This month I will dispel several misconceptions pertaining to tubes that I often
hear talked about by guitarists. These misconceptions, many of them the result
of incorrect information from the Internet, can lead you down the path of needless
expense and poor tone.
1-
Just because a tube is lit up doesn't mean it is
working.
Yes, it is necessary for the tube to be lit. But
there are three to four more voltages you can't
see visual evidence of that must be present
inside the tube for it to operate.Any of these voltages
can be either wrong or not present due to a failed
tube, tube socket or component inside the amp connected
to the tube.
2-Replacing
the tubes will fix my
problem.
We
have a laugh at the
shop whenever we
receive an amp and
its repair ticket
reads " replaced
all tubes and amp
still does not work
right".
Customers have a
tendency to believe
because they can
replace a fuse or
a tube themselves
that this is the
only thing that can
go wrong. They ignore
the fact that the
average amp has several
hundred components
and solder connections
that may fail. It
is entirely possible
(or not) that tube
failure was part of
the problem, but
oftentimes a shorting
tube will draw a
lot of current and
damage other components
connected to it.
When new
tubes are installed
they will sit there
and do nothing as
the circuit the tube
is connected to is
no longer working.
Again, just because
a tube is lit up
does not mean it
is working correctly. |
|
3- Can you test my tubes for
me?
Most tube testing equipment does not test
a tube at the voltages or conditions that occur inside a guitar
amp or simulate the torture a tube is put under when driven hard by a guitar.
It certainly will not test for microphonics. There are new tube testing products
starting to appear on the market such as the Maxi-Matcher that test closer
to real world operating conditions, but they are expensive. Unless you need
to test/match many tubes on a regular basis, they are not an economical choice.
The most simple and economical test is to substitute new tubes into the amp
and see if it improves.
4-Can I install new tubes without adjusting the
bias?
The answer to this question is yes and no. Preamp tubes do not have to
be biased. Output tubes in most amps do. Yes you can put output tubes in
and not bias them but that may mean they are not working optimally.Bias
is analogous to changing the plugs in your car and not adjusting the idle.
If the idle is too high you waste gas, too low and the car will stall.
Same with a guitar amp, biased too hot and the tubes wear out prematurely,
too cold and there is extra distortion (and not the good kind). Setting
the bias also lets the technician take a look at the amp on his test equipment
and verify that everything (including your brand new tubes) is working
correctly. Remember also from our previous discussions that there are two
common "flavors of bias" used in guitar amp tube output stages, fixed
(negative) and cathode bias. Fixed bias often includes a trim pot for a
technician to optimize the bias to different specification tubes. Cathode
bias in theory is self biasing, meaning you can plug in any output tube
(with the same pin configuration) and it will seek its own bias point.
This worked fine during the golden era of tubes when tolerances and specifications
were held to exacting standards, but given today's all over the map (literally)
tube production, you will often experience tubes that will be outside the
usable cathode bias range of a specific amp and they will overheat and
fail especially if you are using a vintage cathode bias amp with new production
tubes . I saw this the other day when retubing a boutique Heritage
Briton amp, The set of EL84 tubes I installed were drawing a whopping 51
milliamps at idle and cooking themselves. Obviously the designer was looking
for a "class A " vibe, but 51 ma at 300 volts plus is an idle dissipation
beyond the tubes maximum rating. I needed to change the output tube's cathode
resistor to bring the tubes to a more reasonable idle current.
Let me also mention to not fall for
the Groove Tube numbered rating system that "gives different
tones" from early distortion to more clean headroom.
This is just so much snake oil salesmanship. It is an
excuse to sell more of the tubes they tested rather than
reject and discard them. Only the tubes with the middle
ratings 4 and 5 should be used. This rating system was
something perpetrated by Aspen Pittman (the P.T. Barnum
of the tube world)
|
.Another aspect of
using Groove Tubes is that if you replace your output
tubes with ones of the same rating they should not
require biasing. Similar types of rating systems
are used by other tube vendors such as Mesa Boogie
whose negative bias amps do not have a trim pot to
optimize bias given their claim that Mesa tubes are
selected and matched for their amps and no adjustment
is required.
