Download Speakers are Important In or Out of the Circuit

Innovation. Amplified.
Chapter 7
Speakers are
Important
In or Out of the
Circuit
by
Hartley Peavey
a serious “mismatch.” Where power is concerned,
proper “matching” of “load impedances” is necessary.
Solid state power amps utilize power transistors which
are high current, low voltage devices that can easily
DIRECTLY drive loudspeakers, which as we now know,
are also high current, low voltage devices. The simple
fact is output transformers are not necessary (in most
cases) with solid state amplifiers.
Speakers are Important In or Out of
the Circuit.
Most guitar players have heard that operating a tube
type guitar amp without a speaker plugged in can
cause problems. Few understand WHY; thus, this brief
discussion.
In years past, most guitar amplifiers utilized vacuum
Speakers
Important
or Out
of the Circuit.
tubes.
Vacuumare
tubes
(valves)Inare
inherently
high
Matching a loudspeaker to a vacuum tube amplifier
voltage, low current devices. This simply means that
generally
requires the use of an output (“matching”)
st guitar players
heardoperate
that operating
a tube
typevoltage
guitar amp
a speaker
tubeshave
typically
at relatively
high
at without
can cause problems.
Few
understand
WHY;
thus,
this
brief
discussion.
transformer
to optimize loading on the tubes, while at the
moderate to low current, which in turn, implies that
same time,achieving optimal current and voltage drive
they are (in essence) “high impedance” devices. By
to the loudspeaker. In a very few examples, tube amps
necessity, audio amplifiers (including guitar amps)
years past, most
guitar
amplifiers
utilized
vacuum
tubes.
Vacuum
tubes
are
have (valves)
been designed
that can drive speakers directly,
deliver their output power to loudspeakers which are
high voltage,
low
current
devices.
This
simply
means
that
tubes
typically
operate
at
but
these
designs
have
not been commercially viable
inherently low voltage, high current devices. One of the
igh voltage challenges
at moderate
to
low
current,
which
in
turn,
implies
that
they
are
(in
because of their high cost and complexity. Therefore,
for audio amp designers has always been
high impedance”
devices.
By necessity,
audio amplifiers
guitar amps)
99.9%
of all tube amps use an output (matching)
the problem
of matching
high voltage,
low current(including
(high
r output power to loudspeakers which are inherently low voltage, high
current to properly match characteristics of the
transformer
impedance) tubes to low voltage, low impedance, high
One of the challenges for audio amp designers has always been thetubes
problem
of loudspeaker.
to the
current loudspeakers. The almost universal “solution”
high voltage, low current (high impedance) tubes to low voltage, low impedance, high
to this “matching” problem has been (and still is) to
dspeakers. The almost universal “solution” to this “matching” problem has been (and
Transformers are relatively efficient devices. The
is often
calledtransformer.”
an “output transformer.”
An
tilize what isutilize
oftenwhat
called
an “output
An output transformer
IS a
science of transformer design and manufacturing has
output
IS a “matching
transformer,”
whichtransformer
has the unique
capabilitytransformer,”
to match twowhich
dissimilar devices (i.e.
had well over 100 years to evolve and develop. Today’s
has the unique capability to match two dissimilar
bes and loudspeakers).
transformers are among the most efficient electrical
devices (i.e. vacuum tubes and loudspeakers).
devices available. Even though transformer design and
manufacture has reached a high level of development,
transformers do exhibit inherent characteristics
that every musician should understand. In order to
appreciate the process that enables a transformer to
work, we’ll have to discuss some basics.
Devices that we call transformers are able to take
one form of energy and convert it into another; i.e.
they “transform” one type of electrical energy into
another…That’s WHY they are called “transformers.”
A power transformer converts the AC (alternating
current) from the mains socket into the appropriate
current and voltage required by the amplifier. In a tube
amp, this generally means a so-called “step up” power
transformer that converts the mains/line voltage to a
higher value to enable proper operation of the tubes.
Remember, tubes are high voltage devices (typically
The advent of solid state guitar amplifiers back in the
e advent of solid state guitar amplifiers back in the 1960’s confused 250
the issue
volts to 500 volts) therefore the tube amp’s power
1960’s confused the issue somewhat, especially since
especially since most solid state guitar amps DO NOT have outputtransformer
transformers.
has to convert the mains/line voltage into a
most solid state guitar amps DO NOT have output
simple (but little understood) reason for this. Both power transistorsHIGHER
and
value.
