reprint of Eimac Amateur Service Newsletter - W6SAI... AS-47 aug 22, 1971






High performance
Power amplifier

This efficient
8877 amplifier
can be run
at 2000 watts
PEP ssb
or 1000 watts cw



     The new Eimac 8877 is a  ceramic/metal high-mu triode rated for use 
up to  250 Mhz.  Operation of this  tube  at  50 Mhz  proved  to  be  so 
satisfactory that other 8877 amplifiers have been designed and built for 
frequencies up to 350Mhz. Two of these amplifiers are of interest to the 
serious vhf operator. One amplifier is designed for the 2-meter band and 
is described here. The other amplifier  covers the range from 150 to 230 
Mhz, and is well suited for use on the  amateur 220-Mhz band; it will be 
described later.
      The 8877 triode has a good division between plate and grid current 
and low intermodulation distortion. It has a plate dissipation rating of 
1500  watts  and  mu  of  approximately  200.  The cathode is indirectly 
heated;  filament  requirements  are 5.0 volts at 10 amps. The tube base 
mates with a standard septar socket.
      This  144-Mhz  8877  linear  amplifier is designed for the serious 
Dxer  who  demands  reliable  service  combined  with good linearity and 
efficiency. The compact grounded-grid design presented here uses a half-
wave  plate  line  and  a  lumped T-network input circuit. The amplifier 
requires no neutralization, is completely stable and free of parasitics,
and is very easy to operate.
      This  amplifier  is  designed for continuous duty operation at the 
1000-watt dc input level, and can  develop  2000 watts PEP input for ssb 
operation with ample reserve. For operation  at 2000 watts PEP the plate 
voltage should be between  2500  and 3000 volts;  under these conditions 
the amplifier will  deliver  1240  watts  output. With the higher plate-
voltage supply, up to 13.8-dB gain can be  obtained  with  an  amplifier 
efficiency of 62%.


C1       1000 pF (Centralab 8585-1000              L1        5 turns no. 14, 3/4" long on
                                                             1/2" diameter form with white
                                                             tuning slug (CTC 1538-4-3)

C2       25 pF variable (Hammarlund                L2        4 turn no. 14, air-wound 3/4" 
         HFA-25B)                                            long 

C3,C4    each consists of 2 parallel               L3,L4     10 turns no. 12 enameled,
         connected 100-pF, 5000-V                            bifilar wound, 5/8" diameter
         capacitors (Centralab 8505-100) 

C5       plate-tuning capacitor (fig. 3)           L5,L6     plate-resonators (see fig. 4)

C6       output loading cap. (fig. 2)              L7        7 turns no. 14 wire, 5/8"
                                                             diameter, 1-3/8" long 
C7       1000 pF, 4 kV feedthrough cap.
         (Erie 2494) 

fig 1. Schematic for the grounded-grid two-meter triode amplifier. Operation bias for the
8877 is supplied by a 12-volt zener diode in the cathode lead.


The circuit
      In the amplifier circuit in fig. 1 the 8877 grid is operated at dc 
ground.  The grid ring at the base of the tube provides a low-inductance 
path  between  the grid element and the chassis. Plate and grid currents 
are measured in the  cathode return lead; a 12-volt, 50 watt zener diode 
in  series  with the  negative  return sets the  desired value of idling 
current. Two additional diodes are shunted  across  the meter circuit to 
protect the instruments.
      Standby  plate  current of the 8877 is reduced to a very low value
by the 10,000-ohm  cathode  resistor;  this resistor is shorted out when
the vox circuit is energized,  permitting  the tube to operate in normal
      A 200-ohm safety resistor insures  that the negative power circuit
of the amplifier does not rise  above ground  potential if the positive-
voltage supply is accidently  grounded.  A second safety resistor across
the 1N3311 zener diode prevents the cathode potential from rising if the
zener should accidently burn open.

input circuit
      The cathode input matching circuit is a T-network which matches  a
50-ohm termination to the input impedance of the tube (about 54  ohms in
parallel with 26 pf).  The network consists of two series-connected ind-
uctors and a shunt capacitor.    One  inductor  and  the  capacitor  are
variable so the network  is  able  to  cover  a  wide range of impedance
      The  variable  inductor  (L1)  is  mounted on the rear wall of the
chassis  and  may  be adjusted from the rear of the amplifier. The input
tuning capacitor (C2) is  adjustable  from the  front  panel.  When  the
network has been properly tuned no adjustment is required over the 4-Mhz
range of the 2-meter band.

fig 2. Structural details of the amplifier show the relative size and position of the
various components. Assembly is made of aluminum panels.