This is in theory not a bad idea
and great marketing for these companies as it gets
you to only buy tubes from them, but there is several
possible things to think about. First, How accurate
is their matching or how much has the tube varied
from this during several thousand miles of UPS
shipping. A tube is a composite of several extremely
fine wire grids wound around metal posts. Vibration
and shock can alter their alignment and therefore
the resulting performance of the tube. Their are
also many amps on the market that do not contain
an adjustable bias control but have a trim pot
that will allow tube matching. |
Many vintage Fenders have this
and many modern Marshalls like the TSL and JVM have
multiple bias pots that let you bias and match the tubes.
You may ask why do I need a tube matching control when
I am buying matched tubes? The answer is that although
the tubes are matched the amp's circuitry they are plugging
into is not. The ability to precisely match the tubes
in the circuit they are in can reduce hum, increase picking
dynamics and create sweeter sounding power chords. When
I was gigging I always installed a matching control into
my amps. To me, it made a big difference in my tone and
gave me the ability to rematch the tubes as they aged
extending their usable life.
So whether or not you should have the bias checked on your amp if you install
new tubes using this type of rating system comes down to how finicky you are
about your tone. Some players are concerned with every little detail of their
equipment, some are not.
5- How long should my output tubes
last?
This is a question with no easy answer. It is dependent upon quite a few variables.
These include:
How often and long the amp is played? Your output tubes go from room temperature
to 700 degrees F whenever you turn the amp on and back to room temperature when
you turn it off. This thermal cycling causes expansion and contraction of the
elements in the tube causing them to drift out of spec. Do you play normally
for twenty minutes at bedroom levels or pound the tubes for four hours at stage
volume?
How much vibration is the amp subject too? Does your amp sit in one place in
your house or do you bounce it in the back of a pickup truck down a dirt road
on the way to a gig. Is your amp a tube combo amp? The combo amp is the hardest
operating environment for a tube. Tubes usually hang upside down which affects
their ability to dissipate heat. They are subject to vibration from the amplifier's
cabinet, being shaken by the speaker and also sit in the vibration field of the
speaker in direct line with the rear sound wave from the speaker. Obviously a
head style amp will have the tubes right side up and affords more decoupling
from speaker vibration even if the head is operated while placed on the speaker cabinet.
What are the operating conditions of the amp
itself? Is everything functioning correctly?
Is it biased right? Are the voltages in the amp
(by design) too much for the tube? Some amps
have 400-450 volts on the output tubes. Some
amps (mostly vintage) can have as high as 550
volts .Additionally many vintage amps with what
were reasonable voltages at the time were
designed to work on the 110 volt AC power common
in the fifties and sixties. Nowadays the common
voltage from the wall is 120 volts AC. A tube
guitar amplifier's power transformer is a step
- up transformer. It takes whatever voltage that
is input to its primary and steps it up by a
factor of roughly four. (110 volt AC becomes
450 Volt DC at the output of the rectifiers and
filter caps). Hooking this 110 volt transformer
to a 120 volt line can add another 40-50 volts
DC to the power supply stressing components and
tubes. In many cases it will exceed the maximum
voltage ratings of the already ancient filter
caps and cause problems for our modern tubes
that are not as hardy as tubes from the vintage
era.
Current production quality of tubes is inconsistent
to say the least. There are only four or five
manufacturers of tubes worldwide. All of the
distribution companies Ruby, Groove Tubes etc.
buy from these manufacturers test and re brand
the tubes with their own name. I find I can
use a brand that works well for a while and then
I start to receive bad tubes. You switch brands
and the cycle repeats itself. It's a gamble at
best.
6-Should
I replace all
my output tubes
if only one fails?
The answer to this has many variables. If you recently replaced your tubes and
one falls out or receives an impact, it is fine to replace just one tube providing
that you use one of the same specifications as in plate current and transconductance. If
the tubes have been in there for a while then replacing the tube with one of
the same specifications may not work as your original tubes have drifted away
from their published specifications and now your new tube is not going match correctly
with them. Therefore for optimum power and tone the entire set should be replaced.
