Continental Electronics 816R - RF Power Amplifier Details

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Figure

3,

816R

PA

DC

Voltage

Distribution

RF

Power

Amplifier,

A12

(Refer

to

Figure

4)

The

RF

power

amplifier

is

driven

by

the

IPA

through

a

matching

network

consisting

of

the

COUPLING

control

(Cl),

the

PA

GRID

TUNE

control

(C2),

and

LI

,2,3.

Capacitor

C3

and

resistor

R1

swamps

the

power

amplifier

grid

circuit

and

provide

a

more

uniform

impedance

to

the

IPA

under

varying

drive

conditions.

Inductor

A12L14

and

the

distributed

capacity

of

resistor

A12R75

couple

A12R75

to

the

cavity,

forming

a

suppressor

that

dampens

the

higher

order

cavity

resonances

that

can

occur

near

the

third

harmonic

of

the

output

frequency.

Cathode

tuning

(or

peaking)

capacitor

A12A3C39

improves

the

bypass

action

at

the

operating

frequency.

Inductor

A12L5

is

the

power

amplifier

grid

bias

feed

choke.

The

power

amplifier

is

a

plate

tuned

tube

that

is

operated

as

Class

C.

The

tube

screen

is

grounded

and

the

cathode

is

placed

750

Volts

(nominal)

negative

with

respect

to

ground

to

provide

screen

voltage,

as

shown

in

Figure

3.

A

fixed

bias

from

the

power

amplifier

bias

power

supply

is

applied

to

the

control

grid

through

terminal

board

A11TB8-19,

resistor

A11R37,

and

terminal

board

A11TB8-20.

When

an

input

signal

is

present,

grid

current

flows

and

develops

grid

leak

bias

across

resistors

A12R35

and

A12R36.

The

increased

negative

potential

on

the

grid

causes

the

diode

in

the

power

amplifier

bias

supply

to

reverse

bias,

preventing

grid

current

flow

through

the

supply.

Hall

effect

probe

A11Z4

monitors

the

amount

of

grid

current

for

control

panel

metering.

The

power

amplifier

plate

circuit

is

coarse-tuned

from

88

to

108

MHz

by

resonating

the

adjustable

coaxial

resonator.

The

resonator

is

the

area

between

the

tube

shelf

and

the

sliding

shorting

plane.

Two

motor-driven

capacitors

permit

more

precise

tuning

(capacitor

A12C51)

and

loading(capacitor

A12C50).

RAISE/LOWER

switches

S3

(PA

TUNING)

and

S4

(PA

LOADING)

on

control

panel

Al,

control

capacitor

drive

motors.

The

de

blocking

capacitor

C2

is

located

between

the

top

of

power

amplifier

tube

and

input

to

the

air

chimney.

6