Conversely, most output transformers
transformers. There’s a simple (but little understood)
ers are inherently low voltage, high current devices. Speakers, by their very nature,
convert a high voltage signal into a low voltage
reason for this.
Both power
transistors
and
coils with impedances
generally
between
4 and
16loudspeakers
ohms. Output tubes optimally
signal (albeit at a significantly higher current) in order
are
inherently
low
voltage,
high
current
devices.
ee matching impedances measured in the thousands of ohms, so it’s easy to
to properly drive loudspeakers. Power and output
by their very
nature,
have voice
coils with Where
d that tubes Speakers,
and loudspeakers
represent
a serious
“mismatch.”
power is
impedances
generally
between is
4 and
16 ohms.Solid
Output
, proper “matching”
of “load
impedances”
necessary.
state transformers
power amps exhibit certain vital characteristics that
have DIRECTLY
come to be appreciated by guitar and bass
tubes
optimally
to low
see voltage
matching
impedances
er transistors
which
are highexpect
current,
devices
that can easily
players.
measured
theknow,
thousands
of ohms,
so it’s low
easyvoltage
to
peakers, which
as we in
now
are also
high current,
devices. The
understand are
thatnot
tubes
and loudspeakers
represent
is output transformers
necessary
(in most cases)
with solid state amplifiers.
2
tching a loudspeaker to a vacuum tube amplifier generally requires the use of an
to work, we’ll
havefew
to discuss
some
basics.
oansformer
the loudspeaker.
In a very
examples,
tube
characteristics
that
have come
to be appreciated by guitar and bass players.
akers directly, but these designs have not been
st and complexity. Therefore, 99.9% of all tube
ces
that wematch
call transformers
areofable
taketo
one
o
properly
characteristics
theto
tubes
theform of energy and convert it into
they “transform” one
type ofpower
electrical
energy into another…That’s
WHY they components
are
the associated
in the power supply.
A typical
transformer.
sformers.” A power transformer converts the AC (alternating current) from the mains
he appropriate current and voltage required by the amplifier. InMuch
a tube
theamp,
samethis
situation exists with output transformers.
eans
a
so-called
“step
up”
power
transformer
that
converts
the
mains/line
voltage
to 50’s and 60’s never envisioned
Amp designers
in the
evices. The science of transformer design and
ue
to
enable
proper
operation
of
the
tubes.
Remember,
tubes
are
high
voltage
that their amps would be used “wide open,” and the
o evolve and develop. Today’s transformers are
ically 250Even
volts though
to 500 volts)
therefore
the tube
has toprocesses re: economics were used
same thought
vailable.
transformer
design
and amp’s power transformer
mains/line transformers
voltage into ado
HIGHER
value. Conversely, most output
transformers
elopment,
exhibit inherent
in regard
to output transformers. The low E on a
gh
voltage
signal
into
a
low
voltage
signal
(albeit
at
a
significantly
higher
current) in
derstand. In order to appreciate the process that
guitar is approximately
80Hz, so that was generally
transformers
(and certain
the associated
perly drive
loudspeakers.
Power and output transformers
exhibit
vital rectifiers and filter capacitors) would suffer some amoun
discuss
some
basics.
the
lowest
frequency
of
interest
in Today,
many (guitar)
amp actually se
(sag) under maximum output conditions.
many musicians
cs that have come to be appreciated by guitar andvoltage
bass drop
players.
output
transformer
designs.(meaning
Becausethat
maximum
output
amps whose power
supply
is poorly regulated
the voltage
drops or “sags” u
full load conditions).
Many players
believe thatonly
this is
a form of “compression”
conditions
were expected,
momentarily,
by most that they fe
able to take one form of energy and convert it intodesirable. In truth,
this amp
kind ofdesigners,
voltage “sag”
wasoutput
not thetransformers
result of “intelligent
guitar
their
were design,” but w
on the power transformer and the associated components in the po
ctrical energy into another…That’s WHY they are attempt to economize
designed with minimal amounts of steel and copper
supply.
onverts the AC (alternating current) from the mains
which produced some interesting and still rarely
ge required by the amplifier. In a tube amp, this
understood results regarding MI amplification.
transformer that converts the mains/line voltage to
Much the same situation exists with output transformers. Amp designers in the 50
Back in
the
1940’s
and 50’s, guitar amp designers never ever thought that their
the tubes. Remember, tubes
are
high
voltage
60’s never envisioned
thatgoing
their amps
woulda beshort
used “wide
open,” and
same thought
Before
further,
discussion
on the
the
“creations”
ever be
played
“wide open.”
