SBE SB2-LA - Circuit Description; Input Circuits; Output Circuits

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for

a

plate

amperes

reading

of

not

more

than

.5.

With

CW

operation,

the

plate

amperes

reading

may

be

increased

to

1.0

(.75

on

15

meters)

provided

that

the

key

is

not

held

down

for

extended

periods

(not

more

than

20

seconds).

NOTE:

Output

meter

(top

meter

in

SB2-LA)

should

be

peaked

for

highest

pos-

sible

meter

reading.

DONOT

EXCEED

1.2

AMPS.

ON

PLATE

METER.

IV.

CIRCUIT

DESCRIPTION

The

Sideband

Engineers'

Model

SB2-LA

linear

amplifier

consists

of

six

type

6JE6

tubes,

parallel-connected,

operated

Class

AB

,

(no

grid

current)

witha

passive

grid

input

circuit

and

a

pi-section

output

network.

Due

to

the

high

perveance

of

the

6JE6

tubes,

efficient

operation

is

obtained

with

low

anode

voltage

and

high

current.

Thus,

most

of

the

expensive

and

bulky

components

normally

required

for

amplifiers

of

this

power

capability

are

eliminated.

Power

supplies

required

for

heater,

bias

and

anode

circuits

of

the

amplifier

are

self-contained.

A.

INPUT

CIRCUITS

Since

the

amplifier

tube

grids

require

no

power

for

operation,

a

bank

of

non-

inductive

resistors

connected

to

the

input

jack

absorb

the

power

generated

by

the

transceiver

(or

exciter).

These

resistors

are

arranged

to

present

an

input

impedance

of

approximately

75

ohms,

essentially

resistive.

Anti-parasitic

resistors

are

provided

in

each

grid

lead.

Fixed

bias

is

shunt-fed

to

the

grids

from

a

small

voltage-doubler

supply

with

an

adjustment

provided

on

the

rear

panel.

BseOuUTPUILZCIREULTS

The

output

network

of

the

amplifier

is

a

conventional

pi-section

coupler

although

the

electrical

(and

physical)

sizes

of

the

various

components

are

unusual.

Be-

cause

of

the

low

plate

load

impedance

required

(about

300

ohms),

the

input

capacitor

required

is

large

in

value.

Likewise,

the

inductance

required

is

small.

Since

the

plate

voltage

is

relatively

low,

a

small

air

gap

is

completely

adequate

for

the

input

capacitor.

The

bandswitch

selects

the

amount

of

inductance

for

each

band,

provides

extra

fixed

amounts

of

load

capacitor

on

80

and

40

meters,

and

adds

a

fixed

input

capacitor

on

80

meters.

A

small

diode,

together

with

a

resistor/capacitor

network,

actuates

the

out-

put

meter,

which

provides

a

relative

indication

of

the

voltage

existing

on

the

output

terminal.