An additional consideration concerning the
shorting of only one tube is if when it shorted
it also pulled down the bias voltage it will
cause the other output tubes to overheat
due to lack of bias and possibly cause
them to fail soon or at least be stressed
out of specification and not sound the way
they should.
7-How
often
should
I
replace
my
preamp
tubes?
Preamp
tubes
unlike
power
tubes
are
not
devices
that
consume
much
power.They
typically
conduct
current
in
the
area
of
one
milliamp
which
is
not
enough
to
stress
anything.
Fortunately when
preamp
tubes
fail
they
often
let
us
know
by audibly
producing
crackling,
hum,or
squealing.
Degradation
in
tone
over
time
is
harder
to
detect.
Notwithstanding
what
is
read
on
the
Internet
concerning
the
difference
in
sound
between
different
brands
of preamp
tubes,
it
is
not
possible
to
hear
subtle
differences
between
tubes
by
listening
and
then
pulling
the
tube
subbing
another
and
then
listening
again.
The
brain
is
not
capable
of
retaining
auditory
information
for
that
long.
The
differences
would
have
to
be
huge
in
order
to
hear
a
difference.
So
in
the
case
of
a
large
difference,
yes
you
will
have
found
a
bad
tube.
But remember
it
is
very
often
your
imagination
that
makes
you
think
one
tube
sounds
different
than
another.
The
only
way
to
actually
hear
the
difference
between
two
tubes
is
to
be
able
to
instantaneously
switch
between
two
tubes
in
the
same
amp while
you
are
playing.
This
is
not
something
that
can
be
done
without
creating
a
custom
test
rig.
I
also
recommend
to
test
using
this
method
as
a
blindfold
type
test
removing
any preconceptions
about
the
tubes
under
test
because
you
don't
know
which
one
you
are
listening
too.
I
have
used
this
type
of
testing
and
it
will
bring
out
extremely
subtle
differences
between whatever
two
things
you
are
comparing. |
|
8- How much better are New Old Stock (NOS) tubes?
There
is plenty
of discussion
on the
Internet
about
this
topic.
I admit
I have
played NOS tubes that feel and sound great. My position as a technician
though is to provide for my customer a good sounding reliable amp that I can
continue to service over time and reproduce the same sound and reliability for
my customer.As a boutique amp builder, I need tubes that will make each of my
amps sound the same and create an identifiable sound for my brand. Using NOS
tubes, this becomes a problem. If I install an expensive set of NOS that sounds
great, what do I do two weeks later when they go down hours before an important
gig. Current production tubes even with their poor reliability at least have
a reasonable expectation of working and sounding good. I can keep a stock of
many on hand to choose between and find something solid to replace any
current production tubes that have failed. With NOS that can't be done. Even
if the NOS tube has really never been used before, what were the conditions of
its storage and handling over the last thirty to forty years. It may have fallen
off a shelf to the floor etc.. And who can prove the tube was never used. It
may look clean and pretty in its vintage box, but that is no assurance it never
has been used. And I have read of several people
who advertise
to buy
used
tubes
that
they
turn
around
and sell
as NOS.
9-
Nothing
is more
misunderstood
in a
guitar
amp than
the output
transformer.
I
have
said
this
in
earlier
articles
and
it
bears
repeating
again,
transformers
are
the
least
likely
component
to
fail
in
the
amplifier.
Unfortunately
due
to
many
techs who don't really understand how an amp works or worse they do
and are unscrupulous there are many expensive and unneeded transformer replacements
taking place. A
case
in
point
Gerald
Weber,
a
self
appointed
Internet
guru
claims
that
it
is
necessary
to
use
the
highest
impedance
tap
on
your
amp
for
highest
efficiency
and
best
sound.
This
means
if
you
have
a
8
ohm
speaker
installed
and
a
16
ohm
tap
on
your
amp,
you
should
replace
the
8
ohm
speaker
with
a
16
ohm
speaker.
