Heathkit W-5M User Manual

This
method of controlling balance
also
permits use of
KT66
tubes with
wider than
average
deviations
in plate
current
and
mutual conductance.
Therefore,
should one
tube fail,
it gener-
ally
will not
be
necessary
to purchase
a
pair
of matched
tubes
to
restore
the high
performance
of
the
amplifier.
One
word
of
caution:
In connectingthe
meter
to
the
"Bass-Bar'
jacks,
avoid contacting
the
chas-
sis
with
either meter
lead.
These terminals
are
about 45
volts
above
chassis.
A
sensitive
meter
connected
from
them to ground
might
be seriously
damaged.
A
second
innovation
in
the Heathkit
W-5M is the inclusion of
a
"Tweeter-Saver.”
Most
serious
audiophiles have
had the
unfortunate
experience
of
losing
one or
more
speakers
because of
voice
coil failure.
High
frequency
drivers
or
"tweeters,
"
have been
particularly
susceptible to
this
trouble.
Speaker manufacturers
have spent a
great deal
of time
in
research
to
isolate the
cause
of these
failures.
One of the principal offenders
has
been high
frequency
oscillation
in
the
power
ampli-
fier
at a
supersonic frequency.
Generally,
this
takes
place
at frequencies
near
the
upper "hump"
in
the
response
curve. This hump
has
been
characteristic
of many
feedback
amplifiers.
In
the
W-5M
amplifier,
particular
pains
have been
taken
to
reduce
the
possibilities
of
instability
and this
peak has
been eliminated.
However,
as
added insurance
the
output of
the
amplifier
is
constantly
monitored
by the
"Tweeter-Saver.
"
Most speaker
systems are nominally
rated
at
4,
8 or
16
impedance
and
present
a load
imped-
ance near these
figures
throughout
most of
the audio
spectrum.
But,
at 50
to 200
kc, these
sys-
tems
show
impedances of
several
time£
the
rated
value;
therefore the
amplifier
is
not properly
terminated,
becomes
unstable
and
tends
to
oscillate
at
supersonic
frequencies.
A
substantial
amount
of high
frequency
power
is fed
to the
speaker, it
begins
to
overheat
and fails.
This
may
take
place even
though
the oscillation
occurs only
intermittently
and
under
continuous
oscillation
even
the
finest
high frequency
drivers fail
in less than
one
minute.
The
"Tweeter-Saver"
will
prevent
these
oscillations,
even
though no
speaker
is
connected.
A
relatively
small
load
resistor,
rated
at only
1/2
watt,
can
be
used safely
in the
circuit,
since
the
purpose
is to
prevent
oscillation,
not
to
absorb power.
The
circuit
does present
a
slight
loading
effect
at very
high
frequencies
but this
is beneficial
since it
improves
transient
characteristics
and
helps
flatten
the
high
frequency
response
of the
amplifier.
In
adjusting
the
pads to
the
several
drivers
of
a
multiple
-speaker
system,
it
is convenient
to
drive the
system
at single
frequency
inputs
while
measurements
are
made with
microphone
or
sound-level
meters.
During
such
tests, if
high
frequency
power is
applied
to the
speaker
ac-
cidentally,
the
"Tweeter-Saver"
will
pickup
a substantial
portion,
thereby
protecting
the
speaker
system.
The
constants
of the
circuit
have
been
selected
to
absorb
power only
at
frequencies
well
above the
sonic
range.
When
making
tests
at high
frequencies
with
large
outputs,
it
is
nor-
mal
for
the resistor
to
heat.
Therefore
such tests
should
be
made
as quickly
as possible.
REPLACEMENTS
Material supplied
with
Heathkits
has
been
carefully
selected
to
meet
design
requirements
and
ordinarily
will
fulfill
its
function
without
difficulty.
Occasionally
improper
instrument
opera-
tion can
be traced to a
faulty
tube
or
component.
Should
inspection
reveal
the
necessity
for
re-
placement, write
to the
Heath
Company
and supply
all
of
the following
information:
A. Thoroughly
identify
the
part in
question
by using the
part
number
and
description
found
in
the
manual
parts
list.
B.
Identify
the
type and
model
number of
kit in
which
it
is
used.
C. Mention
the
order
number
and
date of
purchase.
D.
Describe
the
nature
of defect
or
reason
for
requesting
replacement.
Page
29

Heathkit W-5M Specifications

General

Brand	Heathkit
Model	W-5M
Category	Amplifier
Language	English