As
a result,
both power
efore the tube amp’s
power would
transformer
has
to
processes
re: economics
werethey
usedtypically
in regarddesigned
to output transformers.
The low E on a gui
way
transformers
work
would probably
helpful.
and output
transformers
to be as approximately
economical as
possible
still
performing
task be
80Hz,
so that(while
was generally
the lowestthe
frequency
of interest in many (guita
R value. Conversely,
most output
transformers
Transformers
used
in
guitar
amplifiers
are
typically
intended
by the designers).
Economics
dictated that
both Because
the power
and output
beexpected, only
designs.
maximum
outputtransformers
conditions were
ge signal (albeit at
a significantly
higher current)
inoutput transformer
divided
into
two
basic
categories,
i.e.
either
“power
momentarily,
by
most
guitar
amp
designers,
their
output
transformers
were designed with
as
small
as
reasonably
possible
so
as
to
minimize
the
amount
of
steel
and
copper
used.
They
and output transformers exhibit certain vital
minimal
amountstransformers”
of steel and
copper
which produced
some
interesting
and
or
“output
transformers.”
There
is astill rarely
were
NOT
designing
for
“continuous
maximum
output,”
so
they
accepted
that
their
power
ated by guitar and bass players.
understood results
regarding
amplification.like” device utilized in many
third
type ofMI
“transformer
audio amplifiers called a “filter choke.” However,
3 | P a g ethis
device
is
an
inductor
that
physically
appears
similarwork
to would proba
Before
going
further,
a
short
discussion
on
the
way
transformers
a transformer
but isamplifiers
actuallyare
an typically
inductordivided
that converts
helpful. Transformers
used in guitar
into two basic categor
i.e. either “powerelectricity
transformers”
“output transformers.”
There
into or
“stored”
magnetic energy
inisitsa third
coretype
to of “transform
like” device utilized
in
many
audio
amplifiers
called
a
“filter
choke.”
However,
help “smooth” the “pulsating direct current” delivered this device i
inductor that physically appears similar to a transformer but is actually an inductor that co
from
rectifiers
and
Although
a
k in the 1940’s and 50’s, guitar amp designers never
ever thought
thatthe
their
electricity
into “stored”
magnetic
energy
in itsfilter
corecapacitors.
to help “smooth”
the “pulsating
direct cu
“filter
choke”
looks
similar
to
a
transformer,
it
has
only
delivered
from
the rectifiers
and
filter capacitors. Although a “filter choke” looks similar to
would ever be played “wide open.” As a result, they
typically
designed
both
power
transformer,
it has
only
one
winding,
as opposed
to the
or more
windings present in p
one
winding,
as opposed
to the
twotwo
or more
windings
ransformers to be as economical as possible (while
still performing
the
task
and output transformers.
present
in
power
and
output
transformers.
the designers). Economics dictated that both the power and output transformers be
reasonably possible so as to minimize the amount of steel and copper used. They
esigning for “continuous maximum output,” so they accepted that their power
Back in the 1940’s and 50’s, guitar amp designers
never ever thought that their “creations” would ever
3 | P a g e be played “wide open.” As a result, they typically
designed both power and output transformers to be as
economical as possible (while still performing the task
intended by the designers). Economics dictated that
both the
power
and
outputthat
transformers
be as small as
mp designers
never
ever
thought
their
reasonably
so as to minimize
the amount of
” As a result,
they possible
typically designed
both power
steel (while
and copper
used. They
were
as possible
still performing
the
taskNOT designing for
ted that“continuous
both the power
and output
transformers
be
maximum
output,”
so they accepted
that
imize the
amount
steel and copper
used.
They rectifiers
their
poweroftransformers
(and the
associated
m output,”
theycapacitors)
accepted that
theirsuffer
power
andsofilter
would
some amount of
Choke
voltage drop (sag) under maximum output conditions.