This
is
absolutely
not
true
and
suspiciously
is
coming
from
someone
whose
company
sells
speakers.
Impedance matching between amps and speakers is not completely critical. Matching
an 8 ohm speaker to an 8 ohm tap will provide optimum transfer of power and tone
to the speaker, but any well designed amp will be tolerant of a 100% mismatch(ex.
4 or 16 ohms connected to an 8 ohm tap). There may be conditions where it will
serve a purpose to create a mismatch. If you have an amp you like to play loud
to create distortion from the output stage, an impedance mismatch may serve to
lower the volume coming out of the amp to a more comfortable level. Additionally
an impedance mismatch may alter the amps tone in a way more to your liking( or
not).
10-
Can
I
remove
two
of
my
output
tubes
from
my
amp
to
create
output
stage
distortion
at
a
lower
volume?
The
answer
to
this
is
yes.
I
see
it
written
on
the
Internet
that
removing
one
or
more
tubes
from
the
output
stage
will
overstress
the
others
and
cause
them
to
fail.
This
is
not
true.
Each
tube
serves
as
its
own
independent
amplifier
stage
and
doesn't
care
whether
it
is
in
a
circuit
with
other
tubes
or
not.
It
is
important
which
tubes
you
remove.
A
typical
amp
with
four
output
tubes
is
set
up
as
a
push-
pull
amplifier.
This
means
that
two
of
the
tubes
push
and
two
of
the
tubes
pull
the
speaker
cone
from
its
center
resting
place.
If
you
pull
both
tubes
from
one
side
of
the
push
pull
circuit,
you
won't
hurt
anything
but
you
will
only
be
able
to
push
or
pull
the
speaker
through
fifty
percent
of
the
speaker
cone's
usable
excursion
resulting
in
loss
of
volume
and
horrible
tone.
In
almost
any
amp
if
you
look
at
the
output
tubes
going
from
left
to
right
the
two
tubes
on
the
left
are
one
side
of
the
push
pull
and
the
two
tubes
on
the
right
are
the
other
side
of
the
push-pull.
Therefore
pulling
any
combination
of
1
and
3,
1
and
4
,
2
and
3
or
2
and
4
will
keep
your
push-
pull
circuit
balanced.
It
should
be
noted
though
that
your
results
will
not
be
exactly
the
same
as
an amp
designed
with
two
tubes
instead
of
four
output
tubes.
The
transformer
in
the
larger
amp
is
made
to
handle
larger
power
levels
so
it
may
not
create
as
much
distortion
as
an
amp
originally
designed
with
two
output
tubes
and
a
smaller
transformer.
Also
removing
two
tubes
does
change
the
impedance
at
the
taps
from
what
they
are
with
four
tubes.
A
transformer
changes
what
is
input
to
it
by
a
specific
ratio
determined
by
the
ratio
of
its
primary
to
secondary
windings.
Therefore
if
you
change
what
is
going
into
it
you
change
what
is
coming
out
of
it.
Since
you
are
already
trying
to
alter
power
levels
and
tonal
response
by
removing
two
tubes
you
can
also
experiment
with
the
different
taps
and
see
where
you
like
the
sound
and
volume
best.
11-
Can
I
run
my
amp
with
no
speaker
load
connected?
This is a very misunderstood concept. Never run a tube amp without a speaker
connected. It can generate flyback spikes from the magnetics of the output
transformer that can create much destruction to the amp and transformer. Solid
state amplifiers are different. They can be run all day long on full output with
no speaker connected. Nor do you need to match speaker impedances to a solid
state amp. Lower impedances will pull more power from the amplifier than higher
impedances. You only need to observe the manufacturers minimum load impedance
specification. Going below this may cause the amplifier to overheat , go into
protect or fail completely according to the quality of its design.
That's all for this month. Next month I will provide a checklist of steps to
take when your amp is not working. Fell free to enter your comments and suggestions
below and also you can check out past articles in the Metro Music Mayhem archives.