      Underchassis  layout  of  components  is shown in the photogtraph.
The cathode input circuit is in center compartment. The  slug-tuned coil 
in the  input matching  circuit is  mounted  on the rear wall. Air-wound 
filament chokes are placed in front of the socket. The cathode-heater rf
choke is near the top edge of the enclosure. All of the cathode leads of 
the socket, plus one heater pin (pin 5)  are connected in   parallel and 
driven by the input matching network.
      The ceramic  socket  of  the 8877 is  mounted  one-half inch below 
chassis level by spacers. Four pieces of brass shim  stock (or beryllium
copper) are formed into grounding  clips to make  contact to the control
grid ring.  The  clips are mounted between  the spacers and the chassis.
The aluminum clamps holding  ends of plate lines are visible in the side
compartments.  The filament transformer and dial mechanism are placed in 
the area between enclosure and panel.

plate circuit
      The plate  circuit of the amplifier is  a  transmission-line  type
resonator.  The  line (L5 plus L6)  is one half-wavelength long with the 

Top view of amplifier showing plate compartment. 8877 tube is at center with plate lines
on each side.

tube placed  at  the  center (fig. 2).  This  type of tuned  circuit has 
several advantages.   A quarter-wave circuit would normally be preferred 
because of its greater  bandwidth, but I wanted to use easily obtainable
standard copper water pipe as the center  conductor  of the transmission 
line tank circuit. The resulting high-impedance  transmission line would
make a quarter-wave plate tank circuit physically short and difficult to
      In addition, the heavy rf current that flows on the tube seals and
control grid would, in the process of charging up the output capacitance
to the plate voltage swing,  tend to concentrate on one side of the tube
if a  single-ended  quarter-wave  circuit  were used.  This current con-
centration  would cause  localized  heating of the tube.  The best tuned 
circuit configuration to minimize this effect is a symmetrical cylindar-
ical coaxial cavity.  Unfortunately,  this type of cavity is complex and
difficult to build.

      A practical compromise is to use two quarter-wave lines connecting
to opposite sides of the tube.  It is  interesting  to note that each of
the two quarter-wave lines is physically longer than if only one quarter
wave line were used.  This is  because  only one-half of the tube output 
capacitance loads each of the two lines.

fig. 3. Variable plate portion of plate-tuning capacitor C3. This arrangement permits the
capacitor to be adjusted under full power without "jumpy" tuning as there are no moving or
sliding contacts which carry heavy rf current.

fig. 4. Details of plate lines L5 and L6. Copper tubes are standard water pipe.

      Resonance is established by a moving plate capacitor (C5); antenna
loading is  accomplished by a second  capacitor (C6) placed at the anode 
of the 8877.  Output power is coupled from the plate circuit through the
series capacitor into a 50-ohm output. In the top-view photo tuning cap-
acitor C5 is at the front of the compartment; variable loading capacitor
C6 is at the rear. The plate choke is visible in the front corner. 

      The two-meter amplifier is built in an enclosure measuring  10 1/4
X 12 X 6 1/4 inches. The 8877 socket is centered on a  6 X 6  subchassis 
plate. A centrifugal  blower  forces  cooling air into the under-chassis
area; the air escapes through the 2-5/8-inch diameter socket hole.
      The  plate  tuning  mechanism  is shown  in  fig.  3. This  simple 
apparatus will operate with  any variable  plate capacitor,  providing a 
back and forth movement of about one inch.It is driven by a counter dial 

Rear of amplifier showing blower and coaxial output connector. Amplifier is upside down
in this photograph.

and  provides  a  quick  inexpensive  and  easy  means  of driving a vhf 
capacitor.  The ground return path for the  grounded  capacitor plate is 
through a wide low-inductance beryllium-copper or brass shim stock which 
provides spring tension for the drive mechanism.

fig. 5. Anode clamp assembly for the two-meter linear amplifier.