Choke
Today, many musicians actually seek
3 | P aamps
g e whose
A transformer
works current
by utilizing
currentitsflow
through
power supply is poorly regulated (meaning that Athe
transformer
works by utilizing
flow through
primary
winding to magnet
steel “core.” These
cores are
almost always
composed
of thin
strips of
special steel whos
its primary
winding
to magnetize
a steel
“core.”
These
voltage drops or “sags” under full load conditions).
characteristics
are
optimized
for
use
in
this
application.
As
current
flows
the prim
cores are almost always composed of thin stripsthrough
of
Many players believe that this is a form of “compression”
special steel whose characteristics are optimized for
that they feel is desirable. In truth, this kind of voltage
4 | use in this application. As current flows through the
“sag” was not the result of “intelligent design,”
but
was
primary coil, a magnetic field is created around the core
an attempt to economize on the power transformer and
3
(which itself absorbs the electrical energy as magnetic
energy). This magnetized core alternates up and down
in intensity in direct proportion to the amplitude and
frequency of the voltage and current in the primary.
The changing magnetic field (flux) in the core creates
(by induction) voltage and current in the secondary.
The voltage and current created in the secondary is
DIRECTLY PROPORTIONAL to the voltage current in
the primary in a specific “ratio” which is determined
by the number of turns of wire in the primary and the
number of turns of wire in the secondary.
Simply put, an output transformer operates by utilizing
electrical energy supplied by an amp’s output tubes,
to magnetize the core at the “rate” of the input. As
the core’s magnetic strength varies up and down, it
transfers energy to the secondary by “induction” in the
necessary ratio. Ideally, the output transformer primary
coil is optimized to “match” the tubes being used, and
the secondary (which has NO “direct connection” to
the primary) is “matched” to the speaker used. As
referenced above, there is NO “direct connection”
between the primary and secondary, except by the
“magnetic action” of the transformer’s core. Simply put,
a transformer operates by converting electrical energy
into magnetic energy and then back into electrical
energy.
Generally speaking, one of the greatest limitations of
transformers is the ability of the core to create enough
magnetic energy to produce the desired effect in the
secondary. This is true of both power transformers
and output transformers. A minimally designed power
transformer exhibits a pronounced voltage drop (sag)
when called on for maximum output because the
primary is unable to sufficiently magnetize the core with
enough energy to transfer to the secondary. Typically,
this occurs because the core was “minimized” by
design.
fi amplifiers usually try to operate over extremely wide
frequency ranges (usually from lower than 20Hz to
well above 20kHz) Hi-fi designers have to design tube
amps with power and output transformers significantly
larger than the equivalent guitar amp. The open low
E on a six-string guitar is approximately 80Hz. Most
guitar speakers rarely have significant response above
about 8kHz, so that is the approximate bandwidth that
most guitar amp output transformers are designed to
handle. Because 20Hz is four times lower than 80Hz,
tube hi-fi output transformers tend to be significantly
larger than guitar amp transformers of equivalent
power, i.e. a 100 watt hi-fi amp output transformer
would be typically several times larger (and more
expensive) than a 100 watt guitar amp transformer.
Given this, it should be apparent that a guitar amp’s
output transformer has less “iron” in the core than a
hi-fi amp, and as a result will “saturate” much quicker
than the hi-fi amp’s transformer. “Technically speaking,”
operating a transformer close to its “saturation point” is
a questionable practice. The truth is, much of a tube
guitar amp’s sound has a great deal to do with the
characteristics of the guitar amp’s output transformer!
Hi-Fi Output Transformer
Guitar Amp Outp
Magnetic cores in transformers have a “threshold,”
beyond which, further magnetization is impossible. This Because modern output transformers are usually well designe
realize that they have such a profound effect on the frequency respon
is generally referred to as “core saturation.” Magnetic
known aspect of output transformers is that their frequency response
saturation is determined by a number of factors,
both the high and the low frequency EXTREMES as the signal level a
including size, design, type of materials, etc, etc. Once
the core. This is a “dynamic process” which means that it is CONSTA
the magnetic core of a transformer is “saturated,” no
the frequency and amplitude of the driving signal supplied by the valv
more energy transfer is possible…even if you could
frequency response is usually reasonably flat, but as the signal appro
connect the entire output of Hoover Dam to it! Simply
(maximum output) the extreme ends of the transformer’s bandpass te
put, when a transformer core
but in a “dynamic fashion” (meaning that the transformer’s “bandpass
“saturates,” that’s it!
