503, Newark
electronics catalog number 59F-1521);
notch core with file and snap to break
for parallel operation: overwind third
winding with number-18 AWG enamelled
wire (sea photo)
S1
single-pole,
12-position ceramic switch,
PC1, PC2 three 100-ohm, 2-watt carbon resistors in
parallel (Ohmite "Little Devil"); three
turns number 14 spacewound around one
resistor
RFC1
100 pH, 1A (B&W 800)
homemade versions: A. 112 turns number-
26 AWG, spacewound (wire diameter) on
1" diameter, 6" long ceramic form (Cen-
tralab X-3022H insulator) series resonant
at 24.5 MHz; grid dip with terminals
shorted
7 kV test (Radio Switch Corporation,
Marlboro, New Jersey, type 86A)
s2
Ti
T6
single-pole, six-position ceramic switch
(Centralab PA-2001)
-
T5
see table 1
series filament operation: 10 volts at 15 A
(Thordarson 21F-146); parallel operation:
5 volts at 30 A (Chicago-Stencor P-6468)
8.
136
turns number40 AWG, 1" diameter,
4'14'' long (B&W 3016 or Air-Dux 832T),
slipped over varnished wood dowel for
february
1969
Q
9

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