      The variable output  coupling  capacitor is located at the side of 
the 8877 anode. The type-N coaxial fitting is connected to the  moveable 
plate  of the coupling capacitor.  The fitting is  centered in a special 
tubular  assembly which allows the wole connector to slide in and out of 
the  chassis,  allowing the  variable plate of the coupling capacitor to 
move with respect to the  fixed plate  mounted on the tube anode  clamp. 
When the  final  loading  adjustment has been set the sliding fitting is 
clamped by an arrangement similar to the slider on a variable wire-wound 
      The length  of the plate-line inductors (L5 and L6) is adjusted by
means of dural  blocks  placed at the  shorted end of the line (fig. 4).
The position of the blocks is determined by setting capacitor C5  at its 
lowest  value  and  adjusting  line  lengths  so that  the plate circuit 
resonates at 148 MHz with the 8877 in the socket.
      The plate rf  choke is  mounted  between the junction of one plate 
strap  and a pair  of the  dual  blocking  capacitors;  the high-voltage 
feed-through  capacitor  is  mounted  to the  front  wall  of  the plate 
compartment.  The  blocking  capacitors  are  rated for  rf service, and 
inexpensive tv-type  capacitors  are not recommended for this amplifier. 
A short chimney to  direct cooling air from the socket through the anode 
of the 8877 is made from Teflon and clamped between the chassis deck and 
the anode strap.*

      Amplifier  operation  is completely stable with no parasitics. The
unit  tunes  up  exactly  as if  it were on the "dc bands."  As with all 
grounded-grid amplifiers excitation should never be applied  when  plate 
voltage is removed from the amplifier.

Underchassis view of the two-meter amplifier. The cathode input circuit he center
compartment. Plate lines are visible in the side compartments.

      The first step is to grid-dip  the  input  and  output circuits to
near-resonance with  the 8877  in  the socket.  A  swr  meter  should be 
placed  in  series  with  the  input  line  so  the input network may be 
adjusted for lowest swr.
      Tuning  and  loading  follows  the  same  sequence as any standard
grounded-grid amplifier. Connect a swr indicator at the output and apply
a small amount of rf drive. Quickly tune the plate circuit to resonance.

table 1. Performance data for the 144-MHz power amplifier under the conditions
most suitable for amateur ssb (2000 watts PEP) and cw (1000 watts).

             Plate voltage                    3000 V         2500 V           2500 V
             Plate current (single tone)       667 mA         800 mA         400 mA
             Plate current (idling)             54 mA          44 mA          44 mA
             Grid voltage                      -12 V          -12 v          -12 V
             Grid current (single tone)         46 mA          50 mA          28 mA
             Power input                      2000 W         2000 W         1000 W
             Power output                     1240 W         1230 W          680 W       
             Efficiency (apparent)              62 %           62 %           68 %
             Drive power                        47 W           67 W           19 W
             Power gain                       13.8 dB        12.6 dB        15.5 dB

The cathode circuit should now be resonated. The swr between the exciter
and the amplifier will not necessarily be optimum.  Final  adjustment of
the cathode circuit for  minimum  swr  should  be  done  at  full  power 
because the input impedance  of a cathode-driven amplifier is a function
of the plate current of the tube.
      Increase the rf  drive  in  small  increments  along  with  output
coupling until the  desired  power  level  is reached.  By adjusting the 
drive and loading together it will be possible to  attain  the operating
conditions given in  the performance chart in  table 1.  Always tune for
maximum plate efficiency:  maximum output power for minimum input power.
It is quite easy to load  heavily  and  underdrive  to  get  the desired 
power input but power output will be down if this is done.
     I would like to thank K6DC for his help in adjusting  and determin-
ing the operating conditions of this two-meter amplifier.

*Detailed drawings of the anode clamp, plate resonator and blocking capacitor assembly, 
and variable plate tuning capacitor (C5) are available from R. Sutherland, EIMAC 
Division of Varian, 301 Industrial Way, San Carlos, Califirnia 94070. Ask for drawing 
numbers 168658, 168648, and 168647.


1. R. Sutherland, W6UOV, "Two Kilowatt Linear Amplifier for Six Meters," 
   Ham Radio, February, 1971, page 16.
2. R. Barber, R Rinaudo, W. Orr, R. Sutherland, " Modern Circuit Design for VHF Transmitters," 
   CQ, November, December, 1965.


This circuit may be substituted for the hard to find 1N3311 zener biasing diode shown in the schematic circuit of fig.1

Mount the 2N3055 on a TO3 style heatsink using insulating hardware.