CONSTANTLY CHANGING!).
Tube Hi-fi audio amplifier designers are certainly
familiar with the realities of transformer
design.
Since
hi- Actually, the
Hi-Fi
Output
Transformer
Guitar
Amp
Output Transformer
above
referenced
“dynamic frequency response”
transformer are vital and incredibly helpful characteristics. Most peop
4
tubes/valves
“clip” with rounded edges on clipped output waveforms.
Because modern output transformers
are
usually as
well
designed,albeit
most usually
people asymmetrically
don’t
clip just as “squarely”
transistors,
(wit
Because modern output transformers are usually well
designed, most people don’t realize that they have
such a profound effect on the frequency response of
amplifiers. A little known aspect of output transformers
is that their frequency response tends to drop off rapidly
at both the high and the low frequency EXTREMES as
the signal level approaches “saturation” of the core.
This is a “dynamic process” which means that it is
CONSTANTLY CHANGING with the frequency and
amplitude of the driving signal supplied by the valves.
In normal operation, frequency response is usually
reasonably flat, but as the signal approaches saturation
(maximum output) the extreme ends of the transformer’s
bandpass tend to drop considerably, but in a “dynamic
fashion” (meaning that the transformer’s “bandpass
characteristics” are CONSTANTLY CHANGING!).
There have been many attempts over the years, by
many companies, to design “proper” transformers for
guitar amps. This usually means larger, more expensive
transformers that are (from a technical standpoint)
“better;” i.e, better steel, more laminations, larger size,
etc, etc. Almost every one of these attempts to fit “proper
transformers” to guitar amplifiers has failed because
adding a “proper” (i.e. a hi-fi) transformer often destroys
“the sound.” While it might be argued that many bass
players prefer a hi-fi type of sound (even in valve amps)
guitar players certainly don’t. This is easily proven by
plugging your guitar into a hi-fi amp (which almost
always sounds bad and doesn’t have the “dynamics”
and “feel” of a guitar amp). Unfortunately, guitar amps
are one of the few areas of audio where bigger is NOT
necessarily “better.” If you don’t believe this, I invite you
to plug your guitar into a tube hi-fi amp and draw your
own conclusions.
Actually, the above referenced “dynamic frequency
response” changes of the output transformer are vital
and incredibly helpful characteristics. Most people
believe (wrongly) that tubes/valves “clip” with rounded
edges on clipped output waveforms. This is NOT true!
Tubes clip just as “squarely” as transistors, albeit usually
asymmetrically (with one side clipping before the other).
What causes the “rounded edges” is the action of the
output transformer as it approaches saturation. Happily,
this rapidly changing frequency response of the output
transformer has the overall effect of “rounding off”
square waves generated by the tubes, while at the
same time, it LIMITS the lower end frequency response
coupled to the speaker. This happy and “accidental”
tonal enhancement is caused by the output transformers
unique “dynamic” tone shaping capability. A tube
guitar amplifier’s performance and operation is greatly
enhanced by these characteristics, which only occur at
(or near) maximum output. As the signal approaches
the point of saturation of the output transformer, these
desirable effects are created that play such a vital role in
creating the “sound” of a tube amp.
The above discussion relates to saturation (or near
saturation) and its effect on the output characteristics
of a tube output transformer. Power transformers
encounter many of the same limitations re: the ability of
the magnetic core to transfer energy from the primary
to the secondary. As the core approaches saturation,
the transformer’s ability to transfer additional energy
becomes marginal. Thus, the output voltage tends to
“sag,” which is a characteristic that lots of guitar players
find desirable, seeing this as a sort of “compression.”
This is another area where “bigger is NOT better.”
Larger and more robust power transformers result in
what is TECHNICALLY a “better” unit, having better
regulation, less sag/compression effect, and therefore,
is not necessarily a “better” guitar amp “soundwise” (as
far as most guitar players are concerned).
Tube/valve amps sound like they do for a number of
reasons, which include asymmetrical clipping, which
is mostly because tubes use a “single ended power
supply,” i.e. a high voltage (B+) and ground. Most
high gain amps have the ability to drive the tube’s grids
positive with respect to the cathode, which causes “grid
current” to flow, which in turn creates an effect called
“bias shift.” As the output transformer nears its maximum
design limits, the frequency response at the extremes
vary “dynamically” with significant “rolloffs” at both the
low end and high end. Lastly, some amount of “sag”
in the power supply creates a kind of “compression
effect” familiar to guitarists. These effects work together
in a rather fortunate “choreography” to produce what
we today recognize as a “good” guitar sound.
The fact that solid state amps almost never use output
transformers allows clipping to blast DIRECTLY into the
loudspeaker. This can often result in the characteristic
“rasp” of overdriven solid state amplifiers, unless
special precautions and designs are utilized, such as
with Peavey’s patented TransTube® technology (which
emulates the output transformers unique bandpass
capability as well as other characteristics of overdriven
vacuum tubes, such as asymmetrical clipping, bias
shift, etc, etc.). Overall, much of the “sound” of a tube
amp is attributable to the unique output characteristics
of its output transformer.
5
the loudspeaker.
If (for whatever reason) the SECONDARY of the transformer has no “load,” the magne
energy created by the primary (and “stored” in the core) has NO place to GO! As the alternat
Since solid state amplifiers generallysignal
havein no
output the tube sockets and the associated wiring.
the primary goes down, the magnetic field built up in the core “collapses,” thus induc
transformers, they usually do not clip
asymmetrically,
an extremely high voltage “spike” back into the transformer’s primary windings. Because this
have no grids, and therefore no gridenergy
current
and the and
Remember
that
we learned
highturns
school
physics
is substantial,
because the
primary
coils haveinmany
of wire,
an extremely hi
“transient”
(often
3000 the
volts“law
or more)
can be developed.
The primary
of the output
resulting “bias shift.” It’s certainly not voltage
any “news”
that a
about
of conservation
of energy.”
This taught
transformer
is directly connected
to the tubes
tube “disappear,”
sockets of the it
amp,
solid state amp’s distortion characteristics
are usually
us that energy
can’tand
simply
hasand
to neither
“go the
tubes nor the sockets (and associated wiring) are designed for that kind of voltage. These
much different from the distortion of electrical
a tube amp.
Only
somewhere.”
When
a tubeinside
output
doesn’t
“spikes”
can (and
often do) cause
“arcing”
the transformer
transformer and/or
the tube
by a thorough understanding of what
goes on
inside between
have athe
place
for the
go (i.e.
a the
speaker
or itswiring.
elements,
especially
terminals
onenergy
the tubeto
sockets
and
associated
a tube amp, was Peavey able to emulate all these equivalent) the energy developed in the magnetic core
phenomena with our patented TransTube® technology. “flashes back” into the amplifier, often destroying the
Remember that we learned in high school physics about the “law of conservation of
One might wonder why Peavey still makes tube
amps if output transformer, the tubes, the tube sockets, and/or
energy.” This taught us that energy can’t simply “disappear,” it has to “go somewhere.” Whe
we have achieved such a close replica.
That
answer
is the
associated
wiring.
tube output transformer
doesn’t
have a place
for the energy to go (i.e. a speaker or its
simple! Many of our customers WANTequivalent)
tube amps!
If
that
the energy developed in the magnetic core “flashes back” into the amplifier, often
destroying
the build
output transformer,
the tubes,
the tube NOT
sockets,
and/or the
associated
is what our customer wants, then that’s
what we’ll
This is WHY
you should
operate
a tube
amp wiring.
This
is
WHY
you
should
NOT
operate
a
tube
amp
without
a
“load!”
as long as decent tubes are available. Unfortunately, without a “load!”
that will probably not always be the case.
The above is a rambling discussion about transformers
(both power an output) and their effect on the sound of
a tube guitar amp. I felt it was important to understand
what transformers do (and generally how they do it)
before getting to the final point of this discussion. As
mentioned above, a transformer works by converting
one form of electrical energy into another. In the case of
a tube amp, to achieve the matching, we “trade” high
voltage in the primary, for high current in the secondary
to properly drive the speaker. This is a “two step” process
Ignition Coils
where electrical energy from the tubes magnetize the
core of the transformer at the audio rate.Ignition
Then,coils
the for gasoline engines work essentially the same way. There’s a “seconda
of wire with
many “turns”
of wire
aroundengines
a steel core.
operation, athe
connection
Ignition
coilswrapped
for gasoline
workInessentially
varying magnetic field in the corecoil
induces
voltage
(and current) into the secondary suitable to drive the same way. There’s a “secondary” coil of wire with many 8 | P a
speaker. The important point here is the energy is put “turns” of wire wrapped around a steel core. In operation,
into the transformer through the primary, which converts a connection is made to a power source (battery) then
current flows though the primary coil and magnetizes
that energy into magnetic energy (i.e. it magnetizes the
the iron core. The contacts (points) then open and the
core) and then that energy is reconverted into electrical magnetic field is allowed to “collapse,” which results in a
energy in the secondary windings, which drives the huge “pulse of energy” being inducted into the secondary
loudspeaker.
coil (which is what produces the electrical “pulse” that is
fed to the spark plug). This is also a two step process
where basically the primary coil is energized, producing
a magnetic field, then “de-energized.” As the magnetic
field “collapses” the magnetic energy stored in the core
induces a high voltage into the secondary coil which is
then fed to the spark plug, thus firing the air/fuel mixture.
If (for whatever reason) the SECONDARY of the
transformer has no “load,” the magnetic energy created
by the primary (and “stored” in the core) has NO place to
GO! As the alternating signal in the primary goes down,
the magnetic field built up in the core “collapses,” thus
inducing an extremely high voltage “spike” back into the
transformer’s primary windings. Because this energy is
substantial, and because the primary coils have many
turns of wire, an extremely high voltage “transient” (often
3000 volts or more) can be developed. The primary of
the output transformer is directly connected to the tubes
and tube sockets of the amp, and neither the tubes nor
the sockets (and associated wiring) are designed for
that kind of voltage. These electrical “spikes” can (and
often do) cause “arcing” inside the transformer and/or
the tube elements, especially between the terminals on
An “UNLOADED” tube output transformer produces
high voltage “spikes” in EXACTLY the same manner as
an “ignition coil” does. The difference being that a tube
amp output transformer is NOT designed to operate that
way, but an ignition coil IS! Operating a tube amp with
its transformer unloaded can easily damage or destroy
a perfectly good tube amplifier by the “flyback” action
described above. Hopefully, the above will provide a
little background as to exactly WHY it’s important NOT
to operate an amplifier WITHOUT a proper “load” for the
output transformer’s secondary output terminals.
6
Peavey (recognizing that accidental operation of its
tube amps without a proper load is a possibility) has
endeavored to try to protect the output tubes, sockets,
wiring, and the output transformer against damaging
voltage spikes as described above. Although there is no
100% “fool proof” method to protect an amplifier against
the spikes generated by an unloaded output transformer,
the most effective protection is by use of so-called “flyback
diodes.” “Flyback” is an old term applied to high voltage
systems in tube type TV sets, but it has come to be applied
to any high voltage transient (spike) in electronics. Silicon
diodes have certain characteristics that engineers find
very useful. At a certain voltages, silicon diodes encounter
a “threshold” and conduct energy in the reverse direction
to their normal operation. This “avalanche effect” occurs
at very high voltages and thus inclusion of these into the
output circuit allows some measure of protection for the
tubes, output transformers, sockets, and wiring. If voltages
occur above their “avalanche” level, the diodes conduct
in the “reverse direction,” thus the destructive voltage
transient generated by the unloaded output transformer
is shorted to ground. Although this method has proven
very effective, it should be noted that super high voltage
transients can cause arcing either inside the output
transformer, the output tubes themselves, and especially
between the terminals of the tube sockets. Cheaper plastic
or phenolic output tube sockets are especially prone to
this and can be easily identified because they are usually
BLACK or BROWN. If a voltage transient “arcs” between
the two socket terminals, it usually leaves a “carbonized
path” between the terminals that REMAIN CONDUCTIVE
which can (and often does) render the amp unserviceable.
This is WHY Peavey now utilizes white GLAZED CERAMIC
sockets for our output tubes, since these white ceramic
sockets are MUCH more resistant to arcing between
terminals and the “carbonized path” problem is totally
eliminated.
Hopefully, the above will give players a better idea why
tube amps should never be played or operated without a
speaker load, or a proper termination by a load resistance
of some kind.
7
Innovation. Amplified.
Peavey Electronics Corporation • 5022 Hartley Peavey Drive • Meridian, MS 39305
Phone: (601) 483-5365 • Fax: (601) 486-1278 • www.peavey.com • ©2006 Printed in the U.S.A.